Hello, I am having an extension built, our street is on a gentle slope and our side extension is butting up against a bank of about a meter high. The builder is building a concrete 9″ block retaining wall with 3inch shingle gap on the outside of the wall (between wall and soil) and a tanking material mix on the inside of the wall 1.5 block height above the footing, so not right down to the footing. He assures me it is ok to let the water come in at the bottom (any that comes in through the blocks will run away down the cavity) as it will come in through the side walls anyway. Side walls do not have bank right up to them, but will have a patio both sides, although they are dug down with landscaped retaining walls I will build with sleepers.

So my question is, is it ok not to take the tanking right down to the footing. I buy the idea that the sides do not need to be tanked. I just assumed the more tanking on the slope facing wall the better. But I also think I also understand builders idea that water will come in anyway and the aim is to run it off not stop it coming in. Their will be damp proof and brick work above block wall and on side walls.

I appreciate any knowledgeable views. I will gather some pictures.

What do the approved Building Warrant drawings say?

Always tank where possible on the positive side, tanking on the negative side is not as good. Wall foundation interface is always a weak point. Without appropriate water bar etc it will always fail at that point. The 9″ wall is my concern with the 1.5 off block untanked that means the bottom 300mm will be damp and leaching on to your plaster finish?

That looks fine to me, other than i’d have expected the tanking to go two courses higher at the top.

Imagine the detail if the external ground level was at  or just below the internal floor level, like in most houses.

The portion of the external wall between the external ground level and the foundations  does not normally have any tanking applied to it even though it’s in constant contact with potentially saturated ground.

If it’s passed Building Control with that detail  i’d not worry about.

perchypanther, It sounds like you don’t mean the architects plans/ approved planning drawings? So I have nothing else other than these? No one in the process has mentioned, ‘Building Warrant drawings’.

Aah drawings make it somewhat clearer. I’d be asking for a cavity tray to be installed from the second flat block up to then sit up on to the block beam floor. This will stop the bottom of the insulation from attracting moisture.

But actually looking again that could attract its own capillary issues across the tray? Oh and the fact it’s too late!

The architects plans need to be submitted to the Council’s Building Control department for approval that the build meets the current Building regs.  If you have an Architect they should have done this, speak to them and ask. The council issue you with a Building Warrant to say they’ve checked it and it meets the standards.

It’s a separate process from Planning Consent. Your local council website will have details on this.

As pp says even under permitted development you still need to meet regs.

wrightyson,  did you mean the internal wall? They have put a damp proof course under the beam and block and are putting one above the beam and block. Will this not stop it moving up to internal plasterboard?

thanks perchypanther, it sounds like you think other than going a bit higher with tanking you think this is OK?

why would water come in the side walls? is he not waterproofing those? Usually its best to keep water out.

Is teh cavity drained? where does the water go once inside?

Usually damp proofing is extended down to tie in with waterproofing under teh floor slab and is continuous under and up to at least 150mm above ground level.

Look at the drawings and get your engineer/architect to comment acordingly. (one assumes you have a design engineer/architect?) The form of floor construction (ground bearing/beam and block/PC slab) will affect the waterproofing detail.

Perchypanther I will double check with architect this has been done. thanks. We have an independent inspector recommended by builder overseeing.

thanks perchypanther, it sounds like you think other than going a bit higher with tanking you think this is OK?

It’s not for me to decide if it’s OK, it’s for Building Control 😉

…but yeah,  in isolation and out of context with the rest of the details it looks kinda  like i’d expect.

The reason i’d expect it be two courses higher is that the bottom two  external courses are traditionally wet from rain splash / capillary action  with a DPC above. On your detail it looks like water could potentially travel across the top of the tanking to the insulation in the cavity.

OK I have checked an we have submitted and approved building control drawings, submitted by architect. I have not seen these and am now asking for them.

Yes PP I can see what you are saying and will ask for the tanking to be extended up to the DPC.

VH the side walls that are ground level will not be tanked and will just have a DPC. As PP mentioned isnt it normal for ground level walls to just have DPC rather than tanking?

The block and beam floor is 2 courses above the footing and had a DPC below it (between it and brick underneath) and will have a layer across its entirety above it underneath the floor insulation.

It is increasingly seeming like their is some vagueness about the building control drawings and what the builder is doing? I will try and get on top of this before it is too late. Maybe I should have used council inspector.

one way to do it would be to have water proofing under teh beam and block floor (potentially under the insulation and screed) this would extend under teh inner leaf, up teh cavity and out 150mm above the external ground. You can then have a periscopic air vent to teh sub floor void under/infront of teh DPC to allow teh sub floor void to dry.

if you can vent front and back you may not need to do the side periscope vents.

Personally i`d want to see sonething better than a bit if waterproof screed on the inside of teh outer leaf.

write to your architect saying ‘i`m a bit worried about the waterproofing and long term affects’ and ask for clarifications on how it is detailed and how it works. Get them to site to do this if its easier then you, the builder and the arch can agree and understand it.

The tanking will work fine. However as I said before my main concern would be the insulation absorbing the mortar. As it is there is nothing to stop it dropping in to the bottom of the cavity. If you’re using cavity bat type as in dritherm or similar it will suck moisture like a sponge.

Your builder will call out building control to inspect at various key stages (they give you a form with all the stages on it, or at least ours do), although how fussed about tanking they are I have no idea.

As wrightyson said, ideally tanking should be on the side the water is coming from, so that water pressure doesn’t blow it off, but given that the wall is built (and assuming it’s built to the detail approved by BC) it’s acceptable. However, the point of the gravel is to drain away any water building up outside the wall, so make sure the ends of the gravel can drain, and I’d also put some porous membrane (terram etc) over the top so that it doesn’t get choked with mud etc over the years and stop draining.

Greybeard, good advice (to note builder said he couldnt get down outside as he had to build up to bank) and Thanks all, this has been a real help. I do think the builders are doing a good job. But I don’t think it can hurt to get a second opinion so have called out the building inspector and he is coming today. I worry this will upset the builder, but this is probably a silly thought.

Alan (the builder) has put what I think are telescopic vents in, two on each side, leading out in to brickwork and one in the middle of the tanked wall, but pointing up in to cavity, as it obviously can lead out of wall. Alan has said the tanking material is a special mix that is plastered on (he said its expensive), he said alternative paint on material could have issues with not adhering properly?

Does it matter how much ‘muck’ falls in to the cavity? With the current block courses that have been laid there is already quite a bit in there. So I worry if it will reduce drainage in the cavity?

That first drawing is pretty waste of time showing the tanking, if using bituthene it is meant to be protected by a 100mm block so i would have thought a 330mm retaining wall with the tanking stuck to a primed dry wall all joints filled ie no tip joints, too late it looks built

the buildier should ensure the cavity is clear to prevent moisture bridging from teh damp wall (it will be with teh detail shown) and the inner leaf.

a cementition slurry is OK but not really rated for much hydrostatic pressure. Vandex is the common trade name/terminology but other brands are now available. it seems an expensive way to do it to be honest. Also if its not applied perfectly it will fail. why doesnt he just lap a welded membrane up the cavity from the DPM under teh floor insulation? That said if its all vented it might be OK. I note the extension is in a dip so your builder really needs to ensure the subfloor void doesnt fill up with water. Its difficult to tell from the pics if this is a potential issue or not though.

is the builder laying a land drain agains the outside toes of the retaining wall? if not where will the water go other than into the wall? water finds its path of least resistance.