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    • CommentAuthormarsaday
    • CommentTimeJan 8th 2017
     
    Can i get a bit of help on creating a warm roof for my extension please.

    At the design stage at mo and thinking hard about roof materials to use, plus the main support structure.

    I am going to be making a standard single storey lean to extension. The roof will start under my upper floor window line and fall away from the house at about a 15 degree pitch.

    If i used roof joists/rafters i would pack the insulation between these and lay some on top as well, making 125mm solid insulation. BUT i would like to use oak beams so i get to see these when using the room. This means laying the insulation on top of the beams, plus ply sheeting.

    So it will be oak rafter, then 18mm ply, then 125mm insulation, then 18mm ply, then membrane sheeting.

    After this can i just tile onto the membrane or do i need to use 25mm batons ? So giving me definite lines from which to work and lay the tile. OR can you tile onto the ply and membrane directly. How difficult is this because i imagine you may loose your way a bit.

    Because the pitch is 15 degrees i plan on using a spanish tile which goes down to 15 degrees odd, plus this roof system: http://easyroofs.co.uk

    Any info on the roof detailing would be appreciated.
    • CommentAuthortony
    • CommentTimeJan 8th 2017
     
    No ply needed, no membrane, air/vapour barrier on the warm side of the insulation, insulation between and over the rafters, breather membrane, battens, tiles

    I can get you tiles to go down to 7.5 degrees if needed :)
    • CommentAuthormarsaday
    • CommentTimeJan 8th 2017
     
    Cheers Tony.

    Just been doing some more measurements and definitely below 15 degrees now. The extension at 5.5m = 15 degree pitch, but i need to come out 7m, so pitch likely to be lower.

    Because i am doing the rafters in oak i thought i might not have that much space to fill between the rafter as i want it exposed and seen. If i pack it out with insulation and plaster board i will loose most of the rafter.

    Also don't i need a strong platform on which to work from, hence the ply down. Plus what about maintenance issues later on to clean gutters above and the windows. I will be walking on this area to do all this work.

    I am interested in the tiles at 7.5 degrees, but typing this i might need a different system completely, say fibre glass or zinc.

    So maybe don't need the ply under the insulation BUT don't i need the play above it in order to create a stronger roof. I say this because the window cleaner or myself is going to go up on this roof to clean the windows,
  1.  
    Posted By: marsadayIf i used roof joists/rafters i would pack the insulation between these and lay some on top as well, making 125mm solid insulation. BUT i would like to use oak beams so i get to see these when using the room. This means laying the insulation on top of the beams, plus ply sheeting.

    It sounds like your oak beams will follow the pitch of the roof. Would it not look better if the beams were horizontal and you had a tapered timber filler (I'm sure there is a building term for this) on top of the oak beam to give the pitch (or fall as it sounds like it is almost going to be a flat roof with a 10deg. fall)
    • CommentAuthormarsaday
    • CommentTimeJan 8th 2017
     
    This is what i am doing. The beams will span from left to right, rather than front to back.

    Was going to create my walls on the left and right sides and then sit a timber binder running down the length of each wall. The joist would then sit on this binder.

    I need to speak to an oak frame company to check all this out. The binder will be visible, plus some type of noggin which will sit between each joist. Typing this it sounds like it won't look so good.

    Lots of thinking needed.
  2.  
    By binder I presume you mean wall plate. Both this and the noggin need not be visible, you can set back both from the wall surface enough to make good over them to match the wall surface.

    What will you use for the visible ceiling surface? What about plasterboard on top of the oak joists then
    vcl,
    joists to match the chosen insulation thickness,
    tapered timber filler to give fall (or pitch)
    the roofing material of your choice.

    The plaster board could be replaced with T&G if you wanted a more timbered style.
    • CommentAuthormarsaday
    • CommentTimeJan 8th 2017
     
    Yes wall plate. On the plastered wall side this will not be visible, but i was thinking of doing a feature wall in brick on the opposite side. Any ideas on what happens around the joist area in this instance ?

    The ceiling will be plasterboard in between the joists. VB will lay over the oak timbers and under the first layer of ply.

    I am struggling to see why i would use oak timbers and then use joists above this to match the insulation. I have used furrings before on a flat roof i did and these were placed onto the deck of ply i used. I then ply'ed out over these furrings. These gave me my fall. But for this new build extension i have to build the walls with the correct fall as i don't have room to create another roof with a new fall. I think this is what you are talking about.

    My pitch will be determined by the brick/block walls i will be building. Because the windows above this extension limit how high i can go, i do have a set point from which the new roof must start from.

    Thanks for input.
    • CommentAuthortony
    • CommentTimeJan 9th 2017
     
    You may well be able to tighten things up above the windows/doors on the ground floor to gain a lot more pitch by changing the detail, narrower soffit, notching the rafters to the max over the wall plate, thinning the wall plate, using lower profile beams or lintels, in extreme cases using trimmers in the roof and landing the rafters each side of the openings etc....
    • CommentAuthormarsaday
    • CommentTimeJan 9th 2017
     
    Thanks Tony.

    I have discovered easy slate which allows me to tile down to a 12 degree pitch. What is your tile which goes down to 7.5 degrees. Will find out tomorrow what my pitch will be hopefully.
  3.  
    Posted By: marsadayI am struggling to see why i would use oak timbers and then use joists above this to match the insulation.

    I assumed you would be using a wool type insulation. If you are using a hard board type insulation then the joists above the oak beams would not be needed as the insulation will take the load.

    By the way - why plywood rather than OSB, which would be (much) cheaper

    Will the oak beams be horizontal with the furrings used to give the pitch
    • CommentAuthormarsaday
    • CommentTime7 days ago
     
    Yes OSB.

    I am doing a mono pitched extension, so no furrings. P

    Oak detail is now changing after speaking to architect.
  4.  
    If you're doing a roof as you describe you should have counterbattens beneath the battens to allow run off of any water that does get through and most breathable membranes will need this to satisfy their BBA cert if they aren't going to be draped between rafters by a min of 5mm
    • CommentAuthormarsaday
    • CommentTime6 days ago
     
    Was thinking about this issue and so will see what architect says about it.

    My view on the ply is this, so i may be wrong. At some point in the future i will be on the roof cleaning my windows and gutters out higher up. Do you think the ply will help make the whole roof stronger, or is it wasted expense. I will be using planks to walk on, but the pitch will be about 15 degrees and so i should be able to get on there and work ok.

    There could also be roofing works at some stage of the building life so would ply help if scaffolding was loaded on this roof ?
  5.  
    I don't think that ply will add any significant value over OSB3. If you are using tiles then you will need counter battens and any load will be on the counter battens, which themselves will need support under them. If you are using wool type insulation then some form of joist will be needed to support the battens, if you use hard board insulation then OSB3 will spread the load just a swell as ply. If you don't use tiles but tin or felt or some other flat roofing system then again ply will have no advantage over OSB3

    If later you need scaffolding then I would be removing some tiles and using spreader plates anyway so again no advantage to ply.
    • CommentAuthormarsaday
    • CommentTime6 days ago
     
    Ok, so maybe don't need the ply / OSB. I did mean OSB originally, just put ply as used it for generic term.

    Looking like oak purlins x 4 now.

    Then 125mm rafter laid normally front to back, with insulation of 125mm between rafters.

    Membrane

    Batons

    tile.

    So a more conventional roof.

    Is this roof seen to be warm ? OR does the insulation have to lay over the joists to be warm ?
  6.  
    As I understand it everything has to be below the insulation to be a proper warm roof. So you proposal doesn't qualify. You will have to follow the build up convention for a cold roof.

    125mm is not a lot of insulation about 0.2 U or a worse depending upon the insulation type.

    If it were mine I would have the the (on view) purlins and PB ceiling horizontal.
    • CommentAuthordjh
    • CommentTime6 days ago
     
    Posted By: Peter_in_HungaryIf it were mine I would have the the (on view) purlins and PB ceiling horizontal.

    I'm curious why you prefer a horizontal ceiling. Sloping ceilings are more interesting to my eye - is there some other consideration?
    • CommentAuthormarsaday
    • CommentTime6 days ago
     
    I thought 125mm was normal for new houses and BC. My attic conversion has 125mm celotex.

    What are people aiming for interms of roof insulation. I can add more as the cost is not the issue. My issue is the space i have to create a roof at a decent pitch.

    Purlins will be going from left to right spanning on block walls. The rafters will then be hidden. I was planning on using rafters in oak going left to right, but i need to get some vellum windows in place and so this way if better.

    The ceiling will not be flat, it will follow the pitch contour and so will be more interesting. We will get about 3m of ceiling height next to the back wall of the house and this will fall to about 2m. Still awaiting first drawings.
  7.  
    Most on here reckon 300mm to 400mm of glass wool or equivalent which is about 0.13 to 0.1 U for a roof. The norm (or what the regs require) should be a minimum not a target.
  8.  
    Posted By: djh
    Posted By: Peter_in_HungaryIf it were mine I would have the the (on view) purlins and PB ceiling horizontal.

    I'm curious why you prefer a horizontal ceiling. Sloping ceilings are more interesting to my eye - is there some other consideration?

    is there some other consideration? Nope just preference.
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