Foundation for outbuilding...please help....

[Simon]

Good morning Guys,

 

We are looking to build a cartlodge style garage, with attached storage room, all one storey (the council won't let us have two storey.... ...), timber frame and wall, with pan-tile roof, in an L shape of about 100m2

 

I have asked serached everywhere, and even asked the local BC, but no-one seems to be able to tell me definitively what sort of foundations it needs, i.e. size of trenchfill, or if just a re-inforced slab would pass reg's. We have a clay soil base apparently.

 

Please, could any of you help?

 

Many thanks

Simon

(still mired in planning)

[Ferdinand]

I would think the vendor would tell you that ... technical department?

[Jeremy Harris]

Welcome,

 

The foundation choice depends on so many different factors that it's impossible to give a definitive answer, I'm afraid.  Things that need to be considered in the foundation design (assuming this is a non-habitable building - there are other things that impact buildings that have to comply with Part L1a) would be:

 

-  Topography of the area where the building is going to go, and whether there is made up ground or the need to retain any banks that might arise from levelling the area.

 

-  The loads imposed on the underlying ground by the building, including point loads from something like a frame that had heavy support timbers (for example, traditional oak frame)

 

- The nature of the ground under the building, including its maximum allowable bearing stress and whether it is made of a soil type that may be subject to movement (like some clay soils and heave).

 

-  The local hydrology and depth of the water table.

 

-  The proximity to services, drains, trees and tree roots.

 

On clay soil, it depends on the type of clay.  Some clay soils are subject to heave with changes in moisture content, some aren't, and the foundation design will have to take account of this.  If the frame puts high point loads on parts of the foundation, then it may well be that the foundation design has to make provision for supporting walls that don't impose a uniform load, so that may mean pads where the point loads are.

[Simon]

Thankyou guys,

 

Yes, JS, it is non-habitable, and while I can see it from the garage floor side,  a 1m depth trenchfill, with oversite and a re-inforced 150mm slab on top, does seem a little like overkill for an outbuilding we just want to use as storage space for household items... and it looks like the groundworks are likely to cost three times the cost of the building on top of them...

[Jeremy Harris]

You really need to know exactly what the soil characteristics are like, before anything else.  A slab, with local reinforcement for point loads, might work, or it might not.  Trench foundations may need to be deep, and have clay boards, or they may not.  Without knowing what the soil max bearing allowable stress and heave characteristics are you can't easily design a foundation system.

 

You mention a cart shed type building, so is this a traditional oak frame, or similar, with large section posts that support the structure?  If so, then the point loads that this impose will need to be taken into account.  Much of the static load from a building like this can come from the roofing materials.  Pantiles vary a bit, depending on what they are made of, clay being a bit lighter than concrete, for example.

 

You need to know the imposed loads on the foundations, and they can only really be provided by the building designer.  100m² is a large building, bigger than the footprint of a lot of houses, so this is not an insignificant foundation requirement.

 

[Simon]

Thankyou for all that advice JS, at least I now know what I need to get where I want...

Yes the footprint is quite expansive,, but the building will only stand 3.5m tall, be mostly softwood, with an oak front, and weatherboarding, with reclaimed Norfolk clay pantiles, surely much less weight than a two storey, double brick skinned building with concrete tile roof?

[Jeremy Harris]

It's not the weight, but the imposed load that matters, and to give any meaningful advice we'd need to know what the load applied to the bearing points on the foundation would be, and the properties of the underlying ground (max allowable bearing load, heave characteristics, etc).

 

I'd love to just be able to say "dig out this, add this, and you're done", but it just isn't anywhere near that easy.  The foundations need to be designed to cope with the load and conditions that the building will impose on them, and that's not just the weight of the building, but the loads from wind, snow and ice, the live loads from anything stored in or on the building, etc.  Some of these depend on the location and the degree of exposure to wind, plus design features, like whether the building has large openings, long lengths of wall that may need wind supports, etc..

 

 

Edited June 17, 2017 by JSHarris

[Simon]

Thanks JS,

 

So what you're saying is, the foundations need to be able to support anything that's thrown at them, and as such need to be well over-engineered?

It was so much easier when our house was built....no foundations to speak of, made of used wood, and lumps of clay, and 400 years later it's still here, without so much as a crack!

 

Starting to think we may well forget the whole thing, comfortably into 5 figures just for a base, it's just not worth it.

 

Thankyou for all your help, at least now we know....

[Jeremy Harris]

1 minute ago, Simon said:

Thanks JS,

 

So what you're saying is, the foundations need to be able to support anything that's thrown at them, and as such need to be well over-engineered?

It was so much easier when our house was built....no foundations to speak of, made of used wood, and lumps of clay, and 400 years later it's still here, without so much as a crack!

 

Starting to think we may well forget the whole thing, comfortably into 5 figures just for a base, it's just not worth it.

 

Thankyou for all your help, at least now we know....

 

 

No, not at all.

 

The foundations need to be specifically designed to take the imposed loads and the ground conditions, with an acceptable safety factor.  They absolutely do not need to be over-engineered, that would be foolish and a waste of money.

 

The point is that you can't design the foundations without knowing the imposed loads and the local ground conditions - both vary a great deal, so just guessing could mean you have foundations that are grossly over-engineered, or grossly under-engineered.

[Simon]

Ok, thanks again, wish the other people I have spoken to had been so open about the whole thing, rather than making it sound like a dark art!

 

So what you are saying is, we need to employ a structural engineer? It's a cost we hadn't bargained on to be honest, budget being so tight, we just thought being basically a large shed/barn type building, the foundations would be similarly simple....whereas it's the total opposite...these kit companies specify everything from piles, to just 100mm raft, and everything in between!

The two groundworks quotes we had, didn't seem to worry about loads, just talking about standard sizes, and depths.

It's been further complicated by having to reduce to single storey, so trying to save on some costs, as some costs have nearly doubled, such as the roof, so we hoped the footings could be reduced in spec, from the original two storey building.

Can I take it, these calculations are not possible for the layman?

Once again, thankyou for all your time.

[Jeremy Harris]

Structural engineers are not that expensive, we used one to design a massive, 45m length, 2.5m high, retaining wall, that has tens of tonnes of blocks, steel and concrete in it (see here: http://www.mayfly.eu/2013/07/part-six-there-we-were-digging-this-hole/ and here: http://www.mayfly.eu/2013/07/part-eight-the-wall/  ) and he charged us less than £300, for drawings, concrete and steel specifications etc.

 

A layman can do the calcs, but no insurer will accept them, and the chances are building control may not either.  A few hundred pounds spent with a local SE, who knows the ground conditions in your area would be money well spent.  The chances are you'd save that if he/she comes up with a design that reduces the amount of concrete by a couple of truckloads.

[Declan52]

If it's just going to be used for storage then put the founds in and use wooden joists for the floor similar to some timber framed houses. Will save you on concrete and steel. 9*3 joists and carry a fare bit of weight.

[Simon]

Thankyou all so much for all your great advice, we really had felt like we were  hitting a wall (pardon the pun)..

 

We had been led to believe an SE would cost MUCH more than that....nearly £2k...and as we are probably going to have to get a planning consultant to deal with the council going forwards....anyhow, will def look into getting one on board, I take it you are nowhere near Suffolk JS?!

 

And thankyou for that other idea Declan, it was something we hadn't considered, and could present a useful saving, as the non-garage section of the building is light storage only,,,

 

Cheers guys

[Jeremy Harris]

Sadly I'm not near Suffolk, but it may well be a good idea to just get whoever is supplying the building to provide the foundation design, if they can.  We did this for our self-build, and the timber frame company designed, provided the structural engineer sign off, and installed the foundations, all as a part of the package.  Probably the cheapest way to get the job done, plus it has the big advantage of removing the risk that the foundation dimensions and tolerance may not match the timber frame.

[Temp]

If you are building with oak and a tiled roof it seems a bit mean to scrimp on the slab but that's just me.  If part of it is going to be capable of taking a car I'd lay a 150mm slab over the whole footprint. Online figures put the concrete at £1000 - £1100 but shop around.

 

Perhaps think about raising the timber frame above the slab on a course (or two) of engineering bricks and a DPC. I'd arrange for the cladding to extend beyond the footprint of the slab so that rain drips onto the surrounding ground/gravel rather than onto the top of the slab where it can run under. 

 

I built my outbuilding when loft insulation was still subsidised and filled the walls with it.  Today it's 28C but amazingly cool in there.

 

If putting in electricity consider casting a length of plastic pipe into the slab to make it easy later. 

[Jeremy Harris]

I'd second what @Temp says. 

 

Our timber framed garage/workshop has a 150mm insulated concrete slab floor, and it has a thick layer of insulation inside the walls and ceiling, with a floored storage loft in the roof space, with a hatch and ladder for access.  It's great having it insulate and lined, plus it gives me somewhere to escape to, away from the house.........

[ToughButterCup]

1 hour ago, Simon said:

[...]

We had been led to believe an SE would cost MUCH more than that....nearly £2k...and as we are probably going to have to get a planning consultant to deal with the council going forwards....anyhow, will def look into getting one on board, I take it you are nowhere near Suffolk JS?!

[...]

 

Our SE saved his fee four times over by writing one single sentence. He is a specialist and one of many that we talked to. Get the right SE for the job. Money, tears, aggro saved.

Network, network, network.

[Simon]

Thanks Temp,

 

I get what you are saying, but, we aren't looking to skimp on the slab, it's the 45 m of 1m depth trench under all the supporting walls that is presenting the issue, as that's one hell of a lot of concrete for essentially a large garden shed....not forgetting the oversite..., and we've got to have all that earth grabbed away....the costs really start to add up....

Ooh,  I asked for a quote for concrete, and got the following answer:

 

£79 based on RMX supply.

£45 per m3 un carried below 6m3 and £68/hour waiting time after first 45 minutes on site per load.

 

Can you make sense of it? we can't :-(

[Ferdinand]

29 minutes ago, recoveringacademic said:

 

Our SE saved his fee four times over by writing one single sentence. He is a specialist and one of many that we talked to. Get the right SE for the job. Money, tears, aggro saved.

Network, network, network.

What was that sentence?

[Declan52]

Just kill all the newts and say they emigrated.

[Simon]

1 minute ago, Declan52 said:

Just kill all the newts and say they emigrated.

 

I'm sorry?

[RichS]

31 minutes ago, Simon said:

£79 based on RMX supply.

£45 per m3 un carried below 6m3 and £68/hour waiting time after first 45 minutes on site per load.

 

Can you make sense of it? we can't :-(

 

£79.00 per cubic metre delivered.

£45.00 per cubic metre if you collect, though you won't do that for much over 1m³, and then only with a decent trailer or pickup.

£68.00 per hour for the wagon if it's on site for more than 45 minutes.

[Simon]

5 minutes ago, RichS said:

 

£79.00 per cubic metre delivered.

£45.00 per cubic metre if you collect, though you won't do that for much over 1m³, and then only with a decent trailer or pickup.

£68.00 per hour for the wagon if it's on site for more than 45 minutes.

 

Thanks Rich,

 

That was for C25, delivered to a village 12 miles north of Ipswich....does it sound fair does to you?

 

[Jeremy Harris]

8 minutes ago, Simon said:

 

I'm sorry?

 

 

Sorry, Simon, it's an in-joke here that Ian (@recoveringacademic) had a great deal of hassle and cost from having to deal with a large population of Great Crested Newts on his site..............

 

My guess is that the deep trench foundation that's been called for is just a lazy way out by whoever specified it.  You can look at using a raft foundation, with hardcore beneath and a reinforced steel ring beam under the wall, as that would mean a lot less excavation and use a lot less concrete.  Our house is built on an insulated version of this system, and the photos in this blog entry: http://www.mayfly.eu/2013/10/part-sixteen-fun-and-games-in-the-mud/ may give you an idea about the depth of concrete and amount of steel reinforcement needed (you probably don't need the thick insulation we have).  Our house is built on clay, but it is a fairly hard clay that isn't subject to heave.  However, it is possible to mitigate for a poor soil to some extent by increasing the depth of sub-base under a slab, as this spreads the imposed loads out at 45 deg from the edges of the slab, into the underlying ground, which reduces the bearing load on the soil.  Sub-base is generally a fair bit cheaper than concrete.

[ProDave]

40 minutes ago, Simon said:

Thanks Temp,

 

I get what you are saying, but, we aren't looking to skimp on the slab, it's the 45 m of 1m depth trench under all the supporting walls that is presenting the issue, as that's one hell of a lot of concrete for essentially a large garden shed....not forgetting the oversite..., and we've got to have all that earth grabbed away....the costs really start to add up....

 

That sounds like you are considering tench fill? which is where you dig a foundation strip, then fill it all up again with concrete. That is one possible way but as you have found, expensive.

 

A normal strip foundation is dug to the required depth, then 200mm of concrete poured, blockwork built up from that then soil goes back on the outside and hard infill compacted on the inside.

 

The required depth nobody can answer without seeing the land and without a test pit or 2 dug.

 

A previous house we had in Oxfordshire I built a garage with 600mm strip foundations in nice firm ground.  Where we are now I had to go more than that just to strip the soft black organic soil before I could then dig the strip foundations which were almost a metre down from that, so 1.5 metres overall.