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Building Regulations and Standards: hybrid roof (XPS)

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    • CommentAuthorNovice1
    • CommentTimeDec 3rd 2012
     
    thanks dgh
    looked at vieo but costs were prohibitive over a 360m2 roof.. so I', basically trying to duplicate their system with off the shelf components
  2.  
    I would either fix the firrings on top of the battens forming the ventilated cavity or use tapered insulation.

    The M10 threaded rod sounds like a big cold bridge. Have you looked at fixing the battens forming the ventilated cavity to the structural rafters with heilcal shank fixings designed for warm roof applications? Or screws like those used for EWI & in the Vieo warm roof mentioned by djh? You could then use standard ring shank nails to fix the 18mm ply to the battens.

    David
    • CommentAuthorNovice1
    • CommentTimeDec 3rd 2012
     
    Thanks for the advice david. I'd probably opt for taperred with battens over to form the ventilated cavity.
    Wrt to the fixings
    I was just worried about
    1. sourcing a 440mm+ helical shank or screw.
    2. The Vieo system only goes up to 240 and the fixings seem to stick a hole through the insulation with a screwhead that seems like;y to attract condensation
    3. I've tried to source plastic mushroom head to use with long screws but my structural engineer isn't confident wrt uplift especially with a ventilated cavity

    M10 rod seemed like the safest structural option. I'm new to this so what do you reckon
    • CommentAuthorNovice1
    • CommentTimeDec 3rd 2012
     
    Just spoke to vieo
    maximum that their fixings from EJOT will tolerate is 445mm of insulation (185mm cone+300mm carbon fixing)

    BUT caveats galore
    Worried
    about hitting 50mm rafters
    about 50mm minimum embedding being strong enough

    I'm looking into using threaded rod and trying to find a way to sufficiently insulate external nut or threaded rod with better thermal characteristics
  5.  
    Why do you need a 440mm helical shank fixing? 210mm should be sufficient.

    The helical shank fixings fix 25mm battens forming the ventilated cavity to the structural rafters. They need to be embedded about 35mm into the rafters so need to be 25 + 140 + 9 + 35 = 209mm long. Or have a misunderstood your build up?

    This is on the long side but you can check from below whether any fixings have missed the rafters before fitting the insulation between the rafters.

    David
    •  
      CommentAuthorSteamyTea
    • CommentTimeDec 3rd 2012
     
    Posted By: Novice1M10 rod seemed like the safest structural option. I'm new to this so what do you reckon
    Try this for different bolt materials:
    http://www.engineersedge.com/calculators/machine-design/bolt-preload/bolt-preload-calculation.htm
    • CommentAuthorNovice1
    • CommentTimeDec 3rd 2012
     
    Sorry david my explanation wasnt clear. The pitched part of roof and the lower parts of the flat roof can indeed use standard fixings to hold down the insulation. As we move to the centre / ridge, the insulation will thicken from 140mm to 440 at the centre. The roof is a mansard type with a central flat roof. The flat roof is sort of like a low pitched gambrel roof

    This is the closest image i can find to show how the layers of insulation will thicken towards the centre of the ridge.
    http://www.achfoam.com/News---Media.aspx?gallerytype=images&gallerystyle=detail&ownerid=180&itemid=5089&listname=Roof%20Insulation
  8.  
    OK, I see now.

    Do the fixings for the flat section need to be as strong as those for the pitched sections? Don't you just need to hold down the edges of the flat section? For the rest, couldn't you just build some battens in to the insulation as done by Viking House? These can be perpendicular to the rafters to minimise cold bridging & at whatever spacing available screws or ring shank fixings will allow.

    It may be that the pragmatic solution is a half-way house between tapering the insulation towards the edges & tapering the battens in the ventilated cavity towards the middle.

    David
    • CommentAuthorNovice1
    • CommentTimeDec 5th 2012
     
    Have found a solution from Ejot. 185mm plastic tube washers with climadur-dabo fastener which go up to 300mm plus. Veio source there fasteners from them. Thank for your help guys
    • CommentAuthorNovice1
    • CommentTimeMar 25th 2013
     
    So have read all the threads, spoken to Flat roofers, Tata, ejot, Firestone and Dupont.

    Updated build up (From outside)
    50 degree pitched part of roof
    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 OSB3 on
    4. 140mm Roofmate SL-A on
    5. 9mm ply onto
    6. 175x47mm rafters using silicone and screws All fixed with contersunk flange 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 insulation
    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

    5 degree flat part of roof (180m2)
    1. EPDM onto
    2. 18mm OSB3 on
    3. Breather membrane
    4. 280mm Insulation (x2 staggered layers of 140mm Roofmate SL-A on
    5. 9mm ply onto.....
    the 18mm OSB3, insulation and 9mm OSB3 sandwich held down onto 200x47 rafters with Ejot washers and climador screws

    This gives 280mm insulation in total for the pitched part of the roof and 420mm for the flat part

    Changes
    Have removed the ventilated battens cavity as;-
    1. insulation thickness outside structural deck>>between rafters
    2. 18mm OSB3 will sit in its entirety directly onto insulation to prevent OSB sagging and ponding over time.
    3. External insulation twice that of inbetween rafter insulation which makes a warm roof with little chance of condensation
    4. belt and braces on the VCL with polythene and siliconed 9mm OSB3 layer

    VCL to be lapped from underneath onto the rafters and held in place with glue and the tight fitting in between-rafter rigid insulation +/- taped seams. In that way I can still have up lighters and a decent service void.

    Questions
    1. Does a warm roof need the 25mm ventilated batten cavity
    2, Any criticisms or help please
    • CommentAuthorTimber
    • CommentTimeMar 25th 2013 edited
     
    I always seem to be the 'no' man on these sorts of threads.

    I might seem like a broken record, and I certainly feel like it sometimes. But, here goes nothing....

    I wouldn't do it, but then its not my roof. I would not use an 18mm OSB3 deck below the single ply roof membrane with no ventilation. Not durable and stable enough. In both of those roofs, I would go full warm roof with all insulation outside of structural timber, or I would incorporate ventilation into the structure. Sorry for broken record, but why build such risky constructions? You are asking for condensation problems in both constructions (IMO).

    But, if you can get one/all/any of the people who you have spoken to to give you a guarantee on the buildup (not just on their little bit (with all the associated caveats) but the whole thing) then don't worry what I think.

    I hope this doesn't sound rude, it is not meant to be.

    Edited to add, have you don any condensation risk calcs on the above? That should always be the first thing to do. Anything else is just guessing.
    • CommentAuthorNovice1
    • CommentTimeMar 25th 2013
     
    Thanks Timber esp. for your thoughts on guarantees and caveats
    Is there any warm roof that you wouldn't ventialte?
    • CommentAuthorGreenfish
    • CommentTimeMar 25th 2013
     
    Posted By: Timber... I would go full warm roof with all insulation outside of structural timber, or I would incorporate ventilation into the structure...
    Heeding your warnings, for this case and my own flat roof problem, any suggestions how to attach a thick slab of insulation (e.g. 420mm of XPS) above the structure?
    • CommentAuthorNovice1
    • CommentTimeMar 25th 2013
     
    @Greenfish
    as above (185mm plastic tube washers with climadur-dabo fastener which go up to 300mm plus. Total length of fixing 485mm)

    Did a condensation risk @ 50:50 (50% external 50% between rafters). As predicted by Viking, david and others it was ok. Have now doubled the external insulation?

    Is there any warm roof that does not need ventilation?
    Seems that it all lies in the definition of a warm roof. When does a hybrid roof become a warm roof? Does any insulation internally stop it from being a warm roof?
    • CommentAuthorTimber
    • CommentTimeMar 25th 2013
     
    Yes there are lots of warm roof systems that don't need ventilation - but your roof is not a warm roof!

    Funny, I ran a similar roof to yours earlier and it failed a condensation risk.
    • CommentAuthorNovice1
    • CommentTimeMar 25th 2013
     
    Thanks
    If I were to remove all the in between rafter insulation would that make it a warm roof.
    I am struggling a bit with this
    • CommentAuthorTimber
    • CommentTimeMar 25th 2013 edited
     
    Sort of but not really, there is still structural timber in the buildup of the roof - the deck.

    A true warm roof is

    EDPM (or whatever) bonded to
    Insulation bonded or fixed through
    VCL on
    Deck over
    Joists

    The OSB onto which you are bonding will need to be given a chance. It needs ventilation to stop it being subject to high moisture content. If its moisture content exceeds 20% for any real length of time it will start to delaminate and decay. The roof listed above has no timber between the VCL and the roofing membrane, so there is no vulnerable timber at risk. You may get condensation in the insulation, but there is nothing there that will be badly affected by it.

    Now you can take the above roof and put insulation in between the joists, whilst keeping the rest of the construction the same, but you need to keep an eye on the risk of condensation on the warm side of the VCL.
    • CommentAuthorNovice1
    • CommentTimeMar 25th 2013
     
    Ahh... Now I get it... You have a convert.
    Have always assumed that the structural deck was only the rafters and the inner OSB board.
    Now it seems obvious but the outer OSB is entirely in the cold zone with a waterproof layer above it.
    Even if a liitle vapour escapes the VCL through the life of the building then the water has nowhere to go except into the OSB.
    Thanks.
    Think you just save me a load of hassle and money.
    The ventilated area and the battens are back in.
    Is the depth of a batten enough?
    • CommentAuthorTimber
    • CommentTimeMar 26th 2013
     
    BS5250 states that the ventillation gap should be 50mm - so a 50x50mm batten with 25mm eaves vents should work, but it is limited to spans up to 10m. Any larger and the ventilation path becomes too hard.

    Normally the roofing membrane is just bonded down to the upper layers of insulation - is that not possible with the insulation material you intend on using?
    • CommentAuthorPaulJ
    • CommentTimeMar 26th 2013
     
    How do products like BBA approved Kingspan Thermaroof - which is insulation faced with 6 mm ply - work? When is the ply not structural? Is 6 mm ply less prone to failure than 12 mm or 18 mm?
    • CommentAuthorTimber
    • CommentTimeMar 26th 2013 edited
     
    Don't know to be honest. Plywood is not magic - especially softwood ply. If the moisture content of the plywood exceeds 20% for any length of time, decay can occur. The kingspan thermaroof details that I saw showed no vapour control layer in the construction. I honestly have NO idea how they 'work' in reality (by work I mean have an sort of long term durability). If you are using a plywood with durable timber used in the venteers, then fine, but people use softwood ply which has no natural durability.

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