Tying membrane under a raft into the wall

[steveu]

Hi

I'm building both a garage and house extension both with raft foundations due to the type of ground we have and nearby trees.

I've done the first pour of concrete for the raft and am about to add a ring of blocks around the edge before then doing the second pour to complete the raft. However I'm confused as to how the membrane that us under the raft ties into the walls. One suggestion was to come up the outside and then overlap it with the DPC at that level. The other was to lay the first course of blocks on it and bring it up the cavity and then tie into the DPC.

The problem I have with the first is that it'll be exposed and visible. The problem with the second is that the base of the foundation is resting on membrane rather than directly onto the slab.

Am I missing something ehre? Which is the typical/best route?

Difficult to find anything online that gets specific on this but others must be doing this.

Thanks
in advance.

Steve

 

[jks7492]

I've done the first pour of concrete for the raft and am about to add a ring of blocks around the edge before then doing the second pour to complete the raft.

The problem with the second is that the base of the foundation is resting on membrane rather than directly onto the slab.

Not very easy to understand what you're trying to explain..
Whats the edge detail for your raft? Might be easier if you posted a sketch or some photos?

For a raft i'd normally expect a DPM over the top of the raft (underneath insulation/screed) which then laps into the DPC?

 

[SpecialK]

Any help -

He has a whole series on this build.

 

[steveu]

Not very easy to understand what you're trying to explain..
Whats the edge detail for your raft? Might be easier if you posted a sketch or some photos?

For a raft i'd normally expect a DPM over the top of the raft (underneath insulation/screed) which then laps into the DPC?

No problem, sorry it wasn't clear.

Here is the raft cross section - they're both pretty much the same - and I have membrane between the concrete and the stone:

I've been told that the membrane under the raft should tie into the DPC on the outside wall, and the membrane under the floor insulation to the DPC on the inside wall. Either via this route:

Or via this route:

However none of the books or online examples seem to do either!

 

[steveu]

Any help -

He has a whole series on this build.

Thanks, saw this and some others but none seem to bother about tying the plastic membrane under the slab to anything.

 

[steveu]

Thanks, saw this and some others but none seem to bother about tying the plastic membrane under the slab to anything.

I am now addicted to these though and watching the rest of the series back to back, thanks

 

[jks7492]

 

[steveu]

View attachment 279517

Thanks, I understand that part, and will be doing exactly that.

However, in addition, it’s been suggested that I should look to link the DPM that is between the hardcore and the concrete with the DPC in the outer wall. I can’t see how to sensibly do this and as far as I have found no one else does.

 

[Notch7]

Thanks, I understand that part, and will be doing exactly that.

However, in addition, it’s been suggested that I should look to link the DPM that is between the hardcore and the concrete with the DPC in the outer wall. I can’t see how to sensibly do this and as far as I have found no one else does.

I’m not convinced there is a suitable way of keeping the raft concrete protected from damp.

The options you indicate both have design flaws:

1) DPM at edge of raft is unprotected from damage.

2) they introduce a slip course in the brickwork. (DPM is smooth, DPC is ribbed to grip mortar).

 

[George_engineer]

Thanks, I understand that part, and will be doing exactly that.

However, in addition, it’s been suggested that I should look to link the DPM that is between the hardcore and the concrete with the DPC in the outer wall. I can’t see how to sensibly do this and as far as I have found no one else does.

Plastic sheet beneath the concrete isn't a DPM (but often gets called that as it's the same, or similar material).

Usually the main purpose is to keep the water content of the concrete consistent while it cures.

However, it may also be a gas membrane if you live in a radon or contaminated ground area. In which case check with the specifier on how to terminate it (just to avoid gases getting back inside the house).

 

[^woody^]

Crap design. The DPM should be on top of the raft.

 

[steveu]

Plastic sheet beneath the concrete isn't a DPM (but often gets called that as it's the same, or similar material).

Usually the main purpose is to keep the water content of the concrete consistent while it cures.

However, it may also be a gas membrane if you live in a radon or contaminated ground area. In which case check with the specifier on how to terminate it (just to avoid gases getting back inside the house).

Ah ok, that makes a lot more sense - thanks.

We don't live in a radon area but maybe they're being over cautious. I'll check as you've suggested but logically as the barrier already finishes outside the house as long as it stays there and I don't being it back in we should be good.

 

[steveu]

I’m not convinced there is a suitable way of keeping the raft concrete protected from damp.

The options you indicate both have design flaws:

1) DPM at edge of raft is unprotected from damage.

2) they introduce a slip course in the brickwork. (DPM is smooth, DPC is ribbed to grip mortar).

Yep, agreed.

I think George has hit the nail on the head though in his reply. Think this is two things - radon and damp - being confused/combined.

 

[freddiemercurystwin]

 

[SpecialK]

I am now addicted to these though and watching the rest of the series back to back, thanks

Try also Build With A&E, "Bricklaying With Steve and Alex" and Chris Longhurst - They all have various extension projects. Picked up loads of tips when I built mine!