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Support spacings below 8'x4'x18mm ply substrate?

Discussion in 'Floors, Stairs and Lofts' started by Draughtsman, 19 Oct 2021.

  1. Draughtsman

    Draughtsman

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    The ply substrate is for a click vinyl surface. the cls frame will be sat on rubber pads or low level decking pedestals on rubber pads (on a membrane)

    I was thinking of using a stud type support framework so that ply edges have landings along their full length?

    So does the usual 400mm joist spacings rule apply here?

    Thank you,
    Dain
     
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  3. JobAndKnock

    JobAndKnock

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    If you are building a framework/joists for plasterboard, which almost universally comes in "metric" sizes (2400x 1200mm being a very common size) then the supports need to be at divisors into 1200 (or 2400), namely 400 or 600mm in order to support the board end joints

    if you are dealing with joists which are supporting "Imperial" size boards only (so 8 x 4ft or 2440 x 1220mm) then your 400mm spacing needs to be revised to the Imperial equivalent, namely 16 or 24in (406.7 or 610mm respectively) in order to ensure that joints are adequately supported. This is pretty normal in the construction industry

    The problem comes when you are ply cladding joists which are to be plasterboarded on the undersides. In that case you set the joists out at 400 or 600mm centres and have the joyful task of trimming every plywood board to 2400mm - not so bad with a plunge saw, rail and rail square, but does slow the process down quite a bit

    If you have decent 18mm (or thicker) hardwood plywood for the flooring and your centres are 16in or 400mm then you don't absolutely need the solid strutting or noggins between the joists, at least not to support the intermediate joints, although solid strutting will increase the stiffness of the floor.
     
    Last edited: 20 Oct 2021
  4. Draughtsman

    Draughtsman

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    So the ply still needs a joist every 16" and additional edge support if required would be added to this?
     
  5. JobAndKnock

    JobAndKnock

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    What you are talking should look a bit like this:
    Raised Floor Frame  001.jpg
    The sheets are laid across the short side in the above drawing
    Raised Floor Frame  002.jpg
    Strictly speaking it isn't necessary to support the joints (and if we had to do that on conventional floors I'd go nuts!)

    Whether or not you need the intermediate solid strutting or ends depends on how the framework is fixed down and the cross section of the timber, but the solid strutting (in the middle) really doesn't need to be bang under the joints for 16in centres so long as you support the end to end joints and use sufficient screws. I'd seriously considerglueing to the timber frame and at the edges with a gap filling D4 flooring glue

    One thing to make sure is that your framework ends and the plywood sheets all stop 10mm short of the wall (this gap will be covered by the skirting). This minimimises sound transmission between the floor and wall as well as ensuring that you don't get creaking (ideally) caused by the sub-floor moving under changing loads and rubbing againmst the wall (a real issue which will drive you mad). This gap can be filled with a compressed gap filling tape such as Compraband before the skirtings are applied

    Be aware that rubber pads can compress under load, so you will need to choose a relatively incompressible type to avoid gaps appearing beneath the skirtings over time as the rubber compresses (again, a very real issue with this type of floor structure)
     
    Last edited: 20 Oct 2021
  6. Draughtsman

    Draughtsman

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    Thank you.

    I've seen a few US/Oz forum posts showing ply glued to frame.

    There's a clearance issue I need to create a small shallow step/tray local to the window frame btm, stepping up, to allow all 63x38 CLS sections to be vertical. Snagging. It gets deeper off to the left.

    Build the frames, chock to correct height and see which decking support pedestals are reqd? Or is there a cheaper method? How was it done before decking pedestals arrived on the scene? Pedestal bases can be screwed down.

    How much ply is generally allowable to overhang the outside of the frame?

    I got some of these in, probably made of recycled car tyres, chopped up, wire removed, formed under heat/compression. Quite dense.
    https://www.roofingsuperstore.co.uk...r-base-plate-for-pedestal-200mm-diameter.html

    Conc. base has 2 coats of Sikha Rapid, I'll membrane over it, too.

    DSC03162.JPG
     

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    Last edited: 22 Oct 2021
  7. JobAndKnock

    JobAndKnock

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    Pretty much, yes. TBH if I'm doing a room I set up a laser 1 metre above the required floor level, cut a piece of 2 x 1 or 2 x 2 planed softwood about 1200 to 1300mm long and mark a line around it 1 metre from the bottom. That allows you to set the packing and joists at the right level consistently

    You fixed softwood or plywood packers to the sub floor (SDS, screws and plugs for concrete, just long screws for timber sub-floor) then put two long screws, skewed at an angle from either side, through your joist and into the packings (and the floor as well if it's timber and the packings are thin enough). We did an entire structure (21 large rooms, timber floored, plus 3 landings - 7 former shops on a hill) that way to get level floors throughout for an office conversion about 5 years back. Because the joists were so shallow in some cases pedestals simply weren't viable. The joisting varied from 2 x 2 up to 9 x 2, but we didn't put more than about 70mm of packers under anything

    Thin packings and concrete floors require steel angle brackets to fix the joists to the floor. On large floors provision needs to be made to allow for expansion and contraction, so the angle plates used have slots rather than round holes in them where they tie into the timber joists.

    Try to keep it fairly tight. 50mm is good, 100mm is OK, but beyond that heavy furniture can cause the floor to droop a little. The really important thing is to have a small gap between floor and wall all round
     
    Last edited: 24 Oct 2021
  8. Draughtsman

    Draughtsman

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    Thank you,

    You wouldn't use masonry screws to secure supports to the conc. base? To what depth would you rawlplug/screw?

    What spacings do the frame supports go in at?

    Is a comment missing from your quote of my rubber pads link?

    Dain
     
  9. JobAndKnock

    JobAndKnock

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    Masonry screws are designed to screw into high density concrete, medium to hard brick and stone. They don't work at all well in mortar, low density concrete or screed. They are also expensive in comparison to wood screws and plugs

    You stated that your joisting is 63 x 38 (?). If that is so I'd go for 500 to 600mm centres, no more, as 63 x 38mm stuff isn't the most rigid, especially if it isn't structurally graded

    Quote removed, additional comment (missed) inserted. We've been using similar material to you on a residential conversion (over 100 apartments). It isn't fixed in place at all, if it were the sound proofing effect would be lost, but the joists are full size C16 timber, varying from ex-5 x 2 (120 x 44 finished) up to ex-8 x 2 (190 x 44). On those we are supporting at about 1000mm centres and the structure is tied together with solid strutting and by the flooring (18mm T&G spruce plywood) being glued and screwed. Because the entire structure is glued and screwed together there is no need for it to be tied to the structural floor below
     
    Last edited: 24 Oct 2021
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  11. Draughtsman

    Draughtsman

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    So, unless requested, you wouldn't use rubber pads? In this case noise transmission isn't even an issue.

    Possibly a central (ish) point of fixity reqd? The room is 5mx4m, is this big enough for outlying brackets to need horiz slots?

    Thank you
     
  12. JobAndKnock

    JobAndKnock

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    I'd still go with the rubber "castles" because they offer a degree of thermal separation from the concrete floor. They really don't need fixing down if the joists are tied together with solid strutting and the plywood is glued down
     
  13. Draughtsman

    Draughtsman

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    Is "castle" a generic term for a joist cradle or pedestal? Possibly because some of them resemble a castle on a chessboard?
     
    Last edited: 25 Oct 2021
  14. JobAndKnock

    JobAndKnock

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  15. Draughtsman

    Draughtsman

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  16. JobAndKnock

    JobAndKnock

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    I did a quick search. I couldn't remenber the name of the stuff we've been using (on leave at the moment), but RynoDeck like this is similar:

    RynoDeck 10mm Cradle 1.jpg

    You can see why we call them "castles" now, I hope. The main differences being that the ones we use are a lot deeper - about 10mm - and they are part of a system which includes fixed thickness (10 and 20mm) packers for use beneath the cradle and the manufacturer also supplies 2, 5 and 20mm flat shims which sit inside the tops of the castles

    From the same manufacturer comes this illustration of these in use:

    RynoDeck 10mm Cradle 2.jpg

    They also make a variable height version with a screw adjuster:

    RynoDeck adjustable cradle.jpg
     
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