Conservatory base

jackboy29 · 29 Mar 2019

Hi, could somebody advise me on a conservatory base. My builder was supposed to build a 500mm deep trench around the outside for a 600mm dwarf wall to be built on. Ground conditions are sand which apparently is good. I saw the trench along the front and one side but have returned today and he has cast an 8in slab and the third side has no trench now. He says 8" slab with reinforcing is fine but that isn't what we agreed. The distance from the top of the slab to the bottom of the concrete in the trench for the front wall is 500mm. This now means the trench only sits 200mm below the ground line. I asked him about going belwo the frost line etc but he dismissed it as not relevant in the south. Could anybody advise me whether this is ok. My dad's house build in the 30's was on 9" narrow footings btu things have moved on since then.

Is an 8" reinforced slab ok for a conservatory with a 600mm dwarf wall or should I insist on proper foundations 500mm deep below then ground level?

Thanks

Notch7 · 29 Mar 2019

Most builders I know would use a digger with a 450mm wide bucket for a habitable building -a 300mm cavity wall with 75mm each side.

what do you mean by 8" slab -do mean the width of the foundation, or do mean you have a raft with 8" thick concrete and a skirt 500mm deep?

jackboy29 · 29 Mar 2019

Just one slab to cover the whole site 200mm deep with reinforcing rods within. On one side and the front there is a trench but concrete depth is only 200mm below ground level (so 200mm deep over most of the site and 400mm deep on one side wall and front where the trench is). Return wall there is no trench as they didn't bother digging it so it's just the 200mm slab extending. He says it's strong enough as not much weight but what about subsidence and ground frost etc causing movement of top ground material?
Thanks

Leofric · 29 Mar 2019

Shouldn't foundations project a minimum of 150mm each side of wall so 600mm wide foundations for a 300mm cavity wall ?

KenGMac · 29 Mar 2019

jackboy29, good evening.

Did anyone dig a "trial pit" and possible a bore hole to determine what depth the sand extends to and what is the strata below the sand?

As an aside, in the image posted the left hand edge of the slab "appears" to be curved, OK it could be a tick of the digital image lens?? but?

Ken.

Notch7 · 29 Mar 2019

jackboy29 said:
Just one slab to cover the whole site 200mm deep with reinforcing rods within. On one side and the front there is a trench but concrete depth is only 200mm below ground level (so 200mm deep over most of the site and 400mm deep on one side wall and front where the trench is). Return wall there is no trench as they didn't bother digging it so it's just the 200mm slab extending. He says it's strong enough as not much weight but what about subsidence and ground frost etc causing movement of top ground material?
Thanks

What your builder has built is a sort of raft that has a small skirt around it. Odd construction TBH -most builders would dig a trench, mass fill with concrete then cast a slab in the middle. A raft is only normally built for special ground conditions.

The specification of foundations and slab should be determined by: load bearing capacity of the soil and nearby trees or hedges.

Generally rafts are designed by a structural engineer -a necessity for building regs.

You need to make sure your builder knows how to deal with the damp proof arrangement -its not obvious what to do on a raft, especially a flat raft like you have. Ask him to sketch what he proposes and get it approved on here.

What you have built may be strong enough for a conservatory, but Its a bit on the small size. Maybe a pro will be along to give some advice

themiddleagedun · 29 Mar 2019

The vertical sides of the slab shouldn't be on show like that. It should of been strip foundations with brickwork showing from ground level up to the top of the slab..
It would probably be strong enough, but a bit of a botch..

noseall · 30 Mar 2019

Leofric said:
Shouldn't foundations project a minimum of 150mm each side of wall so 600mm wide foundations for a 300mm cavity wall ?

Proper builders, proper buildings yes. Glorified sheds (connies) - apparently not.

Ian H · 30 Mar 2019

Should a conservatory have insulation in the floor too?

Notch7 · 30 Mar 2019

Ian H said:
Should a conservatory have insulation in the floor too?

Good point, hadnt looked at that.

The concrete slab is only 100mm or so below house finished floor level. No room for insulation plus screed.

Normally the face brickwork continues down to below ground level. In this instance the edge of the slab will be visible.

As Noseall points out - its really a construction for a shed or outbuilding.......and its not been built by a proper builder.

I hope youve not paid them much money.......

tomfe · 30 Mar 2019

Are you planning to build the walls out of pavers?

^woody^ · 31 Mar 2019

Seems like some sort of raft. A raft on sand may well end up floating down the garden.

arkie · 31 Mar 2019

The garden seems to slope up hill, so once back filled i don't think it will be visible, but as said not much room for insulation, 50mm maybe.

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Conservatory base

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