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General: timber frame roof noggin advice

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1 to 18 of 18

  1.  
    • CommentAuthorthebeacon
    • CommentTimeDec 28th 2022 edited
     
    Hi all,

    I have drawn up the framework for my 4x3.5m warm roof garden room. I plan to leave the roof joists unclad internally, partly to save on material and partly to give the illusion of slightly more head height.

    1. How many sets of noggins would you recommend for the roof, and should they be placed central to the walls or central to the overall roof joist length?

    The span will be 3.5m, although the roof overhangs at the front and back, making the roof joist length 4.3m. I am using 47 x 175mm C24 Pressure Treated Timbers for the joists.

    2. Would you recommend making the noggins smaller in depth, for example, using 47x150mm and having a gap between the top of the noggin and the roof deck to allow airflow?

    3. Where the roof joists attach to the top plate of the walls, would you suggest using truss clips (https://www.screwfix.com/p/sabrefix-truss-clips-galvanised-95mm-x-50mm-20-pack/72002) or angle brackets (https://www.screwfix.com/p/sabrefix-heavy-duty-angle-brackets-galvanised-90-x-63mm-25-pack/69361) on each side of a roof joist

    i have uploaded an image to show the design so far.

    Thanks in advance,
      front-view small.jpg
    • CommentAuthortony
    • CommentTimeDec 28th 2022
     
    One row of structural noggins across he middle foe me, no truss clips , wafer screws from underneath
    • CommentAuthorcjard
    • CommentTimeDec 29th 2022
     
    Not sure I'd bother if you're putting staggered boards on the roof deck
    • CommentAuthorCoverley
    • CommentTimeDec 29th 2022
     
    Normal rule of thumb is noggins at max 2.4m which suggests you need one line at midpspan. However it is garden room so reasonable to say you'll not do that because you can opt for less onerous criteria than a house. You will still need to block between rafters over the wall plates.

    Also check the roof is tied roof down! The roof will have uplift in a gale so you need continuous load path into the walls. Either straps from the rafters to studs, or make sure there are robust connections from rafter to wall plate to studs.

    ps - I'm not sure if 'blocks" or 'blocking' is the right phrase? In my mind it's when a piece of timber the same size as the rafter/joist used to close the ends rather like a noggin. It stops the rafters falling over during construction and prevents a weaker mode of failure on the minor axis. See "lateral stability" at the bottom of this link https://www.roymech.co.uk/Useful_Tables/Timber/Timber_Strength_Calcs.html
    • CommentAuthorrevor
    • CommentTimeDec 30th 2022
     
    Posted By: cjardNot sure I'd bother if you're putting staggered boards on the roof deck


    I agree the boards will give you the rigidity you require. However it may be prudent to fix central noggins if your joists are not straight accurately cut noggins will help to pull up the joists so that timbers are all central and so are where you expect them to be when you fix the cover boards. Alternatively you could use herringbone struts if you are concerned about air flow.
    • CommentAuthorthebeacon
    • CommentTimeDec 30th 2022
     
    Thanks for your replies.

    Is it ok not to place the blocking over the top plate? Ideally, I would like to insulate that space with 90mm pir board.

    I was thinking of putting the blocking just off the top plate, use these angle brackets to attach the rafters to the top plate (https://www.screwfix.com/p/sabrefix-heavy-duty-angle-brackets-galvanised-90-x-63mm-25-pack/69361) and then insulate in the gap between the rafters and top plate.

    I have mocked up an example, highlighted in blue, in the image.
      small-blocking.jpg
    • CommentAuthorcjard
    • CommentTimeDec 30th 2022
     
    Will the walls be built, insulated and clad before the roof joists go on? If not, I'd just screw into the joists from below, through the plate, with something like turbo coach screws

    https://www.screwfix.com/p/turbocoach-hex-flange-coach-screws-m6-x-90mm-500-pack/2793x
    • CommentAuthorCoverley
    • CommentTimeDec 31st 2022
     
    It'll be OK to offset the blocking so your insulation is in one plane. Just think about how your cladding line.

    Metal truss clips in the same plane as the insulation will be a lovely cold bridge and condensation magnet. As tony and cjard say, use screws to connect wood to wood.

    Screws will be more than strong enough bearing mind most traditional rafter roofs are held together with ring shank nails. Unless your precise with your hammr you'll find screws easier to position the timbers without them getting knocked out of alignment as you hammer in nails.

    I use Screwfix turbogold because they are lethally sharp and will glide through the timber with an impact driver. They only have a smooth shank for 100mm and above but you'll need that sort of length anyway for skew nailing the rafters to the studs.

    I still think you need a wedge or birdsmouth under the rafters on the wall plate
    • CommentAuthorthebeacon
    • CommentTimeDec 31st 2022 edited
     
    Posted By: CoverleyMetal truss clips in the same plane as the insulation will be a lovely cold bridge and condensation magnet.
    Thanks, this is good to know.

    The walls won't be insulated before i put the roof joists on, but I will have put the osb3 sheathing on the outside. I won't be able to screw the roof joists in from under the top plate because they will be sat directly over the stud joists. That's why I was thinking of using the angle brackets. Would truss clips (https://www.screwfix.com/p/sabrefix-truss-clips-galvanised-95mm-x-50mm-20-pack/72002) result in the same problem? Would using a thermal break tape help?

    Posted By: CoverleyI still think you need a wedge or birdsmouth under the rafters on the wall plate
    Yes, I have now drawn cutting in very shallow birdsmouths under the rafters so they sit flush on the top plate.
    • CommentAuthorCoverley
    • CommentTimeDec 31st 2022
     
    Just skew screw, like skew nails but with screws. You want to visualise a screw going in at 45 degrees on each side of the rafter.

    If you have 45 wide rafters then I'd get a 120mm turbogold screw. Hypotenuse of a 45mm isosceles triangle is angle is 56mm so 120mm should give you roughly 50mm embedding in the wall plate below.

    Hold the screw perp to the rafter side at roughly 40mm up, give it a few turns with an impact driver until it bites maybe 3 to 5mm into the timber, then move the screw to 45 degrees and drill through. That initial bite avoids the screw skidding along the surface if you try to go in at 45 from the start.
      toe-nail.jpg
    • CommentAuthortony
    • CommentTimeDec 31st 2022
     
    Or screw up from underneath
    • CommentAuthorthebeacon
    • CommentTimeJan 2nd 2023
     
    Posted By: CoverleyJust skew screw, like skew nails but with screws. You want to visualise a screw going in at 45 degrees on each side of the rafter.


    Thank you for this advice. Will screw the rafters to the top plate using this technique
    •  
      CommentAuthordjh
    • CommentTimeJan 20th 2023
     
    The side overhangs need to be able to support a person standing on that edge. I'm not sure the short connections to the next joist will do that adequately?
    • CommentAuthorcjard
    • CommentTimeJan 22nd 2023
     
    > need to be able to support a person standing on that edge

    Can't see where anyone said it was going to be a balcony too?
    •  
      CommentAuthordjh
    • CommentTimeJan 22nd 2023
     
    Posted By: cjard> need to be able to support a person standing on that edge

    Can't see where anyone said it was going to be a balcony too?
    It's not a balcony. It's in case somebody accesses the roof for some maintenance purpose, AIUI.
    • CommentAuthorcjard
    • CommentTimeJan 24th 2023
     
    I don't think there will be a problem with it if constructed as detailed
    • CommentAuthorthebeacon
    • CommentTimeJan 31st 2023 edited
     
    Hi everyone,

    I have been looking more closely at roof insulation and waterproofing and came across Kingspan QuadCore Insulated Composite Panels KS1000RW (https://www.steelroofsheets.co.uk/products/kingspan-ks1000rw-quadcore-insulated-composite-panels/). They are Kingspan insulation with a painted metal sheet. They are structural. It could make installing the insulation and waterproofing the roof a quicker and simper job.

    Although they are aimed at industrial use, they have been installed on other garden rooms: https://www.youtube.com/watch?v=VXns2_C2ScI&list=PLjE4kW4I9DKUxeXWQPinMmyrV4EPlo1TL&index=7

    I am considering two options in which they could be installed:

    Option 1:
    140mm Kingspan panel
    Wall top plate

    Sit the Kingspan QuadCore Insulated Composite Panels directly onto the top plate of the walls and fix it in. From looking at their span table, I wouldn’t need joists. This would mean the underside of the panel would be the internal ceiling.

    Option 2:

    140mm Kingspan panel (or slightly less)
    Vapour barrier
    11mm osb
    100mm joist
    Wall top plate

    With option 2 I would still put joists over the roof but 100mm now rather than 150mm, with 11mm osb covering it. This would be because I would prefer a wooden ceiling compared to the underside of the Kingspan QuadCore. The roof joists would only need to take the load of the osb as the quadcore panels would take their own weight.

    Does anyone have experience with these types of insulated panels? Or can see glaring oversights in my thinking?

    Thanks,
    •  
      CommentAuthordjh
    • CommentTimeJan 31st 2023
     
    Posted By: thebeaconWith option 2 I would still put joists over the roof but 100mm now rather than 150mm, with 11mm osb covering it. This would be because I would prefer a wooden ceiling compared to the underside of the Kingspan QuadCore. The roof joists would only need to take the load of the osb as the quadcore panels would take their own weight.
    Sounds like a lot of effort to get a surface finish. Presumably you could just paint the underside of the roof panel? Or maybe glue whatever surface veneer or suchlike to it?

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