SuperPav Posted February 10, 2021 Share Posted February 10, 2021 Can somebody shed some light on this... We're looking at having a approx 7m glazed opening on the ground floor - flat roof with zinc on it. The inside is relatively easy, posijoists will cover that span. The question is on the outside and cladding the aperture around the door where we're intrigued by the attached design we've seen. In our case the stone leaf would be smooth cut ashlar and would only be the height of a small parapet (rather than a wall above the opening), but the question remains - to support the outer stone skin to the parapet (say approx 125mm deep by 300-400mm high), what would one use? I've sketched an option for the section but wonder if there's a more elegant solution as this is only my first thought? Either end of the beam/joist bear will bear on masonry walls. Link to comment Share on other sites More sharing options...
Temp Posted February 11, 2021 Share Posted February 11, 2021 (edited) I wouldn't taper the insulation from 200 to 100. That's an average of 150. I'd go at least 250 to 150 for a 200 average. If the roof is a single pitch perhaps you could also consider sloping the roof front to back to allow you to get rid of the hidden gutter without affecting the appearance (eg Use a conventional gutter around the back)? Edit: What fall does the roof need? A 1:40 fall over 7m is 175mm. Edited February 11, 2021 by Temp Link to comment Share on other sites More sharing options...
Russell griffiths Posted February 11, 2021 Share Posted February 11, 2021 It shows on your drawing a u beam with welded lip, you will need some serious input from a structural engineer for that. And im not sure if 7m is to far for your posi joists, at that span they will need to be very deep. The image doesn’t match the drawing, I know it could just be an image you found on the internet, but I would think the image shows a higher roof that slopes away to the back with the roof hidden. If using zinc have you looked at the minimum fall required. Link to comment Share on other sites More sharing options...
scottishjohn Posted February 11, 2021 Share Posted February 11, 2021 I would have suggested a "catnic lintel this company can do the length you require as they specialise in odd balls 7m is a big opening http://www.harvey-steel-lintels.co.uk/ Link to comment Share on other sites More sharing options...
SuperPav Posted February 11, 2021 Author Share Posted February 11, 2021 Just to clarify that photo is not of OUR proposed design - attached is the flat roof extension that I'm talking about. It's that border frame currently in zinc that we just thought might be nice in stone having seen the photo above. The lintel would only be supporting the decorative outer parapet leaf of stone, all the roof structure would be in the joists. Fall will always be to the outside (front) where the glass is for ease of drainage, since the "back" of the flat roof is against the house walls. In terms of the joist span, the posijoist span tables suggest a PS10 or equivalent 254 section can easily span 7m for a flat roof @400 centres unless I'm misreading something? Ref insulation depth, the house is solid stone walls, so in terms of overall heat loss, 150mm average on top will give say 0.16 U value, which is (unfortunately) more than good enough for us, given the rest of the house. Link to comment Share on other sites More sharing options...
mvincentd Posted February 11, 2021 Share Posted February 11, 2021 Whats your wall construction....is the line drawing right? I'm thinking you've drawn the steel in line with the cavity, but wouldn't it be in line with the inner leaf blockwork? Whats the cavity tray arrangement above the window? So your 7m span is the window width anyway...or is it coincidentally 7m the other way too. Wouldn't you fit the window into a steel goalpost arrangement and run the joists the other way. The top post would have an additional plate welded to underside supporting the 'outer leaf' which in fact is just circa 400mm as you say. I dont know anything about how stone like that pictured is laid but if it was brick/block you'd surely just look to how it was done lower down...ie tying the 2 leafs with wire ties....so i'd imagine Hilti fixing some steel strap into the I-beam. I have broadly this scenario over a 6m wide window but with flint blocks circa 400 high rather than your stone. I imagine (slightly simplified description) the top capping in the photo being a continuation of the box gutter and so acting a bit like a horizontal hook to hold the top of the stone too. On pozis' i have some 6895's that are 147 x 279 @ 400 ctr's but they're supporting a green roof so i'd think you'd be ok finding a 7m pozi. Link to comment Share on other sites More sharing options...
George Posted February 11, 2021 Share Posted February 11, 2021 (edited) I don't think you will get a pozi-joist spanning 7000mm carrying the flat roof. The bi-fold needs something solid above the frame to attach into. You're best off with an RHS steel beam instead of the pozi-joist with a plat or angle welded on to carry the outer leaf/cladding. I wouldn't use actual lump of stone - use stone slips. Edited February 11, 2021 by George Link to comment Share on other sites More sharing options...
Russell griffiths Posted February 11, 2021 Share Posted February 11, 2021 Looking at that pic it makes more sense, I think you would end up with a goal post arrangement with a steel with a large flat plate welded to the bottom to take the stone. Two things I think will come into play. Twisting, with the loading only on the front the steel will want to twist of its mounting points the fall of the roof, will zinc go down to that pitch. 1 Link to comment Share on other sites More sharing options...
SuperPav Posted February 11, 2021 Author Share Posted February 11, 2021 19 minutes ago, Russell griffiths said: Looking at that pic it makes more sense, I think you would end up with a goal post arrangement with a steel with a large flat plate welded to the bottom to take the stone. Two things I think will come into play. Twisting, with the loading only on the front the steel will want to twist of its mounting points the fall of the roof, will zinc go down to that pitch. Either side of the opening is masonry wall, so I don't think there needs to be any goal post, just a steel (or pair of steels) resting across the top of it. The pitch I have relative flexibility with, so can do 1:40 going back towards the house that won't be a problem. My thought/reasoning for putting a steel outboard of the glass rather than using a single beam with lip going to the outside was to reduce the thermal bridging through the steel lip... If the posijoists can't span 7m parallel to the glass, then I'd just put another steel inboard of the glass and run the posijoists perpendicular at shorter lengths (it's just the additional cost of the steel then). Link to comment Share on other sites More sharing options...
George Posted February 11, 2021 Share Posted February 11, 2021 It's a bit wasteful as the outer steel isn't doing much. A larger RHS + plates could do the job of both. If it's a cold bridge you're concerned about, you can insert structural thermal breaks (Farrat packs or similar). Also steel on the outside will need corrosion protection. 1 Link to comment Share on other sites More sharing options...
SuperPav Posted February 11, 2021 Author Share Posted February 11, 2021 Thanks for that idea, yes appreciate that steel would essentially be "overkill" to support a bit of stone, but equally it's not terribly complicated (or expensive) in the grand scheme of things if we do go that route. I think if we stick with zinc cladding, and I get confirmation from the joist supplier that they can span the 7m then that would be the easiest way to go as it means no steels required at all. The technical doc suggests the span is easily achievable, but I've asked the manufacturer to confirm - the attached span limits are based on 0.5kn dead load flat roof (+0.75kn imposed) which would be fine for a zinc roof (no access other than for maintenance). Link to comment Share on other sites More sharing options...
scottishjohn Posted February 11, 2021 Share Posted February 11, 2021 2 hours ago, Russell griffiths said: the fall of the roof, will zinc go down to that pitch No check with makers Link to comment Share on other sites More sharing options...
George Posted February 11, 2021 Share Posted February 11, 2021 It'd span as part of a floor system with joists at 400/600mm centres, but you showed it acting as an edge beam carrying the roof joists spanning left to right. The deflection of the edge beam would be excessive. Your Engineer should be able to advise on these issues. It's common to see pozi-joists supported on steels. Link to comment Share on other sites More sharing options...
Russell griffiths Posted February 11, 2021 Share Posted February 11, 2021 I think you need to re calculate your roof pitch, 1 in 40 is not a lot, it’s certainly not enough for a zinc roof as far as I’m aware. When I looked into mine, it was minimum 5 degrees with no joins in the sheets, if you need to have any head laps the fall needs to increase to something like 12 degrees. My roof is 10 degrees so all my sheets span front to back in one piece. Capillary action is a bugger and will suck water back up a slope if gravity cannot get it away quicker. Link to comment Share on other sites More sharing options...
mvincentd Posted February 12, 2021 Share Posted February 12, 2021 This might help you visualise maybe. Link to comment Share on other sites More sharing options...
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