More foundation fun - straw bale garden room on clay

[Nick Thomas]

I've been following @ToughButterCup's thread with interest, as I'm just starting to put together a garden room. Thanks everyone for sharing so many interesting things there; my case is different enough that I figured a separate thread would be sensible. went on a straw bale building course in August and rather foolishly bought 100 bales to get building with before clearing the site or planning... well, anything, really. They're in the garage, which I'd quite like back! The site is cleared now and I've started digging with a vague plan, but I don't really know what I'm doing so I thought I'd see if anyone had any suggestions or criticisms.

The basic idea is a ~8sqm unheated, buildings-regs-exempt, planning-exempt room in the back of the garden, maximum height ~2.5M. Three square walls (4.6M long side along the wall, ~3M on the short sides), then a gentle curve on the side facing the house. The garden is a ~10Mx10M square, with a brick wall to the back, running northeast. I don't want to interfere with that wall much or rely on it for support, but I don't mind pushing on it (the level of the road behind it is a bit higher than the level of my garden, and there are a line of existing 2" plastic pipes for water coming from behind the wall. Only one intersects the wall; I'm planning to add a length of pipe to divert the water beyond the wall I'm building).

 

Straw bale construction is pretty lightweight compared to masonry (~160kg per linear metre of wall once rendered and plastered), and the resulting walls are nice and flexible - and thick, ~450mm. The straw generally wants to be kept ~300mm above ground level, though. The most northwesterly corner of the room has a high ground level compared to the rest of the site, with a ~4M high apple tree growing in it. The site slopes down both to the south and east; it's about a 140mm difference from NW to SE corner. To fit inside the 2.5M height restriction, I've already decided to minimise ground clearance for the straw in this corner - it won't get 300mm, I'll just belt on many types of water protection to compensate, and if that doesn't work, replace the straw there every few years ^^.

 

Fairly awful screenshot of a model to give pictures to those words:

 

 

The door and window placement is all very provisional, and, uh, I don't have much idea of what the roof is going to be like, other than "lightweight". Probably not flat, but whatever will work with the pitch I can get in 2.5M. Asphalt shingles on some lightweight rafters, perhaps. It's unheated, and I'm not planning to insulate the floor or roof much; the walls will be super-insulating, but I'm after the soundproofing more than that.

 

Since I'm building-regs-exempt, I don't want to overengineer the foundations - I just want them done so I can move on to the fun bits :D. At the same time, I don't want to be jacking up the whole structure in a few years to re-do them because I made bad decisions now.

 

I'd like to minimise cement & concrete use, but not completely averse to using a bit. Straw balers generally love tyre piers rammed with gravel at 2M centres, with a timber box beam / baseplate on top of those to spread the load. I don't really want the tyres leaching their substances into the garden, so have ruled that out ^^.

 

What I've come up with after a few false starts, and after ruling out screw piles on cost grounds, is to have 50cm x 50cm "pads" of pea gravel (I happen to have lots of 20mm hanging around) as deep as is sensible to minimise movement. Compact it, pop hollow concrete blocks on top of those pads to get the height above ground level needed to keep the straw happy. Varying the depth of the holes and the amount of pea gravel allows me to get a level for the timber. I'm aiming for a maximum of 1M between the concrete blocks to minimise the amount of timber needed to span it (a random timber calculator suggests 100x50mm would be more than sufficient for that; I'm planning to over-spec that by having each side baseplate be 100x100).

 

I've been digging along the back wall today, and here's where I've gotten to so far:

 

I've aimed for ~500mm going from the ground level given by the NW corner. As I get further from that, they get shallower due to the slope, of course. The wall is, I think, two concrete blocks of foundation beneath the bricks, and probably poured concrete below that - I've not gotten quite that deep at any point.

 

It's made ground with a lot of clay - these houses are built on an old brickworks that was filled in. My chances of finding undisturbed ground with a shovel are pretty minimal, I think (and there's no access for a mini digger). When it's dry, the surface gets cracks big enough to get your fingers in and becomes impossible to get through. Fortunately, we've had some rain so it's a lot softer right now, but going down even this far it really stiffens up again. There's lots of white stone mixed in, and I'm digging up nuggets of clay like this:

 

 

As far as I can tell it's "expansive clay", and before backfill the whole area was a "luvisol" soil type - so there'd be a fat shelf of clay at some consistent depth if it hadn't all been turned into bricks. I did a soil jar of the topsoil which had plenty of clay, but also a fair bit of silt and organic matter.

 

I did some tissue-paper calculations and I'm pretty confident the soil that's there has the bearing capacity for the structure - it's quite light and the pads are quite broad,so it's well spread. Say ~5 tonnes total for the structure and foundation materials, ~5kN. Spread across 10x500x500mm pads, 2.5sqm, gives 2kN/M², which compares very favourably against the numbers in the table here: http://environment.uwe.ac.uk/geocal/foundations/founbear.htm .

 

The biggest question I have right now is pad depth - would 500mm the way I'm calculating it be enough to minimise other movement (particularly frost heave) in this case, or should I be aiming deeper? 1M? 1.5M? The calculations for concrete trenches get a bit mad when you add clay and trees to the mix, but concrete is really not very tolerant of movement.

 

I've got a nice big bag of hydrated lime and was considering digging some of it into the base of each hole in a highly amateur attempt at soil stabilisation, if anyone has an opinion on that.

 

The 500x500 pad approach could do with a critique as well - assuming I get the depth right, does it have a chance of working? Do the pads need to be bigger? The baseplate and bales are ~450mm wide, so I'm sure I can't go narrower ^^.

 

I read about rubble trench foundations, where the whole length would be excavated, and I guess I'm about halfway there (2M out of 4.6M dug out so far along this wall). I'd be leery of removing all the soil at once with the wall there - what I've removed so far is pretty much at the limit of what I'm comfortable with there - but are there solid reasons why a single long trench filled with the gravel would be better than the individual pads, assuming I keep the concrete blocks at the same spacing either way? I could always dig out the remaining soil to whatever depth is sensible, one section at a time, if it would get a better result.

 

Earthbag builders seem to go with insulating the foundation rather than getting deep enough to avoid frost heave; I wonder if that might be worth it here? I could get away with it on the short and curved sides, but not along the back wall.

 

As I say, four holes dug so far. I figure I've got 6-10 left to do around the perimeter, depending on how curvy I decide to make it and whether I make the short sides more than 3M deep, and four to do internally to provide support for a suspended timber floor.

 

Thanks for any help!

Edited September 15, 2023 by Nick Thomas
NW corner deets

[joe90]

Interesting project, my first ideas on self building were in straw bale many years ago and i still think there is a place for them in house constructions. I too went on a couple of straw builds to get some hands on knowledge of the process. I would have thought angular gravel rather than pea gravel would be better for the founds as they bind and stabilise whereas pea gravel rolls around (used on arrestor beds for runaway lorry’s etc). Line  the holes with membrane to stop stone mixing with soil. You don’t mention finishing the straw bales, lime  render ? I will be following this thread with interest 👍

[PeterW]

Turn blocks on their sides and cement 4 together (2 one way, 2 across) and they will spread the load much better. 

[Nick Thomas]

7 hours ago, PeterW said:

Turn blocks on their sides

 

Thanks, that makes a lot of sense now I stop to think about it. And there are 440x215x215 blocks I could use along the south part where everything is lower (so there's more height to make up). Lovely.

 

15 hours ago, joe90 said:

I would have thought angular gravel rather than pea gravel would be better for the founds as they bind and stabilise whereas pea gravel rolls around

The pea gravel is just what I have lying around. Do you think it makes enough of a difference, once compacted, to be worth using angular instead? For the whole depth, or maybe I'd get the same benefits from just having a layer of it on top of the pea gravel?

 

15 hours ago, joe90 said:

Line  the holes with membrane

Definitely 👍. This was on my mind, but not enough that I remembered to do it in the first test pit. Fortunately, I'll be digging that up in a minute so will get to re-do. Details in a sec...

 

15 hours ago, joe90 said:

You don’t mention finishing the straw bales, lime  render ?

Yes, at least on the three accessible sides, the plan is a nice white lime render. The side that goes up against the brick wall will have a ventilation gap, then some vapour-permeable membrane to protect it from the elements. The bales should also get a pre-installation dip (since they won't be accessible) in a slurry of lime to give some fire resistance. Having the brick wall there is throwing up some fun challenges, for sure, but I don't much fancy demolishing it.

[Nick Thomas]

Overnight I decided to have a go at digging deeper, since the back four pits would be more difficult to access than any of the others in case it turned out they *needed* to be deeper. I'm glad I did - against expectations, I've found a lovely flat hard horizon at about 0.7M below ground level. It seems the brick wall goes at least four concrete blocks down, and I'm around the middle/bottom of the third one down at this point. At 0.5, I was still in topsoil 😬 .

 

 

My spade just bounces off this layer. I can dig into it with a pick - it's mostly compacted gravel, various other bits and pieces mixed in, a few whole bricks and so on. This depth must be fine. SURELY.

 

 

Mind you, this bottom 0.2M is easily as much work as the top 0.5.

 

[saveasteading]

44 minutes ago, Nick Thomas said:

The pea gravel is just what I have lying around.

I'm ok with pea gravel as it will self compact into the base, whereas an angular stone will need whacking.  It's only got to hold up a temporary structure, which will presumably be unoccupied in testing conditions.

do you need it though? hard clay is hard clay and any settlement will presumably be even.

 

Other than that though, its up to you, ie I won't be encouraging straw bale construction. In my opinion it is a gimmick unless you are in a dry climate.

Even there, they don't appear to do anything with masses of straw bales.

I've messed about with adobe, then decided not to us it even for inner garden  wall features.

 

I'd say  you should regard your project as a temporary structure and don't spend much on it 

Good luck with it: I admire the enthusiasm and adventure.

I will click 'follow though', and watch your adventure from safety. I hope experienced  advice comes your way.

 

BUT. do work out how you are going to build it, including roof, floor and openings. it's never great to just start and bodge along.

In your case you need to keep rain and damp off the straw, or it will rot.

 

[Nick Thomas]

On 16/09/2023 at 17:18, saveasteading said:

I'm ok with pea gravel as it will self compact into the base, whereas an angular stone will need whacking.  It's only got to hold up a temporary structure, which will presumably be unoccupied in testing conditions.

do you need it though? hard clay is hard clay and any settlement will presumably be even.

 

As I say, I have no idea what I'm doing ^^. The topsoil definitely swells and contracts quite dramatically so i'd imagine taking that out of the equation can only be a good thing, even if it might cope alright without it. Thanks for the vote of confidence on the pea gravel - I don't have a whacker plate, just big boots and some lumps of heavy stuff!

 

You say temporary structure, but I'll be unhappy if it lasts less than 25 years. The budget is around £2,000, although it's not a strict thing and a lot will depend on what I end up doing for the roof.

 

On 16/09/2023 at 17:18, saveasteading said:

In my opinion it is a gimmick unless you are in a dry climate.

 

Honestly, I appreciate the input, even (especially?) when it's "this seems like a terrible idea" ^^. I'm not here to evangelise straw, just giving it a try. If it's a big wet rotting failure, it will at least be a garden-room-sized one, rather than a house-sized one and if that happens, I can try my hand at straw bale gardening instead. Keeping it dry during construction is a challenge, as you rightly note, but once built - I've been inside straw bale buildings that have lasted plenty longer than 25 years already, in inauspicious conditions, so I don't share the same moisture concerns. Time will tell if I'm just unreasonably optimistic. I have some straw bale specialists weighing in from time to time too, but this scale of project is much smaller than they're used to, I think.

 

Onto a bit of progress - 

 

 

Hole number five, this one in the SW corner (so 3M closer to the house). No made ground here that I can see, just a smooth surface of very hard clay, gently sloping down towards the house. The ground is pretty saturated at the moment and the clayey topsoil is sticky and plastic, but I was pretty much bouncing off this layer. In theory, I'll be digging another hole a bit like this in spring to gather material for the internal clay plaster. A bulk bag of the stuff is £600!

 

 

Here I'm just filling the other four holes with gravel and compacting it down. Still haven't laid the blocks on their sides, but that will come once I get my "L" shape together, which is one pit away. I've gotten some 440x215x215 blocks and should be able to get a decent level by varying the amount of pea gravel in each hole and using different heights of block.

 

This timber is 100x50mm C16, and not strong enough to span the 1M gap between piers according to a random timber calculator. Instead, I've got some 150x47 C24 which the same calculator is happy with. Butt join, 450mm wide, more or less following the "box beam" strawworks detail.

 

I'm currently umm-ing and aa-ing over whether it needs OSB3 top and bottom for the box part of the box beam. Presumably it adds significantly to rigidity against lateral loads, and it gives a space that can be insulated (for this space, the acoustic, rather than thermal, insulation is the important bit), but not much to vertical loading? The sheets aren't cheap, and need a bunch of precise cutting, then gluing and nailing, so it's a saver if I can skip it. Fortunately, I've got plenty of time and a mock-up to run through before that decision needs to be finalised.

[SteamyTea]

2 hours ago, Nick Thomas said:

Honestly, I appreciate the input, even (especially?) when it's "this seems like a terrible idea" ^^. I'm not here to evangelise straw, just giving it a try

I think the use of straw bails is a terrible idea.

But the fact that you are giving it a go is brilliant. 

The foundation side of it is interesting as well, you will get a good 'feel' for what is happening under the ground.

The roof will be the best bit.

I would knock up some simple beams from OSB or ply. But I would say that as I knocked one up just for a laugh one afternoon, just to see what would happen.

 

 

 

[Gus Potter]

On 15/09/2023 at 21:46, Nick Thomas said:

I don't really know what I'm doing so I thought I'd see if anyone had any suggestions or criticisms.

Well done for giving this a go, thanks for posting as very interesting.

 

On 15/09/2023 at 21:46, Nick Thomas said:

Straw bale construction is pretty lightweight compared to masonry (~160kg per linear metre of wall once rendered and plastered), and the resulting walls are nice and flexible - and thick, ~450mm.

Fair enough the walls are light in weight and I assume so will be the roof. How are you going to tie it down to stop the roof and walls lifting up in the wind? Are you going to use the long spikes to pin the roof wall plate down a good bit into the bales?

 

On 15/09/2023 at 21:46, Nick Thomas said:

did some tissue-paper calculations and I'm pretty confident the soil that's there has the bearing capacity for the structure - it's quite light and the pads are quite broad,so it's well spread. Say ~5 tonnes total for the structure and foundation materials, ~5kN. Spread across 10x500x500mm pads, 2.5sqm, gives 2kN/M², which compares very favourably against the numbers in the table here:

Bit of house keeping on the maths mainly for all in case they use the same sums.

 

5 tonnes =  5000kg. Convert to Newtons 5000 x 9.81 (just say 10) / 1000 = 50 kN (not 5). If you have 2.5m^2 of pad area then roughly the pressure at the bottom of the pad will be 50 / 2.5 = 20 kN/m^2.

 

If it is a temporary structure (say snow loading with a return period of 20 years as a ball park figure add some 0.35 kN/m^2 to the roof loading unless you are at a high altitude UK wise.

 

A good bit of that soil you see next to the wall will be backfill (made ground).. ask how are the founds for the retaining wall formed. How far do they protrude into your garden? Is the soil you see just well compacted fill?

 

The main thing here to watch out for is that if you are digging next to a retaining wall is that they are designed for two main things. One that they don't tip over and two that they don't slide into your garden. The sliding is important here. The sliding forces are resisted often by the friction under the found and also by the soil on your side and this can make a big contribution towards sliding resistance. I would avoid disturbing more of the soil close to the wall on your side or lowering the ground level on your side close to the retaining wall.

 

2 hours ago, Nick Thomas said:

I'm currently umm-ing and aa-ing over whether it needs OSB3 top and bottom for the box part of the box beam.

I would give the OSB a swerve as I have my doubts about it performing well in this application.. it is thin and you have fixing issues.. and any long term moisture is not good for it. What about using cheep fencing slats with some 3 -5 mm gap between that are treated, these are more breathable than say OSB? Anyway it keeps the ethos of using more natural rather than highly process materials (lots of glue) such as OSB. If you have doubts get a bit of OSB and leave it out in the rain / in a bucket of water for a while and see what happens to it.

 

As the thing is light weight how are you going to stop it blowing over sideways in the plane of the retaining wall? I assume you are going to make the roof like a stiff beam that transfers the sideways wind load to the rear wall?

 

In high winds you could easily get 100 kg/m^2 wind uplift on the roof unless you live in a very sheltered spot. At the same time you get the wind blowing on the sides of the building wanting to push / suck it over sideways.

 

I think it is these wind stability issues you need to concentrate on rather than the founds, although they are important.. settlement won't generaly hurt anyone (unless left unchecked) where as the thing collapsing or bits blowing off and hitting folk in the wind is a concern.

 

 

 

 

 

 

 

[Gus Potter]

53 minutes ago, SteamyTea said:

I think the use of straw bails is a terrible idea.

But the fact that you are giving it a go is brilliant.

I think it's a great idea @Nick Thomas as a project to do on a budget of 2.0 k.. quite a challenge for the walls and founds, with the roof extra over.

 

Have not dug out my book on bale building but keeping the water out of it is a biggy. That means things like a big eaves overhangs to shelter the top of the wall and mitigating spashing up of rain.. @Nick Thomas mentions 300mm to the bales which is well above a normal spash zone. Also you want to get plenty wind circulating round about to dry things out.. not sure about how close it should be to the retaining wall for that reason.

 

The vertical loads on the bales are not huge.. they will compress over time but it is not like a house designed for 50 years with more onerous loading from say floors, tiled roofs.

 

If it was me I would make sure if the bales start to misbehave then devise a way you can prop the roof / jack it and insert a more traditional timber frame instead so you don't lose your shirt or do the bales again.. tricky as they need to settle in as you build.

 

Normally a garden building like this could cost 15 - 20k (if you went to a turnkey outfit) once serviced with the slabs outside / drainage etc and that would be for a square box. But here we have a bit of character in terms of the shape and form.. and for that you could pay a lot more!

 

Keep posting Nick!

 

 

[Nick Thomas]

3 hours ago, SteamyTea said:

I would knock up some simple beams from OSB or ply. But I would say that as I knocked one up just for a laugh one afternoon, just to see what would happen.

 

I did read and appreciate your thread on that, and there is a box beam detail using timber i-joists: https://strawworks.co.uk/wp-content/uploads/2021/08/05-02-Floor-plate-structural-box-beam-details-with-timber-i-joists.pdf . Thought of going this for, ooh, at least 30 seconds before deciding it was over-complicating things ^^.

 

2 hours ago, Gus Potter said:

How are you going to tie it down to stop the roof and walls lifting up in the wind? Are you going to use the long spikes to pin the roof wall plate down a good bit into the bales?

The roof gets attached to the roofplate with regular fixings, it's just wood->wood. That's both pinned with hazel spikes going into the bales, and held to the bales and floorplate with packing straps. What we did on the course was squash this whole sandwich of baseplate, straw, roofplate, together with trucker's straps, then used packing tape to keep the compression in place. There are also timbers running the full height of the wall to act as posts for the door and window frames. They're not structural, in the sense that the walls support the roof even if they're not there, but they get tied to baseplate (and box beam) and roofplate, so help to hold the whole thing together.

 

How to attach the whole thing to the foundations is a bit more of a question. I was planning on using more of the packing straps to hold the baseplate to the concrete blocks, which adds ~280kg to the overall weight of the structure, but they're just sat on the pea gravel with nothing holding the whole thing down. I was thinking about driving a metal or timber support down and tying that to the blocks, but only vaguely so far. I suppose the friction forces could be calculated and compared to the wind loading, but...

 

2 hours ago, Gus Potter said:

5 tonnes =  5000kg. Convert to Newtons 5000 x 9.81 (just say 10) / 1000 = 50 kN (not 5). If you have 2.5m^2 of pad area then roughly the pressure at the bottom of the pad will be 50 / 2.5 = 20 kN/m^2.

 

Ooof. Thanks for checking my maths here, it's really never been my strong point, but that's a pretty awful fail. I'm still well in range for hard clay soil, at least. You had me hastily re-checking my timber beam calculations, but I don't seem to have made the same mistake there, which is fortunate as I didn't have anywhere near as much margin while cheaping out on the timber!

 

2 hours ago, Gus Potter said:

I would avoid disturbing more of the soil close to the wall on your side or lowering the ground level on your side close to the retaining wall.

 

(And thanks for your other comments about the founds for this wall too). Yeah, I'm well-resolved not to dig any more of this out, and the levels there are fine, fortunately. All the holes are filled again with pea gravel now, so the wall should be back to how it started, more or less. I suppose if I were pushing down really hard on the retaining wall foundation on one side, that could lead to it tipping, but the forces required for that feel... extreme. Still, might have been wiser to give the wall more clearance, with hindsight. I'm scrapping for space, though - it's not a particularly large garden and the bales take up a significant proportion of the footprint.

 

2 hours ago, Gus Potter said:

I would give the OSB a swerve as I have my doubts about it performing well in this application.. it is thin and you have fixing issues.. and any long term moisture is not good for it. What about using cheep fencing slats with some 3 -5 mm gap between that are treated

 

Hmm, hadn't thought of this, thanks. Assuming my maths isn't off again, and ignoring the gaps between timbers for the moment, it's looking like another £450 of timber though (top and bottom of both base and roof boxes, around 15 linear metres of wall, 450mm wide, ~£1/linear metre of 100mm wide fencing slat). Cheaper than the OSB3, but not *super* cheap.

2 hours ago, Gus Potter said:

I think it is these wind stability issues you need to concentrate on

 

Genuinely hadn't thought about this aspect of it, and you're right, it needs attention. Thanks for bringing it up. Fortunately, the site is very well sheltered, although less so to the east - still, we had 40mph winds today and I was playing with a tarp without it being an issue (very scientific measure, is tarp wrangling difficulty). Definitely trying to tackle these catastrophic failure possibilities at the design stage, but I feel I need to do some reading before I can actually interrogate the subject 😬.

 

2 hours ago, Gus Potter said:

@Nick Thomas mentions 300mm to the bales which is well above a normal spash zone. Also you want to get plenty wind circulating round about to dry things out.. not sure about how close it should be to the retaining wall for that reason.

 

300mm minimum - which I'm violating on one wall. They prefer 450mm.

 

I'd put the retaining wall buildup through the ubackus calculator which indicated there'd be an issue with condensation if the bales were hard up against the brick and the delta between inside and out was greater than 10°C ; I'd forgotten to mention it. The same calculator says a 10mm ventilated cavity is enough to manage it even with a 20°C delta; I'm thinking I should go for ~50mm anyway, for practical reasons (I'm going to be fiddling with compression straps and all sorts along there). It's also not a solid brick wall - there are those wooden slat panels from ~1M up, so there will be ventilation from the NE as well, which ubackus didn't know about.

Edited September 19, 2023 by Nick Thomas
s/length/height/

[joe90]

Yes keeping the bales dry during building is a big problem, when I was planning building a bale house years ago I thought of building a scaffold outer with tin roof and sheeting first and effectively building “indoors”, I even priced up buying the scaffold (second hand) and was told as long as I kept the put locks oiled I could sell it afterwards with little loss. I know this is not possible for yours but keeping it dry during the build is very important IMO.

[Nick Thomas]

Aye. My plan at the moment is to try to get the bales up on a single dry day, then protect them with tarps and/or vapour-permeable membrane. As long as I size, dress, notch and dip the bales that need it, and keep them organised, raising the walls should go quickly - and I've got a few helpers I can call on ^^. It's OK if the side of the bales can get wet, as long as they can also dry out - but water from above is death.

[Nick Thomas]

The order: 4200x150x50 C24. The delivery: 4800x150x50 C16.

 

Timber calculator is still happy with it, albeit with finer margins, and honestly I prefer not having to joint two bits of timber to get 4.6M, but... sigh. Will check the order more carefully next time.

 

The C24 has disappeared from their website too, so presumably they've realised they don't have it. Price/metre is about the same either way. Might just let it go.

 

 

[saveasteading]

On 'This Farming Life', on BBC there is a couple living in their self build straw house, and now building straw glamping huts.  May be of interest, but is brief between stuff about animals.

episode 4.

[Redbeard]

"The order: 4200x150x50 C24. The delivery: 4800x150x50 C16."

 

A similar thing happened to me. 'Never mind; bring it back and we'll sort it'. Got it all off the roof-rack. Bloke comes round with a stamp, blanks out C16 and stamps C24: Sorted.

 

The explanation, which one has to believe, of course, is that sometimes supply prices are so close that they buy graded (but not stamped) C24 and stamp as C16 or C24 according to the buyer's request. Alternatively they could of course stamp it all C24 and sell it to you for the C16 price! 

Edited September 21, 2023 by Redbeard
Added quote as continuity lost

[Nick Thomas]

A bit more progress - I have (some of) a structural box beam on top of piers \o/

 

 

Still got a fair bit of levelling to do (might cheat with some packing sheets), noggins to put in over the piers, and prep for the vertical beams the doors and windows will be attached to. The positions of those are pretty much finalised now - where they were in the original plan, but minus the two additional windows on the curved section. The doors I've got lying around are half-glazed, so I reckon there'll be enough light without those - and if it's not, I could always add a rooflight.

 

I also need to get that final, curved side in (two more holes to dig!) To do that, I'm just going to attach straight beams at a 15° angle going out to each of the middle piers, then straight over the central gap. That seems like it'll do the job with only the tiniest bit of overhang here and there, which isn't a problem - the finishes will overhang the beam too.

 

 

So much easier than trying to make glulam beams out of ply 😅.

 

i've concluded that I do need to cover at least the top of the box beam with *something*, mostly to stop critters from getting up into the straw from below. Taking another look at @Gus Potter's suggestion of bits of wood; I happen to have a lot of old decking timber and a few pallets lying around, which might do in a pinch.

 

Wondering if covering the bottom as well would be a good idea from the point of view of resisting the timbers spreading out due to the load coming from above. Since this face won't be in contact with the straw, also wondering if I could just use mouse mesh, which is reasonably cheap and, presumably, more than strong enough to help. I might just be inventing more work for myself, though.

 

No progress on reading up on what the wind will do to the poor thing, but I've belatedly recalled that there was a garden shed sat in a more exposed location last year, with just a timber base, and that went nowhere. It just rotted away from the bottom up. This will be a fair bit heavier than that, and I can ensure the roof stays attached by reference to my solar panel design document which calculated wind loading on the panels for this location \o/. The thing that's left to reason about is whether friction between the concrete block and the pea gravel will be sufficient to stop it from being pushed off the base, or if I'm going to need to anchor it down into the gravel somehow.

 

It's getting dangerously close to playing-with-straw time.

[TonyT]

What are those strange to me at least blocks with holes in them?

not seen them before

[saveasteading]

1 hour ago, TonyT said:

What are those strange to me at least blocks with holes in them?

These are hollow blocks. I've not seen that small  size  recently.

It saves concrete and sometimes cost, lighter to handle,  and allows for the addition of reinforcement in a wall.

Also quite a lot weaker so maybe not a great idea as shown here.

[Nick Thomas]

29 minutes ago, saveasteading said:

Also quite a lot weaker so maybe not a great idea as shown here.

 

They're rated for 7.3N/mm², although I don't know if that's in both orientations or just the voids-vertical one. Easy to break in half with a hammer through the thin walls, anyway (I needed a few halves). I wasn't really thinking about the possibility of them failing, but none of the timbers happen to be placed only above the void space - the weight is always spread across some of the solid bits.

 

Originally they were placed with the voids running top-bottom, but another poster upthread suggested this orientation and I like it for being able to easily strap them to the wood. There were solid concrete "foundation blocks" I could have gotten instead, but I quite like these in this application. Real stone would have been better, but isn't £2.50/piece ^^.

[saveasteading]

50 minutes ago, Nick Thomas said:

They're rated for 7.3N/mm², 

I'm pretty sure that is only when upright. I wouldnt be happy with these and would fill the voids with drypack concrete, though that completely defeats the object of saving concrete.

Otherwise, it probably won't fall down,  and if it did, the bales will crumple and stay standing.

It's your experiment.

[ProDave]

I only worked on one straw bale house.  That was using the straw as insulation not structure.  they basically built a larsen truss timber frame with a massive gap between the inner and outer frame, exactly one bale wide and the bales were stacked in as the insulation but not really doing anything structural.  

 

IIRC the outer part of the larsen truss was erected first and clad in OSB and membrane and the bales stacked from inside before the inner leaf of the larsen truss was fitted, meaning stacking the bales was a dry, indoor job.

 

The big challenge they had was sourcing the bales.  Small bales are not easy to find now, most balers are round, or much larger squares.  They had to find a farmer with an old small baler, and then next challenge was a good weather window to bale them completely dry, and then a dry barn to store them until ready to go into the build.

[Nick Thomas]

2 hours ago, saveasteading said:

I wouldnt be happy with these and would fill the voids with drypack concrete [...] It's your experiment

 

It is, but I'm here for advice and I'm trying to take things people say into account ^^.. Just spent a bit reading about cars and boats crushing these things when laid side-on, which is sobering. I'll flip them upright, re-level, then strap through them vertically. It's not that much harder. Thanks for bringing it up!

 

2 hours ago, ProDave said:

The big challenge they had was sourcing the bales.

 

Yeah, the "flat 8"s seem to be a little niche these days, although it varies from area to area. The farmer I got mine from makes them for horsey people, but they're almost perfect for building with. There's a build going on at the moment with the big ones (hestons?), although I don't have a link offhand.

 

We did pieces on infill and and SIPS, and they both look fine - definitely more suited to mass-market stuff, but more expensive than load-bearing. Also lacking a bit of the charm, at least to me.

[saveasteading]

On 21/09/2023 at 17:07, Redbeard said:

they buy graded (but not stamped) C24 and stamp as C16 or C24 according to the buyer's request

I believe that. There are 5 timber mills within 20 minutes of our project, but they don't produce c24. The local timber is very fast growing and isn't used for structural timber.  I desiged the timber frame and tried c16 assuming better cost. But it wasn't cheap enough compared to imported c24.

So we bought lots of c24 from a major timber supplier. Very good quality in the lack of knots and shakes, but quite wide grained and we don't know the source. 

I expect in Finland,  Estonia or Canada or wherever, they don't stamp the timber, so each end vendor can do as they choose.

Therefore i reckon you are right...if someone wants c16, then some suppliers may stamp it thus to a avoid any complaint.

Meanwhile in the SE, doing a tiny , family project we have c24 that is full of knots and has lumps missing: there is no way it would all have passed a physical  stress test.

 

 

 

[saveasteading]

On the hollow blocks. Upright will be much better. If not too much hassle or cost, can you fill them with gravel?. I couldn't prove any improvement but it would add some stability, especially at any weak points ( which these blocks sometimes have), and would increase the bearing surface if the timber, spreading the load.