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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    I've just ordered a pair of Velux PK10 top hung Windows to go next to each other.

    The existing roof is constructed of 3x2 timbers.

    Double trimmers will be needed to be in-between the 2 Windows. But would I also need double trimmers on the outside and the top and bottom?

    Any thoughts please?
    • CommentTimeJun 11th 2022
    Are you sure you need any trimmers at all between the Veluxes? Their mounting doesn't require a side member. Accessory joining strips and fladshings are available for mounting Veluxes tight together. As to whether doubles are needed anywhere, that's just a structural calculation, for your particular situation.
    Are you sure they don't require a side member? These ones suggest a 40-60mm opening bigger than the frame, and they come with appropriate brackets

    I have a fishing kit for 100mm spacing between the two windows (IE. 75mm X 50mm rafters doubled up)

    I'll take some pictures shortly
    • CommentAuthortony
    • CommentTimeJun 11th 2022
    If Window is nearly a meter wide then you will need a trimmer to carry the middle rafter, and yes two rafters between the windows
    • CommentAuthorrevor
    • CommentTimeJun 11th 2022
    Is it a small roof small span 3x2 seem quite small? What is the rafter spacings if windows are 1m wide then you will need extra doubling up on rafters I reckon. When doing the top and bottom trimmers set the bottom one vertically and the top one horizontally to let more light in even though you only have 3" depth it will help. You can also flare out to the sides if you look closely at the frames you will see the grooves for fitting vapour barrier/ plaster board which are chamfered to aid this. It is surprising how little roofers know about fitting Velux windows my roof trusses were 300 mm deep so I set my horizontal trimmers as described only to be told by my slater I had done them wrong and he had been fitting them for 30 years so he knew what he was talking about!! That is until I showed him the Velux instructions. I have seen Velux TV adverts where they have not followed their own guidance and it evident on many Grand Designs they have not been installed to maximise the light.
    Good tips. The top and bottom trimmers would go on joist hangers so surely they'd need to follow same pitch of the rafters?

    The roof is a traditional terrace circa 1900. Purlin 1/3 up from the eaves

    Existing rafters are approximately 400mm centres, but not quite all the same

    So are we thinking use 3no. Rafters at each end of the two windows, 2 in-between them and then say 2 for the top and bottom on joist hangers?

    Or do you create one big rectangle, and then have rafters in between
    • CommentTimeJun 12th 2022 edited
    Posted By: VictorianecoAre you sure they don't require a side member?
    Can't the brackets be swopped around in the frame slots, to use either side or top/bottom support?

    Whatever the dimensions of a combined opening (for both Veluxes), whether rafter(s) between and/or doubled anywhere is a structural question, not a "... nearly a meter wide then you will need ..." firm rule. Agreed, at 3x2s it might be hard to do without - though the 3x2s are on purlin(s), which might transform them into amply strong rather than spindly members.
    Victorian roofs often seem to have very skinny rafters and chunky purlins, compared to modern roofs. It's stood for 100+ years so far, so clearly this idea works! I think it uses less timber and more labour than modern roofs do.

    When we added insulation the roof ends up much deeper/thicker than the original was, so you can use that extra depth to add chunky modern rafters either side of the opening and across the top and bottom. I think we used 8x2 but not sure. Best to ask a grown-up for help with sizes though, wouldn't rely on what you find on the internet!

    Unless you are lucky, the existing rafter spacing won't be quite right for the dimensions of the Velux, so you are likely to need extra rafter/s anyway.
    • CommentAuthorowlman
    • CommentTimeJun 12th 2022
    It's the rafter depth @ 3" that concerns me.
    Are rooflights being mounted onto upstands?
    Are the rafters being increased to a greater depth internally?
    Is the room habitable?
    How much roof insulation is going to be used and is it enough, what will be the finished roof depth?
    I take revor's good points regarding trimmer mounting angles and light, It's how I did it; but if the 3" rafters are remaining as finished roof depth it hardly matters.
    It's all going to be 3" not sure why you'd mount with upstand?

    Insulation is 2" pir in-between and 3" below
    • CommentAuthorMike1
    • CommentTimeJun 12th 2022 edited
    Posted By: WillInAberdeenVictorian roofs often seem to have very skinny rafters and chunky purlins, compared to modern roofs.
    True - however the Victorian's weren't so concerned about deflection and, arguably, the quality of their timber was better, plus an old roof may well have had its light-weight Welsh slates switched to heavier concrete tiles.

    And a pair of PK10s next to each other is quite a lot of glazing.

    If the roof is well built and the rafters are showing little defection, then rules of thumb may be adequate. If not, it would be wise to consult a structural engineer.
    • CommentAuthorborpin
    • CommentTimeJun 13th 2022
    Posted By: VictorianecoVelux PK10
    Triple Glazed we hope.... :bigsmile:
    • CommentAuthorborpin
    • CommentTimeJun 13th 2022
    For veluxes, I had the top and bottom trimmers set so the rebate can be horizontal/vertical rather than perpendicular to the window. Not everyone does it that way for some reason.
    All in, doubled up the middle rafters, trebled up the outside ones. Nice and secure

    Almost broke my nose though... What's release the spring tension before trying to remove the glass from the frame
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