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More foundation fun - straw bale garden room on clay

Nick Thomas

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Oh yeah, there is a bit of a rush to get the render done now. This little danger ball fell out of the gap between tarp and straw on the outside of the garden room while we were working on the insulation:




There was definitely something gooey inside when I crushed it; unsure if queen or eggs, but I'm very sure I don't want another to show up.


I've been walking around slapping on the tarps every week or two since I got them up, being quite paranoid about the straw's (in)hospitability to critters, but this is the first time something has come of it.


I very much do not like wasps.

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  • 2 weeks later...

Just fiddling around with the window openings.


Turns out the window posts I put in for the "big" window got knocked out of line when I compressed the bales, and I didn't correct it in time. So, they're about 2° out of level :S. Squaring the opening off is fine enough - I was going to build a window box anyway - but means the opening is much narrower now - around 745mm instead of the expected 860(!). I'm just going to order a premade timber unit of the right size. Bit of a blessing in disguise actually, since it'll come with a frame and be double-glazed to boot. Still on the fence about whether I should have a top piece of 2x8 for the box, or just let the window go up to contact the roofplate. It'll open from the bottom, so there's no possible issues with clearance.




Over on the opposite side, I've got that round window to fit. I've decided to build a mostly-circular sleeve out of lengths of  2x8 offcuts, hold that into the wall with more hazel stakes and/or strapping, then screw the bubble window to it on the outside. Simples. I cut the timber today - 12x 121mm long pieces, 15° angles either side. It's glued up and strapped together now, and is hopefully going to work exactly as imagined. Inside, I can have the wall taper in to meet the timber sleeve, which should help to make it feel bigger than it actually is.




I'll need something between the plastic and the timber, and was originally thinking a rubber gasket of some kind, but they turn out to be very expensive. So maybe it'll just end up being a bead of transparent silicone, or maybe some weatherstripping compressed by the screws that'll hold the plastic in place 🤷‍♂️.


In parallel, I've had some lime delivered - I went for 500kg St Astier NHL3.5 in the end, from Lincolnshire Lime, on the strength of a phone call with them. More foolproof than the putty and, after accounting for the fact that the bags of premixed rough stuff contain sand and the bags of lime don't, about the same price. A few of the bags were split on the pallet, but they refunded for those without quibbling, and say it should still be good to use. Fingers crossed.


I still need to order the sand, but regular coarse sharp sand should be OK and I need to finish sorting the walls out and make sure I'm happy with windows and doors first. It's all feeling pretty close now, though.




Oh, and I finally managed to get a persuader - 7.5kg wooden mallet - so the walls will be getting a literal beating as I try to improve how level they are. In particular, the one that's due to take the round window is still pretty far out.

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  • 2 weeks later...

Just adjusting and trimming the bales to prep for rendering. The persuader ended up being very effective - the walls are a lot more level than when I started.


Dug out the hole for the circular window more as well - it's looking pretty convincing, although it's not fixed in place yet.




I got the left hand side done last weekend, and the right hand side (mostly) done this weekend. The hedgetrimmer works (but isn't amazing) for the middle of the wall; the alligator saw works wonders at the corners.




At the corners, you're alternating between bale end (where there's strings) and bale side (where there isn't), so trimming off overhanging straw is a bit involved. For the ends, you can cut the strings, rip a load out with your hands and retie; for the sides, you can cut ~15cm off without encountering them, and not even I'm that far out.


It generates a lot of loose straw; it's very pleasant to be able to take a good proportion of that and stuff it back into gaps in the wall ^^.


More details expose themselves... before I render, I need to cover over the sides of the baseplate and roofplate, where all the raw edges and fixings are, with wood fibre board; put up the wood underside to the roof; sort out eaves ventilation; and get some kind of bead fixed close to the top of the baseplate. I also need to put down chicken wire to cover the gaps, and I was thinking about getting a roll of 600mm wide DPC - like https://mkm.com/product/pvc-damp-proof-course-600mm-x-30m-b012378 - and using it to fashion a skirt, fixing it behind the render bead and allowing it to go down to the ground, maybe with a bit of wood at the bottom of the box beam to encourage it to 'belly out' a bit. That'd hide the chicken wire and the blocks and the void quite tidily, I think.


The replacement square window has been ordered - pine, double-glazed, bottom-opening, toughened glass. Nothing fancy; apparently it will either take ~7-10 days to arrive, or 6-8 weeks. Maybe twice the price of an equivalent uPVC window, but I just couldn't pop one of those in.


Render mesh (10mm openings, which is better for lime apparently) and hessian has also arrived. I've got enough mesh to do a layer over all the walls, inside and out, so will probably do that. I've not rendered before, so I'm looking for every bit of help possible to keep it from shrinking, cracking, and falling straight back off the walls.


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Posted (edited)

I don't really know how obvious the differences are in the photos, but that's all the straw trimmed now. I'm onto boarding along the roofplate.





(The white powder is pyrethrin-based wasp killer, I had a rather persistent visitor in an unfortunate spot ^^).


I've belatedly realised that I need to do... something... above the window. I'm told proper houses generally get seven bales high per storey, so you get a whole bale above the window to hang render off, but I'm six high and so straight up to the roofplate. I need to hide the OSB as well.


Once the T&G roof underside cladding is in place (12.5mm thick, treated pine) there will be ~130mm of the board still showing.  I guess I clad the OSB with offcuts of the same stuff the roof underside is getting, have render bead / a bit of wood covering the edge of that and make that support the render over the span? It'll have the mesh to help it stay together too.


I guess I could skip the render by having the cladding come up vertically too - perhaps double up the board behind it to get it to ~30mm thick, so it sits flush with the render either side. That feels a bit weird though.

Edited by Nick Thomas
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1 hour ago, Nick Thomas said:

Got any space in your garden? ;)

It’s already full of sheds, greenhouse, workshop and garage 🤷‍♂️

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Now we're motoring. Got the underside of the roof clad and finished covering the roofplate with particleboard:




There's a few rough edges to it, but it hid far more of them than it introduced ^^.



Installed bellcast render bead and DPC skirt at the base, all around the perimeter




Really need to trim them back and work out how the render is going to look. The pictured corner is the worst one since most of the bale's "ear" is gone down there. I'll end up sculpting it with lime-straw mix, I imagine.



Finally, got some stainless steel expanded metal lath covering the roofplate and the junction with the straw.




Apparently it's more usual to use hessian or reed mats to cover over wood when lime rendering, but I'm a bit skeptical of it. Time will tell ^^. I've got it (and the beading) fixed with stainless steel screws and washers at 600mm centres, but apparently 100mm (!) is better, to minimise movement of the lath. I'll do that with some staples, I think.


I'm going to experiment with moulding a door and window head using more of the lath; I figure I can make a sort of J shape with the mesh by fixing strips with the screws then pulling up, so the screwhead is on the inside of the lath, and fixing similarly to the roofplate, overlapping the lath that's already there. It should be a pretty shallow angle, and if it doesn't work, I can just cut it off and do the cladding approach, I guess ^^.


I've just ordered the sand for rendering; I'm hoping to get the inside trimmed, then start dubbing out uneven sections and hollows early next week before moving on to a stipple coat.

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Inside (mostly) trimmed.




I managed to break the blade of the alligator saw on a joist hanger, and cut through the extension cable with the hedge trimmer, so a good day's work ^^. The confined space made it a fairly nasty job - ear defenders and face mask, at minimum.


The wall with the circular window in it is starting to look a bit unfortunate; the bale above ends right at the corner of the opening. Will reevaluate once the wooden frame is fixed in place, as that might give it some support - worst case I have to fit a lintel in there somehow, I guess.


The lean on the back wall's corner is also very visible now. Not much I can do about it given the lack of access to the other side though. Call it character.



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Fitted me a window \o/




(^ is from before I fitted the double glazing. If you peer closely you can see some of my experiments in dubbing-out hollows in the wall, too)




Even with all the gappy timber around it, the noise reduction is very noticeable. I guess i'll fill those gaps with expanding foam 🤷‍♂️ then put in the internal sill and some window board, and build up the reveals ready to render.


The window came bare, and I had designs on oiling it, but cheaped out on pine and it turned out to be engineered wood - with lots of finger joints - so I gave in and painted it. White will hopefully blend into the walls once they're limewashed, and if not, it'll be easy to paint another colour. My wife suggested fire engine red...?


For the round window, I experimented a bit with lime rendering the inside of the void, but I'm not convinced it's strong enough to be the finish. So I might need to replace the 20cm timber ring with a 45cm one, to give it a wooden lining. To make that the same way I made the 20cm one, I'll need a table saw *sigh*.

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Plastering / rendering started in earnest today. My arms 😀




(Having another go at the render sill, for better or worse).


That's 1/20 bags on the wall. It's already feeling like I might be a bit short of material ^^.


I used coconut fibre on this batch for reinforcement; it seemed fine, but I'm not convinced I put enough in. More in the next batch.

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39 minutes ago, Onoff said:

Maybe I missed it but what's the plan for lights and power?


There is a plan, but it's pretty minimal. I'll have power, network, and water come in through a couple of conduits along the bottom-left of the wall as seen in the pic above (there's a *cough* intentional *cough* gap two bales up where the front wall meets the side wall) but terminate it all near that corner - I don't plan to run any cables along the walls. I'm thinking very slim units along that wall, with a miniature sink in front of the circular window. Waste out the same way through a third conduit.


Sockets, and a wifi extender plugged into the network cable, will be hidden in the closest unit, where it enters. I don't want a big light or anything like that - it's a small room and the ceiling is pretty low. Not keen on fixed wall lights either. I figure I can always plug in a lamp or scatter some battery-powered lighting around. To start, the electric provision is just an extension cable plugged into the garage, but I'll get it upgraded to a proper cable off the consumer unit at some point. I don't think I'll ever need more than four sockets, but it's always possible I might add more solar panels to the garden room's roof, so I'll size for that.


In general, you can dig out channels in the bales for conduits, and run cables or pipes through them - you just render over the conduit to make it invisible. It's such a tiny room though, I don't see a lot of benefit to it.

Edited by Nick Thomas
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Rendering / plastering continues. Currently settled on a 2:1 sand:nhl3.5 mix, after going as high as 1.5:1 to start - but I don't think it's necessary to be that lime-rich. Today I finally managed to mix up some successful long-straw material too, which is *great* for dubbing out. Most of it went into straightening the corner on the right hand side.


I'm just taking handfuls of the stuff and rubbing it onto the walls at this stage. render-2024-06-17.thumb.jpg.f044f75fefcfd13d4fb255fb7a143e5b.jpg


I mixed the first bag and a half with a paddle mixer; that was enough to convince me to go out and get a cement mixer, which has done the second bag and a half. It does mean I can't have my table saw any time soon, so I'm going again with the rendered sill.


The mix isn't sticky enough to hang on upside down; I'm planning to try a mix that's cut 50:50 with lime putty, which should help. If not, I guess I'll have to get some bendy ply and make a sleeve for the hole to give it some support ^^.


Seventeen bags left. Oh, and corners are an absolute nightmare. The render mesh would help, but is slated to be floated into the next coat - which I'm planning to have go floor to ceiling. Right now I'm just coming to flush with the roofplate.

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OK, basecoat mostly on inside - it's just a few little bits around the windows now. Bag number six ran out slightly before I was finished. Onto the outside next :D.




Also got the window board on for the square window, and clad the subframe There's a 20mm lip so the render can come up flush to it, and some butyl tape around the perimeter of the window for airtightness. Also a very slight bit of wedge-shaped gapping because nothing I do seems to be quite square ^^. I'll just fill that, I suppose.




The windowboard got a bit involved. There's about 60mm between it and the straw bales underneath - it's level with the inside of the window subframe. It's also quite flimsy 18mm furnitureboard, and really wouldn't have been happy with the bales flexing underneath it. The top of the bales needed rendering anyway, for fireproofing, but I wasn't convinced that'd be enough to support it either.


I ended up putting 5x hazel stakes down through the bales and cutting them off at ~50mm below the level of the windowboard, then putting a basecoat of render down on top of the bales to be more-or-less flush with the stakes. Then I put some 2x2 and 2x6 down on top of that, screwed to the subframe, to each other, and also to the hazel. I'd run out of 2x8 ^^. There's a small gap between the render and the wood; I guess I can squash another coat into there once the existing has dried. Windowboard on top of the subframe, 2x2, and 2x6, glued and nailed to all three.


It's feeling pretty sturdy; final job will be to go round it with the router(?) to get a nice curved form instead of the current baby-lethal corners. I plan on doing that after I've finished all the plastering, so I don't accidentally make it too short ^^.


14 bags of lime left. The cement mixer was absolutely worth the money.

Edited by Nick Thomas
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