single skin blockwork outhouse

numpty with a crowbar · 10 Aug 2012

After some advice while I'm in the planning stages for my intended workshop / man-cave:

I'll be getting rid of the shed and replacing it with an (approx) 14ft x 11ft workshop. The plan is to cast a single section concrete base (a raft??) to the right size with a damp proof membrane underneath it, with a depth of about 20cm around the sides. The concrete base will be the workshop floor and not planning on installing a raised floor

Walls will be concrete blocks, which I'll clad in feather-edge timber to soften the look and make it look less like a bomb-shelter.

Height will be 2m at the back rising to just shy of 2.5m at the front

So - after some advice on the following if possible please:
1) Is a 75mm raft with an edge depth of 200mm sufficient?
2) Can I use dense concrete blocks or am I limited to the lightweight concrete blocks (my builders merchants - Doves - has them billed as "7N lightweight solid concrete blocks") - If I've got my numbers right it should be no more than 11 courses of block at one end, and as little as 8 at the other end
3) will lightweight blocks be ok holding up a tiled roof?
4) do I need to add reinforcement to the concrete?

Thanks

numpty with a crowbar · 15 Aug 2012

*bump*

Have edited the original post based on a few changes, and have included a diagram of my idea...

Any thoughts?? Can I get away with 11 courses of dense concrete block on this or should I go with lightweight 7N blocks?

tony1851 · 15 Aug 2012

For the main body of the raft, 3" looks a bit thin - suggest 4". To be extra safe, consider some light mesh- it's not too expensive.
The roof load on the blockwork is not an issue, but the dense block would be more serviceable, eg if you are fitting shelving etc.

numpty with a crowbar · 15 Aug 2012

Cool (& thanks) - adding an extra inch isn't an issue as I'll have to order the ballast in dumpy bags and think for what I'd planned I'd have half a bag spare so will use that to increase the depth.

I'd prefer to use dense blocks anyway (for bolting shelving and wall units to) as long as the raft can take the weight of it ok (max 11 courses at one end and as low as 8 at other). But will look to add some mesh as well to reinforce

Cheers

Andrewmudge · 16 Aug 2012

Srsly pay a structural engineer 200 bicker and have him design the floor appropriate to the load, then you will be sure and it will be guaranteed. Bear in mind we put min 4" under conservatory floors.

tony1851 · 16 Aug 2012

Andrewmudge said:
Srsly pay a structural engineer 200 bicker and have him design the floor appropriate to the load, then you will be sure and it will be guaranteed. Bear in mind we put min 4" under conservatory floors.

Its only like a small garage with no big loads and probably the OP would prefer to spend the £200 on something else.

Andrewmudge · 16 Aug 2012

Although it doesn't have to be a consideration but depending on your local authority stance and other alterations to your home etc. if a building is permanently attached to a foundation plus other variables it may require regs in which case you will be asked for calls anyway. I hope this isn't the case for you but it might be worth checking.

tony1851 · 16 Aug 2012

Andrewmudge said:
Although it doesn't have to be a consideration but depending on your local authority stance and other alterations to your home etc. if a building is permanently attached to a foundation plus other variables it may require regs in which case you will be asked for calls anyway. I hope this isn't the case for you but it might be worth checking.

His building is detached, under 30 sq.m. in floor area and presumably contains no sleeping accommodation so will be exempt from Building Control.

^woody^ · 16 Aug 2012

I'd say go for a reinforced 125mm slab all over, and use 3.5N blocks to keep the edge load down and no need to have an edge beam if you come in 50mm from the slab with the wall

Dense or less dense concrete blocks make no difference to fixing to them, but make all the difference in loads to such a shallow foundation or raft

If you do form thickened edges, then that DPM looks great on a drawing, but will be difficult to do perfect without any voids. You'll need to splay any edge thickening and be very careful with the DPM fitting. It will also need protection where it is exposed around the edge

tony1851 · 16 Aug 2012

[quote="^woody^";p="2485380" no need to have an edge beam [/quote]

Would have thought that a small edge beam might just protect the ground from drying out too much around the edge in a dry summer.

^woody^ · 16 Aug 2012

In a dry summer the whole 450mm of any ground will be dry

tony1851 · 16 Aug 2012

The Oracle has spoken.

EB1caneyeB · 16 Aug 2012

I would do a strip footing 500mm deep and 200mm wide. 11 courses of 7nm blocks is gonna weigh a lot. I would lay a membrane on top of the oversite lapped onto the top of first course of blocks. And do the floor seperately. 125mm thick should be plenty strong enough. I would also put some 10mm movement joint sponge around the edge of floor to allow for expansion and contraction.

This is what i would do. In offering this advice i am not dismissing anybody elses opinion

diynot

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single skin blockwork outhouse

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