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framing floor

Discussion in 'Wood / Woodwork / Carpentry' started by bluebaron, 25 Oct 2021.

  1. bluebaron

    bluebaron

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    I'm planning to build a 6x3m garden room, i'll be framing in 4x2. customer already has a solid concrete base in place and was wondering if I good lay the floor directly on the concrete as we are up against the 2.5m rule.
    Could I get away with 4x2 for the floor joists at 600 centres or should I really be going 6x2? Im guessing there will be little bounce if its on the concrete. I will be insulating 75mm celotex between the joists.

    thanks

    Blue
     
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  3. foxhole

    foxhole

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    Plastic bearers, waterproof.Decking boards are only 25mm thick.
     
  4. bluebaron

    bluebaron

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    they look expensive,

    on reflection I think maybe I use 6x2 for the floor and support on blocks laid dry onto the concrete base with a DPC underneath?

    Ill then fill with celotex at 100-125mm and overlay with 10mm celotex to prevent a cold bridge through the joists and then 18mm ply.

    does sound ok?
     
  5. foxhole

    foxhole

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    Are you paying or the customer ?
     
  6. bluebaron

    bluebaron

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    well the customer is my brother and under strict instructions to keep cost down!
     
  7. conny

    conny

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    How about 4" x 2" @ 400mm centres? Take a little more wood but keep you within the height limit.
     
  8. JobAndKnock

    JobAndKnock

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    You could just lay a DPM, then a layer of PUR insulation, then a floating floor of T&G chipboard (P5) or plywood glued together.
     
    Last edited: 26 Oct 2021
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  9. ^woody^

    ^woody^

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    Why suspend any floor? Put it on the slab above a DPM, or prop it to prevent bounce. It will need noggins too - more if 600 centres.

    And don't have any air gap. Full fill insulation.

    But the floating floor idea is probably best if the slab is flat and level.
     
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  11. bluebaron

    bluebaron

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    so a floating floor is just that. I just lay a DPM,Insulation and then chipboard flooring. What holds it all together then? surly some sort or frame is required?

    Also in my case the slab is oversized so whats to stop the rain from running under the DPM as it has nothing to 'lap up' its just flat on the ground effectively?
     
  12. JobAndKnock

    JobAndKnock

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    Well, you will need to frame the walls out first and clad them, but other than that no framing is required for the floor providing he insulation is tightly fitted and the flooring is pulled tight with the tongues glued

    The DPM for the flooring would be lapped up inside the walls

    In other words build the walls first, then install the flooring.
     
  13. bluebaron

    bluebaron

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    thanks Job,

    Excuse my stupidity but if I lap the DPC inside the wall framing then I would have no DPC on the wall plate?

    I think I am over-engineering this? My brother basically ordered a garden room and built a 150mm concrete slab that he oversized. they were going to slap this new room with its 45mm floor battens straight on the slab and erect the 6x4m room off it.

    I suggested that 45mm floor battens with 12.5mm insulation was woefully inadequate for a building he was going to use year round and his heating bill would be huge. He also had a problem that he wanted a shower room with W/c so not much clearance for waste, (which I got him to install into the concrete).

    In hindsight he doesn't really need the slab as all I could have built it off blocks on the ground, but I assume the best way forward now it to treat the slab as the ground and build a frame of off blocks on the slab (to prevent damp).

    That way I can get then insulate and overboard and use the base as a springboard for a 4x2 stud work?

    I can't really see another way forward?
     
    Last edited: 26 Oct 2021
  14. JobAndKnock

    JobAndKnock

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    If the outside wall frames of your structure go onto DPM roll (available in various widths) lapped up the frames and stapled:

    DPM roll 100mm.jpg

    The walls can be clad and the roof put on, giving you somewhere in the dry to do the interior work. Any wall cladding can be laid over the DPM roll where it is turned up. I'd suggest making at least your sole plate in pressure treated timber. and maybe doubling it up to get a decent lap (using, say 300mm DPM which is under a tenner for a roll)

    By putting down a DPM inside, again lapped up the walls and laying 120mm of PUR (pink, higher density) foam onto it, tightly fitted (i.e. no gaps), it should be possible to float an 18mm T&G P5 chipboard floor on top of that glued at the tongues. That way you get 120mm of insulation instead of 12.5mm, you aren't paying for the joists (which in prolonged direct contact with damp concrete might be prone to rotting), you aren't creating any "hidey holes" under the floor beneath the joists for wild life to nest in, and you can fix the wall frames directly through into the concrete with approprate anchors (although TBH 6.0 x 100mm screws and brown plugs would do the job)

    This is a cheap, fast solution - which is what I thought you wanted
     
  15. bluebaron

    bluebaron

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    ok I think i've got that now.

    I wrap my 4x2 studwork base plate in a 300mm DPC and screw through it to anchor to the concrete, (won't this pierce the DPC?)

    then I fill the internal space with a DPC sheet lapped up the stud work to above ground floor level.

    I'm assuming that I will have to make my internal walls longer to accommodate the extra (below floor) construction.

    Could I not run the internal DPC sheet under the external stud work. (effectively putting the whole structure on a DPC sheet on top of the concrete?)
     
  16. JobAndKnock

    JobAndKnock

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    Yes, but don't worry about that. It's accepted practice to fix through a DPM - it still protects 99% of the material, which should in any case be treated timber, otherwise how do you get a structural fix?

    I think your sequencing is a bit out, here. I'd think in terms of building the wall frames and applying a moisture barrier on the cold side of the insulation (i.e. the outside of the framing), which should be a vapour permeable breather membrane such as Tyvek or roofing membrane, cladding the outside (consider battening off the framework to get a ventilation space, too) and sealing it to get it relatively weather tight, then getting the roof frame on, applying a moisture barrier to that, adding cladding that and felting it (or whatever weatherproofing system you require, such as fibreglass or Sarnafil, etc) so that you have a relatively weatherproof structure to work in. Next I'd put in window frames, door frames, glazing and doors. This gets you the main structure.

    One minor wrinkle - consider adding in a pattress on the inside of the wall frames where the skirting will go. Rips of 150 x 12mm OSB will do the job. It will make the job of fixing the skirting later on far, far easier

    At that stage I'd run any wiring and other services I require, then insulating the gaps between the wall studs (for preference I'd use mineral wool batting or possibly sheep's wool - better for soundproofing), fix a vapour control layer on the warm side of the insulation (this can be 1000 or 1200 gauge Visqueen - same stuff as you'll use on the floor - or a purpose made VCL) and finally cladding the inside with plasterboard/timber, etc. Once the walls are in the floor membrane and PUR insulation can goes down in a relatively dry environment. Try doing it before the structure is up and all the tracking about with wall insulation, membranes, wiring, ladders (the roof, remember?), etc is likely to shred it and make it useless, hence avoiding doing it in one hit. The PUR I'm suggesting is fairly rigid (hell, we lay it onto concrete slabs and pour 50mm cement screed in over the top of it - and it supports commercial kitchen equipment) and has good insulation properties, although not as good as blown PIR insulation like Kingspan or Cellotex

    BTW I'm assuming a lot about the structure of the wall there, but it's something like what I'd expect to see on a build. That said I also don't know if you are going for a warm roof or a cold roof, hence the lack of detail about the roof (you obviously need to consider ventilation, etc)

    If anyone has a cheaper way of doing this I'd be interested to know about it
     
  17. bluebaron

    bluebaron

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    Thanks for the comprehensive reply,

    I hadn’t considered doing the floor last, most of the stuff I’ve seen you start the frame off the edge of the floor/structure.

    I get what you mean now so will definitely consider that method.

    I’ll probably end up with a cold roof as I think the warm roof method is quite an eyesore, being twice as think as a cold one.

    good tip for the skirting as well
     
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