We are having an extension built and I have some concerns over how the joiner is constructing the suspended floor.
The house is constructed with a vented void where the dpc is about three/four couses above ground level, then above the dpc are air bricks. The extension has been constructed the same way with the airbricks and dpc at the same levels as the house.
The joiner to make the suspended floor has bolted timbers to each side of the extension, these timbers start below the dpc and finish above the dpc, also the timbers cover the airbricks. He has however drilled lots of holes where the airbricks are behind the timbers. Attached to these timbers are joist hangers. This gives the same floor level as the main house.
My concern is that the timbers bridge the dpc, surely this can't be right?
The person to collar is not the poor tradesman who has been forced to bodge it to get the job done and for whom any delay is expense, collar the person who drew the plans with insufficient detail, who isn't making himself available to answer this kind of query, or the person who is overseeing the trades whose job it is to examine the plans and anticipate any problem like this before you get there.
Unless of course the carpenter is the builder and you don't have any plans. In which case you need to stop everything immediately and call Building Control.
Thanks for your reply, the builder is the joiner. I agree with you that is sounds like a balls up, but will it work? Or will the section of timber below the dpc soak up water and/or transfer it above the dpc?
Nothing should span the DPC. The timber should have been specified as treated, (usually green in colour). The normal practice is for the hangers to be cemented into the wall and above the DPC. The hangers used would be much thicker metal for this puspose.
But anyway. It's quite unsual to have suspended floors at ground level these days. Haven't seen one for years. Normal practice is to have concrete (with insulation and DP membrane below. This a much better system, nothing to rot and warmer too.
There's two problems here, the first being that the airbricks are at the wrong height - they should have been 2 or 3 courses below their present level, so that you get an unhindered crossflow of fresh air under the floor.
The second problem is the DPC is at the wrong level too, it shouldn't be
75mm below floor level, it should be at floor level, which inevitably means that the joists are usually below the dpc, to get around this, builders usually incorporated two dpc, one underneath the joists, then laid 2 or 3 courses, then the secondary dpc at floor level, are you sure there's not another dpc under the one visible in the pictures?
Either way, the vents will need moving - why they put them in the way of the dpc remains a mystery.
Thanks for your reply, I have uploaded two more pictures to:
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first new picture shows where the builders have knocked out an existing airbrick to find the floor level, the original dpc is just under this airbrick. They have replicated this in the extension with the airbricks at the same level and the dpc under the airbricks. So maybe this is OK?
The second new picture shows (hopefully) the position of the existing joists, this is looking into the airbrick they knocked out. The joist is sitting on top of the dpc.
So is the problem just that the new joists are of a different height to the old and are too large to sit above the dpc and give the same floor level?
BCO was happy with position of airbricks and dpm, external ground level means air bricks can't be any lower, builder needs to change them to telescopic air bricks. Instead of timber bolted to wall use embedded joist hangers.
He also had a good look around, no other problems. Had a private chat to him after, says work is OK, in some places builder has gone further than he needed to. Would recommened not changing builder but to keep a close eye on him. BCO will make extra inspections before plastering.
Hello Hugo, by sub-floor do you mean the concrete on top of the dpm sheet, or the suspended timber floor. Either way what are the consequences of the sub-floor being below the adjoining ground level?
The sub-floor is the covering over the ground; in your case, concrete. If it is below the adjoining ground level, there is the risk that any water that gets inside cannot drain away and will sit under the floor.
If the ground around the building is sloping, part of the sub-floor can be below ground level provided it falls away to the lowest edge of the building where it can drain away. I have accepted a 'french drain' around the building or offending part that is able to drain away, i.e., to a gully or a soakaway.
Graham I concur with the majority here that this is a builder who most certainly has not a clue about new build or even how to repair old.
He could have a) put the joists in joist hangers or even BAT Joist angles (Jangles) if he could find any b) put a vertical DPC against the wall before installing the so-called wall plate
What he should have done and what I would now absolutely insist on is
1) 4" concrete on the oversite ( a building reg or used to be )
2) brick or concrete block piers or honeycombed sleeper wall inside the line of the room at the perimeter at about 8 ft intervals intermediately (depending on size of joist)
3) the wall plate laid flat on a polymeric or LL dpc
the airbricks are in exactly the right place - under no circumstances move them but do put in a telescopic connection to under the insulation (to be)
What really concerns me most is the apparent lack of use of tanalised timber . Is it even VAC VAC treated? which is not so good as tanalised due to the absence of combination of the preservative with the hydroxyl groups on the timber
My advise is to swallow hard and
a) get rid of the timber floor with all the complications of having air and insulation under and underneath put in 4" or bigger if you can ducts to the outside vents from the existing vents to provide through flow of air b) If there IS concrete laid and it is rough determine if it is ok for structural purposes either on its own or by adding to it - incorporate ducts below such as to allow sufficient mass above or put in a reinforced 4" concrete floor with vent ducts below c) put in as much insulation as possible not less than 4" preferably more up to 12" max d) a 3" screed with UFH
I think the problem is that there is no requirement for the builder to do these things. For example are tanalised joists a requirement? The BCO has seen timber used for the joists and didn't question it. "the wall plate laid flat on a polymeric or LL dpc" again is this a requirement? Why would any builder spend extra time and money on things that are not required by the regs?
The Building Regulations are only about health, safety and welfare. They are not about property maintenance or longevity of materials, and are certainly not about the quality of materials or workmanship, except so far as is needed to ensure continued compliance with the requirements.
Its not the builder's time and money being spent on these things, it's yours. You've either contracted for a reasonable quality of work or a legally minimal job. Either way, you should get what you've paid for.
Thanks Hugo, I understand. Can you tell me in your experience the percentage of jobs you have seen that use treated timber (joists/roof) as opposed to untreated.
Don't know, 'cos it's not something I check for. I would imagine it's approaching 0%, which is more to do with the quality of the work I have to deal with. A properly constructed floor or roof shouldn't need treated timber; it shouldn't be getting damp enough, or be ventilated enough to get rid of it.
As we always have to tell people; we're not Clerks of Works. we only control enough to ensure the health & safety of the finished work.
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