Foundation for a brick shed

[Shinogi]

Hi, I'm just starting a brick shed build and I need a little advice for the foundations.

The brick shed will be 30m2, roughly 4.50m(w) x 6.50m(L) x 2.50m(h), cavity insulated wall with brick and block and a flat or slightly pitched roof. The ground consists of 0.42m of top soil in which it goes into pure compact clay at that depth, which I can push my thumb into with a lot of pressure but it only slowly sinks into the clay. The clay feels slightly damp but not saturated.

What depth and width should I be looking at to dig for a trench fill foundation to support this? Rebar recommended or fine without? It will be built right at the bottom of the garden away from the house if that contributes to any replies. I presume a strip foundation wouldn't be advised due to being on clay?

I presume I won't need rebar but if I can get away with not digging as deep if it is used, i'd prefer to add rebar as I'd like to mix up the foundation myself but currently, it's looking like the job is too big to mix it and premixed might be the way to go to get it laid in one day.

Any help and advice is appreciated, thank you

2 holes dug so far to see what ground i'm dealing with.

 

[pinenot]

Depth of dig, to the undisturbed clay line allowing you a width comparable to thickness of the wall composite (if twin wall etc.)+ 300mm. You wont require rebar but if available cut some 6 -9 mm mesh 100mm less than the strip found width (50mm clear of each side) push this in to the newly poured concrete until half way down, lap any joints x 200mm. Allow 3 days to set before building your solum...pinenot

 

[Shinogi]

Thank you pinenot. May I just confirm by digging to undisturbed clay, that you mean to where the clay starts at around 450mm? Is that my depth or is it preferable to dig into the clay slightly? Just making sure I'm clear on the depth.

I haven't got any rebar lying around but if it's going to be make the foundation stronger, I'll get some anyway.

Thank you again.

 

[fmck]

You say its too big a job to mix yourself but you are aware you don't need to pour the foundations in one go. Likewise if its a concrete floor you can split the floor under stud partitions and allows for expansion on your slab.

 

[Shinogi]

I presumed that it would be best laid in one go since it's on clay. Most information on the net shows clay is the hardest to build on due to it moving but others say it's the best as it's good ground.

So you're saying, If I presumably use rebar mesh, I can lay the foundations in two parts? The main floor will be hardcore, Sand, DPC, insulation boards then topped with concrete.

 

[fmck]

Think about it do you think they build foundations for health centres, shopping malls, schools, etc in one pour?

No just tie it in similar if you were sticking an extension on to a existing building. Leave some dowels or mesh sticking through tie it together.

 

[Shinogi]

I guess you are right, thank you. I've been thinking about these foundations for ages and I guess I've just made things too complicated going over different information. It would be perfect if I could do it in two stages.

Lovely, that makes my job so much easier as I would prefer to do it myself but the site is a fair 35m+ from the road and limited space for storing all 1 tonne bags I need for the job.

Quick question, do I leave the first half of the foundation to fully set for 3+ days or can I pour in the second remaining half the next day?

 

[fmck]

I am building something similar on the projects section here on diynot. It's a similar sub structure but will be a timber frame construction with textured versapanals and ship lap.

You can pour the following day or if you want shift your stop end if you want continue on if you feel up to it. It's easy to put a stop end in, a piece of 50mm timber under the dowels/mesh and a bit of jammed plywood on top. Move it where you want that's standard practice for a small founds like this.

I work as a construction engineer and have worked on small builds to large scale civils projects for the past 20 years. It won't matter wether you want to split 3-4 pours. Just ensure you tie it in.

 

[Shinogi]

Lovely, thank you. I can do it myself then if that is the case. When I do it in sections, I'm presuming that there will be small gaps where I join them up. Do I just skim over them with concrete to make them watertight?

 

[pinenot]

Strip foundations don't have to be waterproof, there simply have to be load bearing. The next day pours will basically bond with the day before pours with the mesh providing a homogeneous link between the two.

If you have access to the foundation pouring site, or you can make temporary access for a concrete bowser to get to the pours, all you have to do is shovel/rake the cement reasonably level, any discrepancy being made good with the first brick/block course.

 

[banjodeano]

Sorry for jumping in....but, how can you tie one section of concrete in with the next.? sure the concrete section will have boards all the way round it, so how can you have mesh protruding.?

 

[Shinogi]

You use two pieces of wood. Use one thin piece which would be the same height as the mesh and use a second piece to slide on top of the mesh above the first piece of wood leaving the mesh protruding between the wood.

 

[pinenot]

Strip foundations are in essence concrete poured into dug channels in the ground and some reinforcing mesh thrown into the mix when it's wet. Stop ends or day joints are where the pour ends one day and recommences the next. These can be stepped i.e. under mesh protrudes 50mm forward of the above mesh concrete, held there by wood battens which are removed the next day. The second days pour then flows around this and the protruding mesh to form a homogeneous strip...seemples...pinenot

 

[Shinogi]

Can you please confirm to me that digging to the undisturbed clay line means that I only have to dig to where the clay starts which is 420mm or is it wise to dig into it a little?

Just want to be sure, I'm quite looking forward to this and would like to get it right

 

[pinenot]

Work out the depth required to build the substructure up to dpc level, and dig to that depth into the clay (remember you'll have mortar joint forbye the bricks/blocks height to add on)...pinenot

To banjodeo - there are no shutters so to speak forming side walls for strip founds, you simply use the trench walls i.e. a 600 strip is nominal