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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!
13 Answers from MyBuilder Damp Proofing Specialists
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
Answered 12th Sep 2011
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 14th Sep 2011
Sleeper wall, maybe best. Joist need to be kept off the wall, it's a bit bodgy though
Answered 15th Sep 2011
Manchester • Member since 10 Jul 2012 • 15 jobs, 100% positive feedback
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 18th Aug 2012
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
I would advise getting in contact with building control now for a site a visit and explain the situation to them, that way the solution will be to their satisfaction.
P.s As above definitely sack the builder. this has only come to light because you noticed it, otherwise it would not of been mentioned, obviously the guy is not very honest.
Answered 3rd May 2018
I would be persuing the builder legally , as the job is not fit for purpose ! with the DPC above the floor level , wet weather will almost guarantee the blockwork below DPC will be damp , and by the sound of it , directly behind the skirting ! The job needs doing properly , there is no easy fix . Not too long ago I did remedial work to an extension with the exact same mistake . The job is time consuming , but not hard ! It may be that the finished ground level around the extension may need addressing also , to maintain 150mm below the correctly installed DPC . As for the joists ? Dwarf walls , laid to a footing and honeycombed , with airbricks allowing a crossdraft . They make for a firmer floor than hangers imho. Your builder hasn't notified building control when he should have , they would have picked up on this and am fairly sure they will demand it done right when they see it , They certainly would where I live ! If as you say , he is a reputable builder , it would certainly be in his interest to wipe his mouth and put this right . I'm pretty sure the courts would see it this way too ? good luck !
Answered 26th Jun 2019
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 13th Sep 2011
Quite simply get building control involved.
Answered 9th May 2020
If i was paying a builder for something i'd want it done correct. get him back and tell him to knock it down [ at his expense ] some of us time served builders may be a bit more expensive but you get what you pay for .its one of lifes little lessons. hope this helps
Answered 29th Dec 2020
Well dpc not 150mm above external ground level I’ve not heard of that even by the worst so called Builders so that needs a dpc correctly installed higher up lapping into existing House dpc so continuation of dpc the joist if you’re talking suspended timber floor should be a min distance from bottom to floor beneath and wall timber they are hung from will need dpc behind that is attached to floor dpc I suspect building control will ask for this or advise what should be done before proceeding
Answered 30th Jun 2018
Exposed Timber should never be touching any bricks below DPC this will result in damp rotting timber and fungal growth.
Answered 15th Aug 2023
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