Blockwork Shed Foundation Issues?

[pfm401]

Hello

We had a blockwork outbuilding built last year (about 4m x 3.5m). I've previously asked for advice on water ingress but another thought has come to mind on the construction of the shed. I can't say for sure but I think it's built as follows (pics attached):

- Hardcore (not sure how deep or what type)
- Concrete slab (not sure how deep or what type
- 4 courses of blue engineering bricks
- Medium density blocks

I got concerned when the builder cut out the overhang of the concrete base as he said that might be why it was leaking. Another builder says there should be a 6 inch overhang for foundations, my builder says that is only for "actually for foundations when building an extension or house".

Has the shed got an adequate base?

Many thanks
Paul.

 

[Mr Chibs]

Do you have problems with this, or are you just asking for future?

Has it been built off slab or a proper trench foundation?

 

[^woody^]

The toe on a raft foundation serves several purposes - design, structural, aesthetic, but the omission of the projection does not necessarily mean there will be a problem.

However both your builders are wrong in there understanding of building.

 

[pfm401]

Do you have problems with this, or are you just asking for future?

Has it been built off slab or a proper trench foundation?

Asking for the future, no observable issues now but only 1 year old. As there's issues with the leaking through mortar at the bottom, and issues with the roof, if this is an issue I'd consider getting a surveyor to review before making payment on another job the guy is doing.

It's a slab I'm sure, but I'm basing this on the picture. Sorry to be thick but what would go in a trench foundation - concrete or brick?

 

[pfm401]

The toe on a raft foundation serves several purposes - design, structural, aesthetic, but the omission of the projection does not necessarily mean there will be a problem.

However both your builders are wrong in there understanding of building.

Thanks. To be fair it might be me misunderstanding what they're saying! I did check with the "another builder" who said that he would have done 150mm hardcore, 150mm concrete with mesh reinforcement because the building is quite light

 

[Mr Chibs]

I would have thought it better to dig footing, fill with depth of concrete, then build off that, engineering brick, then blocks... similar to houses/extensions.

However, bothers house is built on a raft, has very high water table and it's reclaimed ground from mining etc, so it is possible.

I would have thought it's difficult to get a single skin blockwork water-tight. Depending on what you are using if for, perhaps you could clad it outside to shed the rain... lot's of options.

What roof type are you having?

PS, I'm not sure I'd worry about the lack of footing projection, it will make no difference to what you have.

 

[RonnyRaygun]

The slab doesn't need to protrude past the face of the bricks. It's common practice to form an edge thickening with a toe that ensures the brickwork begins below ground level and the concrete slab isn't visible. It's not essential though and does require a lot more work which adds a lot of cost for a shed base. Even with a toe the brickwork usually sits flush with the edge of the slab.

The six inch overhang is because the standard is 600mm wide trench fill foundations and 300mm wide cavity walls -it doesn't apply to a raft (it's not essential with trench fill either so long as the foundations are wide enough to spread the load to the ground below.

The builder should have put a couple of layers of steel mesh in the slab to help spread the load as without it there is a risk of the slab cracking if the ground moves in the future.

The lack of overhang won't have caused any leaks. I assume you've got a DPM on the slab lapped into the masonry? You might still get penetrating damp through the single skin of blockwork though.