DPM options for new garage on slab foundation

svicente · 20 Jan 2010

Hi there. First post!

I'm currently in the process of attempting to design/build a new garage at home. I've opted for a rc slab with the brick/blockwork commencing at the top of the slab which is roughly ground level also.

What is the best way to lay/route the DPM? I originally thought about putting it under the slab but i'm now thinking about raising the floor inside the garage (for insulation) and this gives me an option of putting it above the slab.

From looking at the attached drawing. What would be the best way to route the DPM and tie into the DPC.

Untitled

svicente
20 Jan 2010
1

Thanks for any assitance.

Simon

Chukka63 · 20 Jan 2010

Firstly i would form the concrete with a 150mm step around the perimeter to allow the 150mm difference between the outside ground level and the DPC.you then need to cut some blocks down to 150mm or use 2 course of bricks on the internal leaf.then lay 150mm DPC in the brick/blockwork so 50 mm is protruding into the garage..
lay your insulation and then you DPM and lap the 2 together.you will need more than 50mm screed if youre driving a car over it.

svicente · 20 Jan 2010

Thanks for a swift reply.

Only problem is i'd be reducing the depth of my slab at the edges to 150mm if i read you right? Would this not undermine the ability of the slab to hold the weight of the building? It is fairly stiff clay underneath mind (150mm 2" crusher run/hardcore mix under the slab).

Also, i don't plan on leaving a car in the garage, but i assume someone may at some point. What would be a good depth for a screed for a car?

Cheers

Chukka63 · 20 Jan 2010

you can just thicken the slab around the walled perimiters.
why do you need insulation in a garage???,why not just finish the concrete to a good standard.

noseall · 20 Jan 2010

I always look at this design and wonder, WHY?

Strip foundations (at depth) with an enclosed slab is a far better, easier, and more practical design that will afford you a dry interior.

What you are showing is good for a shed only.

If keeping out the damp is what you want to achieve then you need to redesign.

Chukka63 · 20 Jan 2010

mine is not to reason why........

lostinfens · 20 Jan 2010

Nose says it all - why on earth go to all the faff when a strip found is a doddle. Gives a lot more leeway, allows for membrane & insulation & is probably quicker in the long run.

JohnD · 21 Jan 2010

garage appears to have brick and block cavity wall, as well as an insulated floor.

Perhaps it is going to get converted to a granny annexe once the planning permission has been granted.

hotrod · 21 Jan 2010

As already said above, no real need for insulation in a garage.

svicente said:
From looking at the attached drawing. What would be the best way to route the DPM and tie into the DPC.

See DPM / DPC detail below:-

hth

JohnD · 21 Jan 2010

hotrod, in your diagram, are the strip and the floor poured together, or strip and dwarf wall first?

I thought the wall strip would be deeper/thicker than the floor?

svicente · 21 Jan 2010

Thanks for all the replies!

I understand that strip footings are more practical and offer a better solution. But given the lack of time i have to do the work, which is about 8hrs a week if im lucky (how long to hand dig strips for a 6m x 3m garage?), and on a shoe string budget. I had to go with the simplest solution.

Re the "shed" comment. Does anyone not think a 300mm slab with C35 concrete (i can get it relatively cheap) and A242 mesh will work for a cavity walled garage?

The insulation is because the garage will be a workshop for my bike, and no doubt as our 1 year old grows up and i cant get my football on the telly any more, it'll more likely turn into a bolt hole for me! The beer fridge is waiting in the loft for a good home.

Thanks again!

noseall · 21 Jan 2010

svicente said:
Re the "shed" comment. Does anyone not think a 300mm slab with C35 concrete (i can get it relatively cheap) and A242 mesh will work for a cavity walled garage?

Yes it probably would but why?

You still need to dig the slab deep enough as to remove all the fertile matter near the surface.

It will only need be 300mm around the load bearing edges though this should also contain thicker re-bar tied into the mesh.

The whole job will be better off built on strip foundations.

lostinfens · 22 Jan 2010

Given a lack of time??? wtf are you talking about? If time is of the essence , then get a builder in. If your doing-it-yourself then time must be a secondary consideration. It may come as a shock to you but most builders are on a time limit by virtue of the price which they offer - more often than not in competition with another builder.
Digging and concreting a raft is more difficult than a conventional strip foundation, if you're going to do it right. Can't tell you how long it'll take to dig either, but a weekend should do it with a strip found & no mesh reqd.

hotrod · 22 Jan 2010

JohnD said:
hotrod, in your diagram, are the strip and the floor poured together, or strip and dwarf wall first?

Hi John, that diagram is purely a DPM / DPC detail and makes no detailing for foundation design as this would vary dependent on design (e.g. raft / trench block / trench fill etc). You are correct though, in that a traditional strip the trench would be deeper than the slab, so in that respect the diagram could be misread. But, like I said, it is, and is notated as, a DPC overlap, so the foundation detailing would never be used on site as a working drawing. Hope that clarifies.

svicente · 22 Jan 2010

Noseall: There was an existing garage which was crumbling/leaking with an asbestos roof which i demolished. Once i broke up the original slab, which was not even 100mm thick. I then tried digging out the original hardcore to reduce the level a little. This hardcore was in excellent condition (original builder must have had a lot of surplus left over). It was deep and of good grading. I then gave this a wacker and touch up with some type 1 and here we are at the present situation.

I'll see if i can caj some re-bar from an old site i was on to stiffen up the edges for shear. Thanks.

Lostinfens: I have no money to get a builder in. I work for a civils company and can get materials at cost (pretty much). I can get plant for free, just usually not when i want it. I spent £45k on an extension last year, and the builder quoted me £16k for a new garage. I've had other quotes and none cheaper than £8k. I can do this myself with a little specialist help when required for about £5.5k and this includes some little luxuries like insulation and electrics. I'm in no major hurry to finsh, just before the summer, so baby can go into garden. But have little time available to actually do the work except weekends. I've already gone down the road of a slab footing (see first para), so no going back now.

Cheers for replies guys.

diynot

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local

DPM options for new garage on slab foundation

hotrod

hotrod

Similar threads