No-concrete passive foundations

[sam]

I've read what I can on this and did ask on the forum some years back but shall now try on my own thread.

 

I want to build a smallish (c. 4 metre square) garden room, single story and timber construction. This style of building is often built without conventional foundations, maybe on 'shed base' or paving slabs.

I'm curious about the following design which has no concrete.

 

I’ve consider the potential issues with this design which might not be present with conventional foundations, a passive slabs or a suspended floor. I've identified the following potential issues:

 

1. The floor is unsupported: This is a somewhat standard floating floor arrangement and might have a little bit of a spring to it.
2. The EPS under the walls could be compressed: It takes 45kn/sqm to compress EPS100 1%. The 145mm wide base plate around the perimeter has a surface area of 2.2sqm so if the building weighed 10 tonnes this would reach that level of pressure and compress the 100mm EPS to 99mm. I think the whole building will be significantly lighter than this. I know passive slabs often specify higher grade EPS but that would seem unnecessary for a light weight building.
3. The walls could distort laterally: This is the same as a timber frame building on dwarf brick walls. Additionally the top EPS layer would prevent the walls moving inwards. If this was considered a risk the walls could be fixed to the flooring or the base plate could be connected to a strap between the EPS layers.

 

Am I missing something - why wouldn’t this be a good idea?

 

[JohnMo]

This is how I did it

https://forum.buildhub.org.uk/topic/31017-summer-house-insulation/

[Nick Thomas]

So you'd to dig out the whole footprint and fill it with rubble?

 

One thing I looked at for my own garden room build -

 

was to dig out the footprint and pour foamglass into the hole, topped with limecrete. I didn't fancy the digging *at all* though, so ended up with very (too? time will tell) minimal columns of pea gravel, and plans for a suspended timber floor which I'm just coming round to now. I lost headroom by doing it that way, of course.

 

One thing I didn't properly appreciate until it was pointed out to me was the risk associated with the building not being tied to the foundation in any way. More of an issue the more lightweight your construction is - but especially with a flat roof and a timber frame, you don't want the whole thing taking off.

 

Rubble *trench* foundations are totally a thing, so I can't see why digging out the whole footprint wouldn't work, anyway. Weren't a lot of pre-20th century houses built on just that?

[Nick Thomas]

A bit more on that limecrete floor option: https://www.mikewye.co.uk/product-category/lime-products/limecrete-floor-systems/

 

It doesn't envision standing the walls on top, but if they're lightweight, no particular reason why not, right?

[Russell griffiths]

@sam

I’m not keen on your walls being so low in the ground, I would put both layers of insulation inline on top of each other so the outer edge is flush, then fit the sole plate on top of both layers. 
 

I would look at a denser insulation for the perimeter 

maybe xps300 or eps 300. 
I would stagger all the joins in the insulation and glue it all together to form one big slab. I would also glue the chipboard flooring down so it all acts like one big chipboard insulation sandwich, sort of.  
 

your bigger problem will be stopping it blowing away. 
 

1m long threaded rod buried in the ground below your drainage layer, coming up through your sole plate could fix this. 
 

what do you want to do in there as 4m square is rather small. 
and the cost per m will be  extremely high, it won’t cost much more to make it a bit bigger. 

[oliwoodings]

To expand on @Russell griffiths's suggestion of 1m threaded rod, perhaps you could bolt a square flat plate on the bottom of the rod which then gets buried and compacted down. If you were open to a little concrete you could even postcrete these in.

 

[saveasteading]

As Russell. The timber must be higher out of the ground.

I don't like the eps  under the walls.

It will compress a lot as the building will lift /rock in the wind and put all the load on the other side. And rocking will make a repeated point load. The eps is supported laterally  by stone as shown but that could change.

Seems odd having a permeable sub base with perforated drains. This will be wet. Why?

 

Uplift and sliding must be dealt with. We can return to that.

 

Not using concrete is a great ambition. I'd use a concrete slab though. The carbon will be offset by not having to rebuild this after the first storm.

 

 

[mike2016]

Groundscrews are perfect for this. Watch Robin Clevetts garden room series on YouTube. No concrete!

[Russell griffiths]

Really this is all a bit pointless without knowing what it’s to be used for. 
it may well be better to put it up on 4 ground screws with a timber floor with insulation between the floor joists. 

[Bramco]

2 minutes ago, Russell griffiths said:

it may well be better to put it up on 4 ground screws with a timber floor with insulation between the floor joists.

+1 to that - just helped my son build his shed which is exactly how he did it.  Does depend on the ground conditions of course.

 

Simon

[saveasteading]

Do groundscrews ever come out again?

 

Even easier is pads of stone with a paving slab or 2, or 3 on top. Then timber baulks as sole plate/ beams. 

 

Then a shed with internal insulation.

 

Telling us the purpose might spoil the fun but hasten a solution.

 

[joe90]

I was considering a summer house on a slope here and using ground screws to anchor it and create lift on the low side plus no foundations really appeal.

[sam]

Thanks all - some really interesting points.

 

The building will be an insulated garden room - for a garden office or children's play room. Building control would not be required. I can get a digger on site but not ready-mix concrete. It is on a slope, but I don't that overly relevant for this discussion.

 

A suspended floor design (on pads or ground screws) would be the simpler solution and is what I designed and got approved for my planning permission (in order to be taller than permitted development allows). For aesthetic reasons I'm keen not to bring the building up higher than I need to, hence exploring other options. I come on this forum because the wisdom should put my mind at rest that I've tried and failed to find a better solution or to give me the confidence to proceed!

 

My question is also partly theoretical. I'm aware of 'passive foundation' designs (diagram below) which are very similar to my diagram above but with the addition of a reinforced concrete slab above the insulation - even if access was easy is this necessary? It adds rigidity to the base of the the building (preventing lateral movement but I don't feel this to be an issue) and adds mass (reducing rocking/lifting, which might be an issue).

 

I am concerned by the lifting/rocking issue. I don't think the increased rigidity of this concrete slab would help, as the weight would still be on a point if it rocked. It would add mass but only about 6 tonnes if 100mm thick - so not sure it would substantially change the situation.

[joe90]

Perhaps i am getting lazy (old) but the thought of lumping concrete about for a garden room does not appeal when ground screws are an option, yes no good for a garage etc but IMO perfect for a garden room. 🤷‍♂️

Edited March 23 by joe90

[George]

On 23/03/2024 at 10:44, sam said:

Thanks all - some really interesting points.

 

The building will be an insulated garden room - for a garden office or children's play room. Building control would not be required. I can get a digger on site but not ready-mix concrete. It is on a slope, but I don't that overly relevant for this discussion.

 

A suspended floor design (on pads or ground screws) would be the simpler solution and is what I designed and got approved for my planning permission (in order to be taller than permitted development allows). For aesthetic reasons I'm keen not to bring the building up higher than I need to, hence exploring other options. I come on this forum because the wisdom should put my mind at rest that I've tried and failed to find a better solution or to give me the confidence to proceed!

 

My question is also partly theoretical. I'm aware of 'passive foundation' designs (diagram below) which are very similar to my diagram above but with the addition of a reinforced concrete slab above the insulation - even if access was easy is this necessary? It adds rigidity to the base of the the building (preventing lateral movement but I don't feel this to be an issue) and adds mass (reducing rocking/lifting, which might be an issue).

 

I am concerned by the lifting/rocking issue. I don't think the increased rigidity of this concrete slab would help, as the weight would still be on a point if it rocked. It would add mass but only about 6 tonnes if 100mm thick - so not sure it would substantially change the situation.

The slab is designed to allow for any settlement that might occur in the subbase.

 

However I've got some ideas based on the 'traditional' no concrete foundation (redeveloped by Frank Lloyd Wright) which I'll sketch up when I'm on my computer.

[saveasteading]

5 minutes ago, George said:

any settlement that might occur in the subbase.

Or you could compact the sub base properly then the slab doesn't have to be a bridge.

7 minutes ago, George said:

foundation (redeveloped by Frank Lloyd Wright) which I'll sketch

Interesting, and im looking forward to it. I've never heard of that but it's likely to be as crazy as his other innovations.

Worth reading about this madman as an example of what not to do. 

 

Did he not believe in physics or simply in hoc to his  own 'genius". Do they still praise him in Architectural circles? It's worrying.

[Iceverge]

If you want to have a durable low profile foundation you are stuck with masonry of some description.

 

I built a small pumphouse by burying tyres in the ground and using them as the formwork foundations for piers that eventually held some M24 threaded bar to hold a suspended timber floor. The whole thing ended up very high up though.