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Building Regulations and Standards: hybrid roof (XPS)

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    • CommentAuthorNovice1
    • CommentTimeSep 4th 2012
     
    In the interest of up-cycling and cost I have access to v. cheap 140mm Roofmate SL-A (Styrofoam XPS - usually used in inverted roofs). We have planned a hybrid mansard roof.

    From outside
    1. Tata courus uban seam (standing seam metal roof) onto
    2. Breather membrane (tata does not specify a void as they want the metal roof fully supported by the 18mm ply)
    3. 18mm ply,
    4. 140mm Roofmate SL-A on
    5. 9mm ply onto
    6. 175x47mm rafters using silicone and screws All fixed with contersunk phillips head wood screws
    7. VCL layer fiited from below. Edges taped and then secured/wedged in place in between rafters with tight fitting 140mm Roofmate SL-A
    8. 12mm Plaster board secured with screws and possibly a bead of silicone to seal the holes where the plaster board screws puncture the VCL as they go into the rafters

    This gives 280mm insulation in total and I'm assuming as the xps is denser than the EPS better sound abatment (we live near a busy road)

    I have tried running this through Build desk U but I'm struggling either because of lack of talent or because it is such an unusual build-up, I'm stuggling to get the components from Build Desk U picking list. I have spoken to Dow technical department, who don't see problem in using a product intended for inverted roofs in a ply sandwich with a metal roof on top.. as long as VCL in place.

    Questions
    1. Will the VCL eliminate the condensation risk of replacing the EPS with XPS
    2. Can I do without a void?
    3. Is the VCL in the best position
    4. In the breather membrane in the best position
    5. How would you improve the build-up

    6. Can I use the 140mm Roofmate SL-A as EWI onto a rendered blockwork cavity wall? As far as I know XPS is used as EWI on the continent.

    All critic and advice welcome.
    Cheers
  2.  
    1. Will the VCL eliminate the condensation risk of replacing the EPS with XPS

    EPS or XPS should make little pratical difference, there might still be a slight risk over the winter, but probably will be a net -ve over the year.

    2. Can I do without a void?

    most standing seams dont have one, and you're on the warm side of a hybrid roof anyway.

    3. Is the VCL in the best position

    I'd put it under the "inner" roofmate, directly behind the plasterboard, with service void if needed.

    4. In the breather membrane in the best position

    It's just temporary weather protection prior to standing seam install, then a slip sheet after, so yeah. It wont breathe once it has steel on top in any case.

    5. How would you improve the build-up

    As long as your VCL is decent (i'd maybe use a foil based one as belt and braces) and is fully sealed and not damaged it looks ok to me. Might raise an eyebrow with an overzealous BCO though, and can't say how it would affect product warranties on the steel (if thats relevant)

    AFAIK, the EWI XPS boards have different facings to key the render on, while Roofmate-SL has a skin surface for water resistance in inv. roofs. So you might have issues getting render to stick properly to it. If you've got plenty spare SL, then fixing ply and/or metal lath over the SL might solve that, but probably at your own risk as wouldn't be tested/bba'd/guaranteed etc
  3.  
    How will you fix the 18mm plywood to the rafters through 140mm of insulation? Helical shank fixings require a lot more than 18mm of timber at the top end. If you're going to use screw fixings then you need to make sure they have an appropriate shear rating.

    David
    • CommentAuthorNovice1
    • CommentTimeSep 4th 2012
     
    @Graeme
    Brilliant!! Thanks for that.. all makes sense

    @David
    as usual impressed with attention to detail
    1. Do you have any fixings in mind that you would use?
    2. Could I supplement with a contact adhesive to stick the 9mm ply to the outer 140mm xps and also to the 18mm ply? ie. a DIY SIPs panel.. Chould help with uplift and sheer?
    3. Would a mushroom insulation fixings help eg. http://www.rjfixings.com/mushroom-insulation-fixing-35x50.html

    Thanks
  5.  
    I would use something like the 200mm Heco Topix flange head fixings, but check with a structural engineer & whether you need stainless steel fixings.

    David
    • CommentAuthorTimber
    • CommentTimeSep 4th 2012
     
    Use decent plywood!

    I persoanally would introduce some ventillation in there too by fitting battens ontop of the outer insulation before the 18mm ply, breather membrane and metal roofing.
    • CommentAuthorNovice1
    • CommentTimeSep 4th 2012
     
    Thanks David. I will chase structural engineered

    @Timber
    Thanks for advice. My timber merchant says that exterior shuttering ply has the same water resistance as most other exterior grade ply and that even when using marine ply he advises a coating of proton wood preserver to the exposed face and the edges. Is that right? What would you use?

    I take it that the 200mm fixings would fix the battens through to the rafters and then the 18mm ply could be glued and screwed either into the battens or again through to the rafters?

    Does introducing that void increase the risk of wind uplift?

    @Graeme
    In your experience have you ever had any trouble or advice wrt standing seam roofs onto ply (without any void)?
    Cheers
  8.  
    I wouldn't use a vapour control layer on the inside in your case, its a bit better to have it but I think its overkill. At least half of your insulation is outside the roof structure so the rafters will never be cold enough for condensation to occur. Seal the perimeter of the plasterboard with silicone might be a cheaper way to create a secondary airtight layer. Air-tightness is the key, don't worry so much about diffusion.
    If you are fitting spotlights in the ceiling then I might change the above opinion, but they would introduce the melting risk of a membrane, so in that case I might suggest compartmentalizing the area around the spots.

    If you triangulate the timbers and battens at the soffit you could reduce the shear effect, you could also fix the battens using screws perpendicular to the roof and alternative screws 20 degrees off perpendicular with the point facing up the roof.
    • CommentAuthorNovice1
    • CommentTimeSep 4th 2012
     
    Thanks Viking. What do you mean by  triangulate the timbers and battens at the soffit ?

    Would you suggest as Timber has of introducing a void and is there a risk of wind uplift with standing seam metal shot?

    Thanks
    • CommentAuthorTimber
    • CommentTimeSep 4th 2012
     
    Be careful what plywood you buy. WBP doesn't exist as a standard anymore (although a lot of places sell 'WBP Plywood').

    You are looking for a plywood with a Class 3 glue bond to EN636. I would advise buying a good quality plywood from a well know manufacturer (like finforest or similar).

    Shuttering ply is likely to have lots of voids in the laminations and won't be as good as a structural ply with a class 3 bond.

    A brush applied perservative won't do much in the long run, but sealing the edges with an edge sealing product might help (but not totally necessary).

    I don't think that the void would increase the chance of uplift if vented at eaves and ridge, but it will definately help with the long term durability of the plywood below the metal roofing.
    • CommentAuthorNovice1
    • CommentTimeSep 5th 2012
     
    Thanks Timber. All makes sense. Will definately take that on board.. saves worry for not alot of extra outlay.
    Cheers
  12.  
    Tyvek Metal is a breathable membrane specifically developed for standing seam roofs. It claims to allow condensation forming on the underside of the metal sheet to drain away and appears not to require an air gap. No idea if it work though as in the end a metal roof proved too expensive for our project.
    • CommentAuthorNovice1
    • CommentTimeSep 5th 2012 edited
     
    Thanks Cav8andrew... nice to know I have options if BCO becomes difficult.. did you compare its price v. 'standard' breather membrane?
  14.  
    Sorry, no, the initial costing for the metal element of the roof were, in our case, so prohibitive that we went no further.
    • CommentAuthorTimber
    • CommentTimeSep 5th 2012
     
    Cav8 - I know the product. It is essentially a breather membrane with a furry plastic fibre chest wig on the outer face. This 'wig' creates a small gap between the membrane and the metal roofing. It would certainly be preferable to a standard membrane, but I honestly don't know how much by.

    As always it becomes a pricing exercise!
    • CommentAuthorborpin
    • CommentTimeSep 5th 2012
     
    Posted By: TimberBe careful what plywood you buy. WBP doesn't exist as a standard anymore (although a lot of places sell 'WBP Plywood').

    You are looking for a plywood with a Class 3 glue bond to EN636. I would advise buying a good quality plywood from a well know manufacturer (like finforest or similar).
    Ah just the information I needed!
    •  
      CommentAuthordjh
    • CommentTimeSep 6th 2012
     
    Posted By: Cav8andrewNo idea if it work though as in the end a metal roof proved too expensive for our project.

    What kind of cost are metal roofs?
    • CommentAuthorborpin
    • CommentTimeSep 6th 2012
     
    Posted By: borpin
    Posted By: TimberBe careful what plywood you buy. WBP doesn't exist as a standard anymore (although a lot of places sell 'WBP Plywood').

    You are looking for a plywood with a Class 3 glue bond to EN636. I would advise buying a good quality plywood from a well know manufacturer (like finforest or similar).
    Ah just the information I needed!
    Again thanks for this. I very nearly bought 'Marine Ply' but asking for this precise thing gave me Malayan Ply to the right standard for 2/3 cost of Marine.
    • CommentAuthorborpin
    • CommentTimeSep 6th 2012
     
    I also fond this on ply which explained the differences nicely http://www.tradewood.co.uk/media/c345d690feca47abb74ee560931b1ecbPlywood%20Standards.pdf
    •  
      CommentAuthordjh
    • CommentTimeSep 6th 2012
     
    Posted By: borpinI very nearly bought 'Marine Ply' but asking for this precise thing gave me Malayan Ply to the right standard for 2/3 cost of Marine.

    Is it FSC certified? Malaysia's a nice place but there are some wild frontiers nearby.
    • CommentAuthorborpin
    • CommentTimeSep 6th 2012 edited
     
    Posted By: djh
    Posted By: borpinI very nearly bought 'Marine Ply' but asking for this precise thing gave me Malayan Ply to the right standard for 2/3 cost of Marine.

    Is it FSC certified? Malaysia's a nice place but there are some wild frontiers nearby.
    I'd guess so as it is from a reputable local merchant.
  22.  
    djh, this may be totally unrepresentative but I was quoted £100 m2, (pre weathered zinc sheet) which on a 288 m2 area was beyond my budget. I have chosen to go with a Nulok slate roof which was somewhere near half the cost and gave me the option to self install, if finances required. As I say I did not investigate further (so not a very comprehensive overview) as I could not imagine making the kind of savings I would require to make viable.
    • CommentAuthorNovice1
    • CommentTimeSep 7th 2012
     
    @Cav
    Tata courus urban seam £15/m2 supplied and you can become certified to install for free in a day. 40 year guaruntee on material.

    @Viking.
    What do you mean by  triangulate the timbers and battens at the soffit ? 

    Have decided on belt and braces ie. Void + best breather + vcl + vikings silicone to edges of inner layer of ply etc

    Any one soured or priced
    A. tyvek metal breather membrane? I need 300m2
    B. Best price EN636 Class 3 ply. I need no. 153 18mm sheets?
    C. 200mm Heco Topix flange head fixings?

    Cheers
    • CommentAuthorCav8andrew
    • CommentTimeSep 7th 2012 edited
     
    Novice1, unfortunately planners would not go with coated steel only a surface that would weather/patinate and as it's over 8m in length and 1.1/2 storey high I cant just do what the hell I want !
    Cant see how you would install this with a ventilation gap on battens,It appears to need to sit directly onto a solid base, or have I misunderstood.
    Yes I misunderstood, air gap under 18mm ply was being proposed.
  25.  
    •  
      CommentAuthordjh
    • CommentTimeSep 7th 2012
     
    Thanks cav8, novice1. I need to try not to get caught in that trap!
    • CommentAuthorNovice1
    • CommentTimeDec 3rd 2012
     
    Update and Upping the spec
    Just thought I'd get some additional thoughts
    My current build-up on our mansard roof is;-
    From outside
    1. Tata courus uban seam (standing seam metal roof) onto
    2. Breather membrane (tata does not specify a void as they want the metal roof fully supported by the 18mm ply)
    3. 18mm ply with 25mm gap underneath,
    4. 140mm Roofmate SL-A on
    5. 9mm ply onto
    6. 175x47mm rafters using silicone and screws All fixed with contersunk phillips head wood screws
    7. 140mm insulation in-between rafters with VCL layer fiited from below with battens to create service void. Edges taped and then secured/wedged in place in between rafters with tight fitting 140mm Roofmate SL-A
    8. 12mm Plaster board secured with screws and possibly a bead of silicone to seal the holes where the plaster board screws puncture the VCL as they go into the rafters

    This gives 280mm insulation in total
    To avoid summer over heating and to increase thickness of above-structure insulation I was considering additional insulation on the flat roof segment of the mansard roof. (taperred insulation up to the level of the firrings). We have a 4-5 degree fall over 5m from centre height of 300mm above structral rafters

    Challenges and questions
    1. Is it worth the extra effort as the insulation thickness will not be continous over the whole flat roof (ie. maximum 440mm above rafters at the centre to 140mm at the edges and onto the pitched roof
    Thanks
  28.  
    The key thing is to avoid any unventilated voids. All voids should either be filled with insulation or ventilated.

    What's the build-up/finish on the flat roof section? Where are the firrings? Are they on top of the rafters & below the ply? If so, I would definitely insulate that area.

    David
    •  
      CommentAuthordjh
    • CommentTimeDec 3rd 2012
     
    Posted By: Novice1From outside
    1. Tata courus uban seam (standing seam metal roof) onto
    2. Breather membrane (tata does not specify a void as they want the metal roof fully supported by the 18mm ply)
    3. 18mm ply with 25mm gap underneath,
    4. 140mm Roofmate SL-A on
    5. 9mm ply onto

    You could probably replace this lot with a Vieo warm roof build-up, still using the same covering. I think it would simplify the build, but I don't know what it would do to the cost.
    • CommentAuthorNovice1
    • CommentTimeDec 3rd 2012
     
    Thanks again David
    On the flat roof the firrings would be fixed to the 9mm ply with the structural rafters underneath the 9mm ply. They would be inbetween the external insulation at 1200 crs and the 18mm (external) ply fixed alternatively onto them and then insulation bearing battens at 600crs.

    Sounds complicated but trying to minimise thermal bridging from the firrings

    An alternative we are considering is to do away with the firrings and just sandwich tapering insulatiion in between the 9mm and 18mm ply using M10 threaded rod through the structural rafters and then through the 18mm ply using nuts and recessed washers. We worry about condensation along the fixings in this set up

    Thoughts

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