Damp Proofing Question

How worried should i be about the timber joists and flooring in a new extension touching the wall below the dpc?

I'm having a single storey extension built (approx 4m x 5.8m). The builder inserted the air bricks and dpc too high which meant that, when he put in the floor joists (hung off ledger boards bolted directly to the blockwork), the ledger boards covered the air bricks and all the timbers were below the dpc.
He admitted this was wrong when I pointed out to him that this couldn't possibly be right.
They have now put in new air bricks lower down so they vent underneath the floor joists.
With the dpc the builder suggested either injecting a dpc lower down or inserting a length of damp proof membrane between the ledger boards and the wall. I sought advice from a surveyor friend who said that neither of these options was acceptable (the injection system isn't that reliable and adding a membrane between the timber and walls was flawed because every bolt hole would puncture the membrane and allow damp through the hole). He said the ledger boards should be scrapped and the joists should be hung from the blockwork using S type joist hangers so that at no point does any wood touch the walls below the dpc. This is what the builder has now done.
However, the joists are so tight in the hangers that some still touch the blockwork and none of them are more than 1-2mm away from the blockwork. How worried should I be over this? What's the minimum gap you would recommend between the wall and the joists? And between the wall and the floorboards themselves (as they will also be below the dpc)?
Would a solution be to build a sleeper wall (with dpc and wall plate) at each end and rest the floor joists on this so that they are not connected to the blockwork at all? As the sleeper walls will then be taking the full weight of the floor, do they need some sort of foundation or can they just be built directly onto the oversite concrete?
I'd be really grateful for any advice please.
P.S. I'm on the west coast of Scotland if that makes any difference in terms of building regs, etc.
P.P.S. Before anyone else assumes I went for the cheapest dodgy builder, he's a well-established local firm, recommended by the architect & was main contractor on building works at a private school in the area!

6 Answers

Best Answer

Sounds like your builder made a big mistake, you shouldnt get the dpc and floor levels wrong.
However, if you go down the route of sleeper walls, then you will need some decent foundations for them.
An alternative, as he has now fitted hangers, you can either get joist end caps, used as if they were built in, or wrap the ends in dpc, both prior to fitting into hangers.

Answered 13th Sep 2011

B J D BUILDING/ROOFING

Member since 29 Oct 2008

i would be more worried about getting final cert ,if this builder cannot get a simple thing like damp course level correct ,what other horrors are going to crop up through the build ,i gather he was the cheaper offer ,beware of cheap imitations ,said it a hundred times there are no bargains to be had in building
good luck

Answered 11th Sep 2011

substructure

Member since 7 Jun 2008

The other way is to take out the suspended floor, lay in damp proof membrane (lapped up to DPC height), sand blind it and put in concrete floor screed.

Answered 19th Oct 2012

ATS Building Services

Member since 16 Oct 2009

Sleeper wall, maybe best. Joist need to be kept off the wall, it's a bit bodgy though

Answered 14th Sep 2011

Rebel Carpenter

Member since 24 Sep 2008

Best option is right first time.dpc 150 above FINISHED ground level.That is the regs and any arc would put that an plans.Get it put right now save you loads of problems later.Leave existing damp in place,get a good brickie to install a new one lower even if you use a sleeper wall you will still get rising damp

Answered 17th Aug 2012

l.mcguinness building and joinery

Member since 10 Jul 2012

I would be inclined to incorporate a sleeper wall set up, but you will have to survey the oversite first. If your builder has made a lash up of D.P.C installation then yes, keep the suspended floor independant from the blockwork.
Thats the only way to stop and future potential rot and decay to joist ends.
last thing you need is Dry-rot accuring.
Sack your builder!!

Answered 12th Sep 2011

NLC Carpentry & Joinery

Member since 2 Jul 2011

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