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General: direction of floor and roof joists on garden room build

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  1.  
    • CommentAuthorthebeacon
    • CommentTimeAug 7th 2022
     
    Hello,

    I'm planning on building a garden room and currently drawing up a design. I will have ground screw foundations installed and have asked the company also to build the subframe platform using 45x150mm joists. The garden room will be 4x3.5 metres, there will be 12 screws, and the front door will be on the 4m wall. The 4m wall will also be the highest point of the sloping roof. The roof slops from 4m wall to 4m walls.

    The ground screw company sent me a plan of the timber subframe (see attached image), It's not included in the plan, but the parimeter will be double wrapped in joists. The majority of the joists run from the 3.5m wall to the other 3.5m wall. When I look at building designs, the floor joists run parallel with the roof joists. Is there a reason why the floor and roof joists should run parallel? Would it be ok to have them running perpendicular?

    Thanks in advance,
    Tom
      4000 x 3500 subframe.png
  2.  
    Tom, in our house the floor, ceiling and roof joists run at 90⁰ to each other.

    The drawing doesn't show where your screw piles go, but I guess there are 4 down each of the long edges, and 4 more across the middle of the building? Those middle ones will provide a mid-point support to the primary (doubled) joists, so those only have to span halfway across the building, 1.7m. Then the secondary (single) joists only infill between the primary joists, each spanning 1.3m. Those short span lengths are how you get away with relatively skinny joists without sagging.

    The ceiling has no midpoint support, so the joists will likely span the shortest direction. They’ll likely all run wall-to-wall so they all share out the load, rather than transfering it into primary joists.

    Can't remember if we previously discussed how to fit enough floor insulation into the thickness of the joists layer - what's your thinking around this?
  3.  
    I suspect the screw piles will be in the position as per WinA thoughts.
    In that case the 3500 joists will be supporting joists for the 1335 joists. IMO the 1335 need only be 45 x 100 but I suspect that it will all be constructed from 45 x 150 for ease of construction. If the ground screw co. are providing the subframe then I suspect the units would be made off site and arrive ready to place on the ground screws.

    Unfortunately the dimensions do not lend themselves easily to sheets of OSB which are 1250 x 2500 however for floor insulation I would put OSB on the underside of the floor (I suspect that the flooring will be constructed in 3 discrete units then placed on the piles) as it is being constructed, that is OSB screwed to the to be underside of the joists the units then flipped over and fixed to the piles. Insulation can then be put in to the space created and then the floor placed on top.

    It might be worth adjusting the short joists so that multiples of 1250 mm are obtained although that may already have been done because I see the top most short joist spacing is smaller than the rest. Check the expected dimensions.

    There is no need to have the floor joists and the roof joists in the same direction.
    •  
      CommentAuthordjh
    • CommentTimeAug 7th 2022
     
    Typically joists run in whatever direction allows them to be shortest. Often that means the floor and ceiling joists run in the same direction. But there's no necessity for that. Rafters normally run up and down the slope of the roof.
  5.  
    <blockquote><cite>Posted By: djh</cite> Rafters normally run up and down the slope of the roof.</blockquote>

    Isn't that so that the slate battens can run at 90 degrees, ie in the direction of the slate courses?

    They could I suppose run along the top of each rafter, but that would for all time dictate the size of slate that could be used, and probably also that the slates had to have an even spacing rather than reducing in size the higher up the roof.
  6.  
    Posted By: Cliff Pope
    Posted By: djh Rafters normally run up and down the slope of the roof.


    Isn't that so that the slate battens can run at 90 degrees, ie in the direction of the slate courses?

    They could I suppose run along the top of each rafter, but that would for all time dictate the size of slate that could be used, and probably also that the slates had to have an even spacing rather than reducing in size the higher up the roof.

    only if you are using slates.
    This is a garden room with a single pitch roof with probably a shallow pitch so fit the rafters in any direction to best suit the roof type.
    • CommentAuthorthebeacon
    • CommentTimeAug 8th 2022
     
    Hi,

    Thank you for your feedback and comments. Its greatly appreciated.

    Posted By: WillInAberdeenin our house the floor, ceiling and roof joists run at 90⁰ to each other.
    Thats great to know.

    The screws run as you have described. Attached is a plan.

    Posted By: Peter_in_HungaryIt might be worth adjusting the short joists so that multiples of 1250 mm are obtained although that may already have been done because I see the top most short joist spacing is smaller than the rest. Check the expected dimensions.
    The centres of the smallest joists are at 400, so I believe it will work with fitting the OSB sheets, but I will check with the ground screw people. They might need to be adjusted to take into effect the double-wrapped perimeter joist that's not currently on the plans.

    Posted By: WillInAberdeenCan't remember if we previously discussed how to fit enough floor insulation into the thickness of the joists layer - what's your thinking around this?
    I don't think we did talk about how to install the insulation. I was planning on attaching battens into the joists to sit at least 100mm pir insulation board on.
      4000 x 3500 Screw Layout.png
  8.  
    Just a thought, before you change any joist centres. I'm not familiar with 2500x1250 sheets of anything in the UK. Any OSB I've ever used in building TF houses is 2400x1200. Sometimes plywood comes in 2440x1220. Maybe it's a European thing, with the UK still stuck with imperial conversions. 4.8m lengths of 6"x2" :confused:
  9.  
    Posted By: GreenPaddyJust a thought, before you change any joist centres. I'm not familiar with 2500x1250 sheets of anything in the UK.

    You could be right about UK dimensions. best check local availability.
    •  
      CommentAuthordjh
    • CommentTimeAug 9th 2022
     
    Seems a strange set of dimensions for a UK building. Sheet materials would fit best with the long axis horizontal on plan, then six sheets all trimmed the same would work, or six sheets all trimmed to the same length with two further trimmed for narrower width. But the supports aren't laid out for those plans?
  11.  
    I suspect that the building size was not chosen for easy fit, waste free, minimum cutting of sheet boards.

    BTW our OSB is 1250mm x 250mm or 600mm x 2500mm for T&G flooring OSB and our plaster board is 1200mm x 2000mm How does that compare to the UK standard sizes ?
    • CommentAuthorthebeacon
    • CommentTimeAug 10th 2022
     
    Posted By: djhSeems a strange set of dimensions for a UK building



    Posted By: Peter_in_HungaryI suspect that the building size was not chosen for easy fit, waste free, minimum cutting of sheet boards.

    You are both correct, i designed it for what i could fit in my garden.

    Posted By: djhheet materials would fit best with the long axis horizontal the on plan, then six sheets all trimmed the same would work
    i have drawn a rough plan for the boards, laying the longest side on the horizontal axis. The board edges are in green pencil, the framework is in grey pencil. Where the board edges meet (i marked the green line with an X), should i put in joists running from the X to X for the board edges to land on?


    Posted By: Peter_in_HungaryBTW our OSB is 1250mm x 250mm or 600mm x 2500mm for T&G flooring OSB and our plaster board is 1200mm x 2000mm How does that compare to the UK standard sizes ?
    UK standard square edge is 2440x1220mm, UK T+G is 2400x600mm.
      IMG_8289.jpg
    • CommentAuthorGreenPaddy
    • CommentTimeAug 10th 2022
     
    PiH, we're still tied to 8ftx4ft, or chunks of 2ft (600).

    So, almost all sheet 2400 long, then can vary in width 300,400, 600, 1200. Plasterboard does come in longer options, I think 2700, and maybe 3000? Floor to ceiling is usually 2400, hence plasterboard being 2400. 2000 plasterboard would be no use, unless the boards were run horizontally. The UK building industry would go into meltdown, if you suggested horizontal boards - way too openminded.

    To be fair, with taper edged p/brd, for tape and fill, vert boards mean you tend to get very few butt joints, which are more work to disguise (ceilings excepted).
    • CommentAuthorGreenPaddy
    • CommentTimeAug 10th 2022
     
    thebeacon,

    yes, you need timber to catch the board edges, a row of noggins (dwangs north of Hadrain's wall) at point X-X.

    For the timber frame, your dims actually work out not badly, versus waste. Assuming 4.8m timber, then 3.5m leaves 1.3m, and your cross timbers will be less than 1.3m. The 4m dimension leaves 0.8m, which will do for the row of noggins at 0.4m.

    I would personally use T&G moisture resist chipboard for the floor, which are 2.4x0.6, run parallel to the 3.5 sides. However, if you use 2.4x1.2, then I would brick bond it, so start 2.4 sheet bottom left, then 2.4 sheet mid right. Just install the noggins at the board edge position. Lots of ways to skin a cat - apologies to cat lovers :shocked:
  15.  
    Posted By: GreenPaddyyes, you need timber to catch the board edges, a row of noggins (

    +1
    Unless you use t&g chipboard when noggins are not needed (but get the long edge on a joist)


    Posted By: GreenPaddyI would personally use T&G moisture resist chipboard for the floor,

    Would work, chipboard tends to work out cheaper than OSB although square edge OSB 3 is cheaper than t&g chipboard but for a garden room I would go for OSB3 as being better at resisting moisture / water. Chipboard has a smoother finish but both will abrade over time. (will there be a floor covering on top of the sheet material?)

    To match up the sheet joints you could move the joist closest to the X-X line 230mm to match the sheet joint (and perhaps adjust the other joist 150mm to be midway) Note a 47 X 97 joist at 400 centres will span 1.9M in a domestic building so unless you are putting in machinery the loading should be OK.
    • CommentAuthorSimonD
    • CommentTimeAug 10th 2022
     
    You can buy 3050 x 1525 plywood sheets. Available at most good timber merchants. 8ft x 4ft plywood sheets are technically 2400 x 1200 sheets as you allow for trimming factory edges.
  17.  
    Except that plywood is usually silly money compared to chipboard or OSB
    • CommentAuthorSimonD
    • CommentTimeAug 10th 2022
     
    Posted By: Peter_in_HungaryExcept that plywood is usually silly money compared to chipboard or OSB


    I can get B/BB exterior grade plywood at a better price than osb3 right now. But prices have been coming back down again. Birch plywood is the real stinger right now.
    • CommentAuthorthebeacon
    • CommentTimeAug 11th 2022
     
    Posted By: GreenPaddyHowever, if you use 2.4x1.2, then I would brick bond it, so start 2.4 sheet bottom left, then 2.4 sheet mid right.
    that's a great suggestion, thanks. If using square edge OSB3, in addition to screwing to the timber frame, would you suggest using glue at the edges of the boards?

    Posted By: Peter_in_Hungary(will there be a floor covering on top of the sheet material?)
    Yes i will lay a floor covering. As it will be a workshop/studio, it will most likely be a sheet material such as ply.

    When installing the PIR insulation between the joists, would you recommend taping the edges of the pir to the joists
    , with foil tape? I will be installing a vapour control membrane between the insulation and the OSB subfloor.
    • CommentAuthorJeff B
    • CommentTimeAug 11th 2022
     
    Posted By: thebeacon
    Posted By: GreenPaddyHowever, if you use 2.4x1.2, then I would brick bond it, so start 2.4 sheet bottom left, then 2.4 sheet mid right.
    that's a great suggestion, thanks. If using square edge OSB3, in addition to screwing to the timber frame, would you suggest using glue at the edges of the boards?

    Posted By: Peter_in_Hungary(will there be a floor covering on top of the sheet material?)
    Yes i will lay a floor covering. As it will be a workshop/studio, it will most likely be a sheet material such as ply.

    When installing the PIR insulation between the joists, would you recommend taping the edges of the pir to the joists
    , with foil tape? I will be installing a vapour control membrane between the insulation and the OSB subfloor.


    1. Re: glue on the edges - I wouldn't bother personally but others may have a different opinion.
    2. Re: PIR insulation. As you intend to use 100mm sheets and you have 150mm joists I would fit the insulation so that it is a few mm's below the top of the joists rather than flush and use low expansion foam to seal the gaps. This way you do not need to cut the insulation to ensure a perfect friction fit between the joists. I have plenty of experience with this!
    • CommentAuthorEd Davies
    • CommentTimeAug 16th 2022 edited
     
    Posted By: Peter_in_HungaryUnless you use t&g chipboard when noggins are not needed (but get the long edge on a joist)
    The CaberFloor manufacturer shows the long edges not supported in their own illustration at the top of the page:

    https://uk.westfraser.com/our-products/caberfloor/caberfloor-p5/

    But their FAQs says the short edge should be supported:

    https://uk.westfraser.com/resources/help-advice/faqs/

    I’m using 22mm CaberFloor T&G with 450mm joist centres – is it OK if the short edge overhangs the joist?
    No – all short ends must be supported by a joist.
    This makes sense to me; long edges are strung across multiple joists and so are, sort of, in tension whereas short edges are only supported by the sheets on each side and can easily pull out so will be a lot more squishy.
  22.  
    Posted By: Ed DaviesPosted By: Peter_in_HungaryUnless you use t&g chipboard when noggins are not needed (but get the long edge on a joist)

    When I made that comment I was assuming that the long edges would be along the joist. When I have put in t&g chip board long edge across the (400 centres) joists I have not used any additional supports but the chipboard was laid brick fashion so no in line short edges.
  23.  
    Posted By: Ed Davieslong edges are strung across multiple joists and so are, sort of, in tension whereas short edges are only supported by the sheets on each side and can easily pull out so will be a lot more squishy

    That's right, the 2400x600 boards are stiffest if they span perpendicular across several joists (so with an end joint landing on every 4th or 6th joist). Rather than running parallel to the joists, with an edge joint along every joist. Or with unsupported ends.

    The point where the floor runs over a joist, is a point of concentrated bending moment (like snapping a stick by bending it over your knee). If there is a t&g joint at that same point, that joint design cannot resist bending so well. So the sheet bends more and sags deeper inbetween the joists. We'd prefer to only have those weak joints at every 6th joist.


    The mechanics for a sheet running perpendicular over many joists, are similar to the equation for a beam "fixed" at both ends:

    Sag = A

    (Where A = loading x span^4 x elasticity x thickness^3 x some constants)

    If running parallel to joists with the long edges along two neighbouring joists, it's shape is similar to a "supported" beam:

    Sag = 5 x A

    And if the short end stuck out unsupported, "cantilevered", it could be even saggier:

    Sag = 48 x B (B is ≤ A because it sticks out ≤ the whole span)
  24.  
    Posted By: thebeacon
    Posted By: WillInAberdeenCan't remember if we previously discussed how to fit enough floor insulation into the thickness of the joists layer - what's your thinking around this?
    I don't think we did talk about how to install the insulation. I was planning on attaching battens into the joists to sit at least 100mm pir insulation board on.

    Sounds good. As an alternative, you have enough depth of framing to get the same insulation value by fully filling with 0.032 glass wool, which might be easier/cheaper and with lower embodied carbon. There are advantages to doing it either way.

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