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General: Ridge detail of loft conversion

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    • CommentAuthorShevek
    • CommentTimeFeb 18th 2012 edited
     
    This is section through my model of a loft conversion at the ridge. What do the builders think about stuffing the mineral wool between the PIR coming up the pitched roof and the EPS coming along the flat roof? Is this how you'd do it? Or would you perhaps cut the EPS around the trimmed rafters, tight against the PIR? Or something else?

    I'm also wondering whether I might be better to pitch the joists of the flat roof in line with the roof rather than laying them flat?
  2.  
    Yes that's a tricky intersection. One of the reasons why I have perservered with cold roofs in conversions.

    Is there any ventilation in either the flat or pitched roof? Or are you going for fully filling between the structural timbers?
    • CommentAuthorShevek
    • CommentTimeFeb 18th 2012
     
    Yeah, the pitched roof will have a 50 mm gap for ventilation, with 50 + 50 mm PIR retrofitted between and under the rafters. That reminds me, I need to make the ridge tile a ventilated type rather than the bedded type I've shown.

    With the flat roof I was thinking, warm roof, joists fully filled with mineral wool and 150 EPS on top of that.
  4.  
    I'm not sure building regs will allow an unvented flat roof with insulation between the joists. There's a lot of heated debate about this kind of thing on here. Others will say there is no condensation risk but I'm not a fan of hybrid roofs. The last conversion I did has details of how to overcome the problem you face. [though with a vented flat roof]
    http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=7057&page=2#Item_15
    • CommentAuthortony
    • CommentTimeFeb 18th 2012
     
    I always fitted the steel with a shaped 100x50 and touched it tight under the rafters, the new ceiling joists then run under it, hung if necessary to form a strong triangle and no silly notching in, noggs and no cracking afterwards either.
  6.  
    Yes, that's a good point. I think its as low as it is shown in the section to accommodate the over joist insulation. This problem doesn't arise with the cold roof method as the steel is usually tight to the ridge to maximise headroom
    • CommentAuthorCWatters
    • CommentTimeFeb 20th 2012 edited
     
    Cold deck/roof = Structural elements on the cold side of insulation where condensation may form => requirement to ventilate.

    Warm deck/roof = Structural elements are far enough to the warm side of insulation that condensation cannot occur on them. Interstitial condensation risk analysis required if structural members aren't entirely on the warm side.

    Many Building Control Departments work to this advice note...

    http://www.woking.gov.uk/planning/building/guidance/flatroofs
    • CommentAuthorShevek
    • CommentTimeJul 15th 2012 edited
     
    Any thoughts on where I go from here? I've firmed up heights and this is the resulting detail. Talk about rock and a hard place.

    I can't go any lower with ceiling (already at 2030 AFL). And once I add a 1:60 fall in some fashion I've got about 30 mm before I reach the 150 mm limit from the last tile. And I've still got to get my top layer of insulation + another layer of ply on yet. Clearly I need an alternative approach. (We're trying to hit Passivhaus)
    • CommentAuthorTimber
    • CommentTimeJul 15th 2012
     
    Cold unvented flat roofs are bad. I have seen a few catastrophic failures through my work.

    I personally would never consider it.
    • CommentAuthorShevek
    • CommentTimeJul 15th 2012 edited
     
    If you're talking about the pitched roof, it's insulated between and under rafters. 100 K7 between and 82.5 K18 under. Granted I need to add some counter battens (and show membranes) but this is a standard Kingspan detail with a BBA, no?
    • CommentAuthorShevek
    • CommentTimeJul 15th 2012 edited
     
    Just double-checked Kingspan's site.
    http://www.kingspaninsulation.co.uk/getattachment/78f5cba0-5188-4b8a-a9ec-a0708ccc61be/Kooltherm-K7-Pitched-Roof-Board.aspx (first option on page 2)

    But then if you look at the BBA it only shows a ventilated version (figure 4)!
    http://www.kingspaninsulation.co.uk/getattachment/f11c1a16-dae0-4424-a50c-dd1f3555a1dd/BBA-Certificate-K7.aspx?disposition=attachment

    Boy this is difficult. I'm not sure we can reach passivhaus.
    • CommentAuthortony
    • CommentTimeJul 15th 2012
     
    1 in 40 fall for me and I go to within 150 of ridge.
    • CommentAuthorShevek
    • CommentTimeJul 15th 2012 edited
     
    Thanks guys.

    I guess one way I could get the u-value of the pitched roof up while also providing a 50 mm ventilation gap would be to add a 50 mm batten along the underside of the existing rafters, effectively increasing the depth of the rafters. Then partial fill with 100 mm between and 82.5 mm under.

    Posted By: tonygo to within 150 of ridge.

    Problem with this is that I wouldn't then have three layers of roof tiles, so they wouldn't be water tight.
    • CommentAuthorCWatters
    • CommentTimeJul 16th 2012
     
    Posted By: Shevek Problem with this is that I wouldn't then have three layers of roof tiles, so they wouldn't be water tight.


    I wonder if no tiles is an option. eg flat roof with flashing up under the ridge tile. Perhaps not using mortar but a dry ridge system?
    • CommentAuthorMike George
    • CommentTimeJul 16th 2012 edited
     
    Hi Shevek,

    Not sure if I'm getting this right. Have you decided to go with an unvented flat roof?

    I've done several of these. I vent the flat roof with counter battens running in line with the ridge. The insulation is carried through on a horizontal plane to intersect with the pitched front roof. Clear 50mm gap also intersects with gap on the pitched roof. Ridge vented with dry system.

    Yes, the top row of tiles is omitted. I use GRP. You can get pre-shaped sections for this. Some points on that

    1. Add a bouble batten at the highest point to create an upstand to provide some protectrion against snow and also support the ridge.
    2. Where the flat roof meets the vertical walls of the convesion is a very tricky detail also.
    • CommentAuthorShevek
    • CommentTimeJul 16th 2012
     
    Thanks guys.

    Mike, yes, I was aiming to do a warm deck unventilated flat roof. It's a relief to know you've successfully done it the way you describe. I'll try that, thanks very much. I assume building control/NHBC has been fine with this approach (i.e. running inline with the ridge)
    • CommentAuthorShevek
    • CommentTimeJul 16th 2012 edited
     
    Mike, one other thing, have you been doing ventilated flat roofs with insulation between and over roof joists or between and under? Is there such a thing as having a ventilated flat roof with insulation between and over the roof joists?
    • CommentAuthorShevek
    • CommentTimeJul 16th 2012 edited
     
    Had a crack at a detail. Am I barking up the wrong tree trying to keep insulation above joists? I probably need to raise joist height and go between and under joists, to eliminate that no-mans land.

    And what to do at the ends near the ridge, where the tiles to either side of the dormer will actually be higher than the flat roof waterproof membrane?
    • CommentAuthortony
    • CommentTimeJul 16th 2012
     
    Always join insulation up fill both white triangles with insulation -- I wouldn't use warmcel on a flat roof.

    No vent to rhs of ridge only lhs,pack out titling fillet to touch ridge tile
    • CommentAuthorShevek
    • CommentTimeJul 16th 2012
     
    Thanks tony, why would you omit venting to rhs? Wouldn't that stop cross-ventilation taking place through the flat roof?
    • CommentAuthortony
    • CommentTimeJul 17th 2012
     
    No because you make a clear path so that it still will, I would worry about water getting blown up under the ridge from the flat roof side in storms.
    • CommentAuthorShevek
    • CommentTimeJul 17th 2012 edited
     
    Ah right, with you now. Good idea. Maybe I could go further and pack it tight with something to seal it? Foam?
    • CommentAuthorTimSmall
    • CommentTimeJul 17th 2012
     
    Change your 50mm parallel battens on underside to cross battens to reduce thermal bridge (can also then space them to suit plasterboard and insulation board sizing).

    Where is the steel going, and what are the details to reduce thermal bridging through the steel upwards and at both ends (and to airtight - this can be tricky).
    • CommentAuthorShevek
    • CommentTimeJul 17th 2012
     
    Posted By: TimSmallChange your 50mm parallel battens on underside to cross battens to reduce thermal bridge (can also then space them to suit plasterboard and insulation board sizing).

    Brilliant, cheers.

    Posted By: TimSmallWhere is the steel going, and what are the details to reduce thermal bridging through the steel upwards and at both ends (and to airtight - this can be tricky).

    No steel in the upper roof. Engineer managed to design it out. We're using that trebled up 220 mm timber instead.

    We will have two steels going across between party walls in the new floor though. Haven't thought about those details yet.
    • CommentAuthorShevek
    • CommentTimeJul 17th 2012
     
    Posted By: TimSmallChange your 50mm parallel battens on underside to cross battens to reduce thermal bridge (can also then space them to suit plasterboard and insulation board sizing).

    Actually, just realised this won't work, the idea of the parallel batten to underside of rafters is to increase the depth of the rafter so we can fit thicker insulation between rafters than the insulation under the rafters.
    • CommentAuthortony
    • CommentTimeJul 17th 2012
     
    use deeper joists?
    • CommentAuthorShevek
    • CommentTimeJul 18th 2012
     
    This is to do with the existing rafters to pitched roof rather than the new flat roof.
  28.  
    Posted By: ShevekActually, just realised this won't work, the idea of the parallel batten to underside of rafters is to increase the depth of the rafter so we can fit thicker insulation between rafters than the insulation under the rafters.
    Its true that the side effect of reducing the thermal bridging will be to reduce the temperature of the rafters, thereby increasing the condensation risk. However, do you need to worry about this if you're ventilating above the insulation?

    It seems a much better use of the materials & deepening the rafters so that you can conform which some rule of thumb for the amount of insulation between & below the rafters just seems daft.

    David
    • CommentAuthorShevek
    • CommentTimeJul 18th 2012
     
    Yeah fair point. Best thing to do is probably to get dew point calcs done.
    • CommentAuthorTimber
    • CommentTimeJul 18th 2012
     
    Well I went and inspected a building today with a failed flat roof. It was a cold, unvented flat roof that had suffered both localised leaks and problems with conensation (no vcl). Needless to say it was propper knackered (at the ripe old age of 3!).

    Warm flat roofs are the only choice in my mind.

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