Concrete free foundations - combining ground screws with a solid compacted floor

[MarkBentley]

We are building a 30m2 timberframe single storey annex and want to minimise our use of concrete. We want to include a wet underfloor heating system.

 

For the flooring we are thinking the layers would be:

• compacted ballast
• compacted sand
• PIR insulation board
• UFH pipes
• screed
• wooden flooring

Thinking that we don't need a concrete slab for stability because if it shifts a little and the screed cracks the wooden flooring will cover over those defects.

 

The question is how to constrain the flooring?

• Can you use blocks without foundations, but supported by earth on the external side, to create the footprint of the building and then compact and build up the flooring within the fooprint?

 

The timberframe structure can be supported by ground screws. All the examples of using ground screws that I've seen have the building suspended slightly above the ground - is that essential?

• Can you position the ground screws so that the base of the timber frame sits ontop of the footprint blocks but the weight is supported by the ground screws?

 

Is there a better way to combine a single storey timberframe build with a solid floor whilst still using minimal concrete?

Edited April 6, 2022 by MarkBentley
clarity

[Mr Punter]

If this is a proper building needing building regs I think you may struggle.  Blocks without foundations would be a no-no.  You can't have your timber frame walls starting less than 150mm above ground.  You may do better going for a system that is already in place for the type of building you propose.

 

You may notice that most of the ground screw solutions are for "garden room", "home office",  "decking" and so on.  No good for a proper habitable room under building regs.

[Conor]

I think an insualted raft is your friend here. You'd get away with a single 100mm thick slab on EPS insualtion. If you do the calcs I wouldn't be surprised if the embedded carbon of the EPS and concrete is less than the PIR and screed. Remember, screed has a lot of cement in it as well.

 

Or, you can go for a fully timber structure (BC will be tougher) with suspended timber floor with an over floor heating system. Then ground screws / hardcore / stone might be enough.

 

[markocosic]

 

Some thoughts...not tried either as yet but wish I'd thought about it / she'd asked for it before we laid the floor!

https://www.youtube.com/watch?v=Z4jShqK6qNQ

https://www.youtube.com/watch?v=1-TrFIcxPjU

 

 

 

Though to be honest for 30 m2 and insulated well might you just...

https://www.saturnsales.co.uk/Panasonic-Etherea-CS-Z20XKEW-Inverter-Air-Conditioning.html

 

More than 1 metre from the boundary and less than 29.9 m2 largely solves building control I think.

 

 

Dig down (scrape off topsoil) then pop your screws in and suspended floor above if you're trying to keep overall height down?

 

 

This is chuffing heavy and hasn't sunk yet FWIW. I think (I hope!) they're fine for houses not just 

 

 

 

Screw piles and ratings from here:

http://pamataisraigtiniai.lt/en/

 

0.75 tonne each for sand; 1.5 tonne in better soil; for the larger screws

 

 

In a UK context I'd look at how you make block paved driveways. Scrape off mud. Geotextile. Wackerplate crushed rubbish / hardcore. That in itself isn't moving. Some blocks on top to lift the wood off the ground. DPC. Then wooden building on top?

[JohnMo]

If you don't want concrete, why use a screed, which is concrete.

 

Use the proportion of concrete you were going to use in the screed and do strip foundation and insulated suspended wooden floor. Or passive slab.

 

Was told not to use screw piles on a building as you make it worthless, screw piles may have a 50 year life, but a normal building should be there for 100s.  How do you replace a corroded screw pile?

[SuperPav]

I have to question why, but then again most things I do someone would quite reasonably question why If this is "just because I want to prove we can build this thing without concrete", then that's fair enough and crack on with it. However, if you want to reduce the carbon footprint etc. it's not as clear cut as just cutting out concrete completely.

 

So if you want to proceed without concrete, just look at how houses were built 300 years ago, and repeat? 

 

Dig down strip trenches until you hit hard ground, then use bricks or (if you're made of money) large stone over a wide footing, build up to ground level with lime mortar, then have a suspended timber floor. This can be easily insulated from below, and you can overlay a UFH system in either boards, or if you want a bit of thermal inertia, you can simply fit the pipes between joists in a layer of sand.

 

Alternatively if you're okay with lime, then do the same for the footings, but instead of suspended timber floor, whack down some glapor (or equivalent), membrane, then UFH in limecrete.

[dpmiller]

screed can be gypsum too if you prefer

[markocosic]

I don't think you need strip foundations.

 

Dig hole. Ram with rocks. Move over. 6-10 feet probably works best. Dig hole. Ram with rocks. Big rock or a block on top to finish. Build your frame up off of these piers. 

 

For single storey 30m2 it's very unlikely to blow away unless in an exposed area and made of nothing. Sand between joists for ufh would help see to that too.

 

One could probably also use mud even. Nice clay bake!

 

I don't think it needs ufh in a UK climate if the insulation is good though. Is it internal for area that counts for the 30m2 rule? So no incentive for thin walls where space allows?

[Iceverge]

I've been thinking about this. Continuing my never ending love hate affair with concrete. 

 

Why not do an insulated raft foundation but instead of a concrete slab you have a "timber slab". A lattice frame of say 140*45mm placed inside an EPS tray.

 

If constructed properly it should have all the mechanical properties of a concrete slab. 

 

With the insulation outboard it would be able to breathe inwards and not decay. Dried sand inside the frame and topped with OSB. Build your timber frame up from there. 

[markc]

I agree, UK seems obsessed with concrete for foundations and slabs whereas many places around the world build on timber founds, posts and floors

[markocosic]

Concrete is reasonably idiot proof is why not; in the main part.

 

And clay; clay is rubbish for bearing strength.

 

And banks; banks want properties that remain saleable at any point during the loan period. Even if it's theoretically ok if they were worth zero once the loan was repaid...

 

http://www.geotechdata.info/parameter/bearing-capacity

 

100 kPa means 10 tonnes per square metre. Or a 3.5 tonne transit van on a 60x60 cm paving slab.

 

A few 60*60 holes rammed full of rocks happily carry 30m2. Spacing limit is probably how bendy your floor beams are.

[George]

For the walls you may be able to use a rubble trench foundation (https://en.wikipedia.org/wiki/Rubble_trench_foundation) - you'll likely need an engineer to sign it off to satisfy building regulations.

 

For the floors, either a suspended timber floor or a leca insulated limecrete slab.

[Bozza]

My project included planning to build a garage amongst tree roots.  What was agreed was the structure, timber or metal framed, would be supported via ground screws to form effectively a canopy.  The base of the garage was permeable geoweb with tarmac on top to form a solid floor.

 

in end didn’t build the garage but that was what was approved as build method.

 

 

[MarkBentley]

On 08/04/2022 at 00:45, Iceverge said:

Why not do an insulated raft foundation but instead of a concrete slab you have a "timber slab". A lattice frame of say 140*45mm placed inside an EPS tray.

 

If constructed properly it should have all the mechanical properties of a concrete slab. 

 

With the insulation outboard it would be able to breathe inwards and not decay. Dried sand inside the frame and topped with OSB. Build your timber frame up from there. 

That seems like a sound idea. I'll look into it a bit more. Thanks

[MarkBentley]

On 10/04/2022 at 07:41, George said:

For the walls you may be able to use a rubble trench foundation (https://en.wikipedia.org/wiki/Rubble_trench_foundation) - you'll likely need an engineer to sign it off to satisfy building regulations.

With being 30m2 or less we are hoping to do this via permitted development and therefore not have to adhere strictly to building regs. I'd not heard of rubble trench foundations - that could be a viable option

[George]

OK - well I'd still get an engineer to look. 

 

Just something to check - I think if you have sleeping accommodation in the building then the 30sqm exemption can't be used. But I'm not 100%

[MarkBentley]

This is a response from a company that does insulated raft slabs, about @Iceverge's idea about a timber insulated raft foundation:
 

Quote

A couple of things to consider,

 

At 30m2 you may find you still to comply with building regs, irrespective of the planning position.

 

The timber would be below ground level and careful consideration of the damp proofing would be required.

 

The timber would need to spread the load out over a wide area, carrying the load on a grid on narrow timbers made lead to the loads being too concentrated. (Stiletto v snow shoe). Fixing a sheet of ply to the underside of the timber lattice may help.

 

A sheet of CLT laid flat may be more effective.

 

I'm not sure what the sand would add apart from thermal mass.