Outbuilding Raft Foundation

[KeenDiyer1111]

Hi All!

I'm now in the process of digging the foundations out for my garden room. The plan is to make this from SIPS construction which will sit on top of rows of 140mm wide block (which is sunken underground and will be waterproofed!) up to ground level with the raft foundation slab acting as the floor (40cm below ground level) Anyway, I had a question about the raft foundation.

While digging I've realised that the clay subsoil does not start until around 70cm into the ground. The plan currently is to dig 80cm deep, lay 200mm of MOT type 1, another 200mm of concrete which will be reinforced with mesh. For the edges of the slab I will dig 1m deep and the concrete will be 400mm deep and 300mm wide.

I do have a couple of trees close to the foundations (within 1m). One is a cooking apple tree and the other a plum tree. Does the plan above sound viable? I'm trying to avoid subsidence/heave and hoping the plan above can support the load from the building.

 

[^woody^]

What you are describing with deep edge foundation strips is not a raft, and appear to be unnecessary.

 

[stevie888]

You don't say how big the room is, but I've seen normal 2 storey brick built houses founded on a 200 raft with thickened edges (still a raft!), so structurally should be fine.

I'd be more concerned about water ingress: I assume you're dropping the floor to keep overall height within PD limits but maintain decent headroom? If it was mine I think I would make the slab flat bottomed and form my base walls from RC. This will give the same structural stregth but bea much better proposition for a proper tanking exercise.

 

[mrrusty]

Here's a raft I did for a garage. It's 150mm in the centre with an approx 300x450 edge "beam". My C.Eng MICE son sprinkled holy water on my design and said it would be fine, as it has proved. https://community.screwfix.com/threads/garage-work-in-progress.222282/ The stepped edge has proved 100% watertight

I've also done a SIPs garden room - this one on easypads, but our clay layer is at just 400mm. Again this has proved solid so far. (its PB's and skimmed internally - no cracks!) https://community.screwfix.com/threads/garden-room.256842/

 

[KeenDiyer1111]

You don't say how big the room is, but I've seen normal 2 storey brick built houses founded on a 200 raft with thickened edges (still a raft!), so structurally should be fine.

I'd be more concerned about water ingress: I assume you're dropping the floor to keep overall height within PD limits but maintain decent headroom? If it was mine I think I would make the slab flat bottomed and form my base walls from RC. This will give the same structural stregth but bea much better proposition for a proper tanking exercise.

Ah sorry, forgot to include. The room is 5m x 2.5m and like you said I was dropping the floor to keep it within PD limits and have headroom! So from what I understand you are saying that I should not thicken the edges of the raft and keep this 200mm edge to edge but rather make the walls that are underground out of reinforced concrete instead of blocks. Will it still be structurally stable without the deep edges on the clay soil near trees?

Also how would you go about making the walls out of reinforced concrete. Would you make the walls first and then tie this into the slab or do the slab the whole way and then make the walls on top with concrete?

 

[KeenDiyer1111]

Here's a raft I did for a garage. It's 150mm in the centre with an approx 300x450 edge "beam". My C.Eng MICE son sprinkled holy water on my design and said it would be fine, as it has proved. https://community.screwfix.com/threads/garage-work-in-progress.222282/ The stepped edge has proved 100% watertight

I've also done a SIPs garden room - this one on easypads, but our clay layer is at just 400mm. Again this has proved solid so far. (its PB's and skimmed internally - no cracks!) https://community.screwfix.com/threads/garden-room.256842/

Thank you for this! Both projects look good! I'm glad to know that your foundations have stood the test of time! Really reassuring for my case!

 

[KeenDiyer1111]

What you are describing with deep edge foundation strips is not a raft, and appear to be unnecessary.

Okay, so would you just do 200mm edge to edge of the raft foundation, no need to make a thicker "beam" around the edges?

 

[stevie888]

Ah sorry, forgot to include. The room is 5m x 2.5m and like you said I was dropping the floor to keep it within PD limits and have headroom! So from what I understand you are saying that I should not thicken the edges of the raft and keep this 200mm edge to edge but rather make the walls that are underground out of reinforced concrete instead of blocks. Will it still be structurally stable without the deep edges on the clay soil near trees?

Also how would you go about making the walls out of reinforced concrete. Would you make the walls first and then tie this into the slab or do the slab the whole way and then make the walls on top with concrete?

Rafts are just what the say on the tin: they can move as the soil m/c alters. The trick is to make them strong enough not to break their back so the structure doesn't suffer differential movement. This is in direct contrast to strip or piled foundations where we strive to get down to ground that is stable: generally 1000 but if trees are nearby considerably more until economics and safety issues mandate piles rather than simple trench fill.

The thickened edge is purely to help distribute the line loads produced by the structure and with a lightweight structure such as yours probably not necessary but if you're spending a lot of hard-earned then there's a lot to be said for erring on the side of overdoing things.

On this tack, concrete is pretty waterproof stuff, but we would generally tank a below ground structure like this to be 100% sure. So cast the slab over a heavy duty dpm leaving enough to turn up the sides and joint a vertical layer to, Form the slab with a "kicker" which is simply a section of your wall maybe 75 high that you use to align your wall shutters.

Altternatively, and maybe more diy friendly, you could use hollow concrete blocks with rebars drilled into the base and the voids concrete filled. Not as rigid as full RC but more so than nowmal blocks simply mortared.

A coiuple of other thoughts:

1. Your access will need thought
2. It may be worth casting a wee sump in the slab so that if you ever do get flooded it's easier to pump out.