PeterW Posted June 14, 2016 Share Posted June 14, 2016 Just finalising some roof details and come across an odd one that I can't seem to get a consistent answer on. Proposed roof build up is warm roof, insulation TBC but most likely a full fill plus PIR under for a foil VCL. To give the roof some strength (and because I've got a cheap pallet load...) there is an OSB3 sarking layer. On top I was expecting the breather membrane held down with counter battens and then tile batten and tile. Talking to a roofer he has said he would drape the membrane over the counter battens as the membrane doesn't need to touch the OSB but that doesn't seem right to me ..? My expectation is that it's the same for the dormer cheeks as it is for the roof and the membrane goes tight up against the OSB..? help !! Link to comment Share on other sites More sharing options...
Crofter Posted June 14, 2016 Share Posted June 14, 2016 Usually the breather membrane goes straight onto the sarking. Check the BBA certificate allows this application (most do). Your guy might be thinking of effectively ignoring the sarking and fitted the membrane draped. I think in practise you would need much deeper battens for that to actually work, as unless it is pulled pretty tight it is going to touch the sarking anyway. Sarking is good. I've just fitted mine and the stiffness it has added to the structure is incredible. Link to comment Share on other sites More sharing options...
ProDave Posted June 14, 2016 Share Posted June 14, 2016 My roof membrane is straight on the sarking board and that's normal up here. To do that is must be a non tenting membrane. Draping it over the counter battens makes it more vunerable to damage and is something you would only do if by mistake you had bought a tenting membrane so it could not touch the OSB. As already noted, Sarking board is a building regs requirement in Scotland, I would never want to go back to the English way of not using it. Link to comment Share on other sites More sharing options...
TerryE Posted June 14, 2016 Share Posted June 14, 2016 12 hours ago, PeterW said: There is an OSB3 sarking layer. On top I was expecting the breather membrane held down with counter battens and then tile batten and tile. +1. Just make sure that your breather membrane is a decent non-tenting variety. Also remember that you will need breather access at the eaves and ridge. Link to comment Share on other sites More sharing options...
Crofter Posted June 14, 2016 Share Posted June 14, 2016 1 hour ago, TerryE said: +1. Just make sure that your breather membrane is a decent non-tenting variety. Also remember that you will need breather access at the eaves and ridge. Could you elaborate on that bit please? (I'm about to fit my breather and don't to do it wrong!) Link to comment Share on other sites More sharing options...
PeterW Posted June 15, 2016 Author Share Posted June 15, 2016 I plan on using a dry vented ridge and the instructions on that are pretty clear about how the venting works however its the eaves like you that I'm looking at as suggested approach is OSB / eaves protector / membrane over the top, but where does the ventilator go..? Link to comment Share on other sites More sharing options...
Crofter Posted June 15, 2016 Share Posted June 15, 2016 Hmm. So can I not just have the breather felt go up and over the ridge? Does it need some additional ventilation up there? My roof covering is steel sheeting, with a ridge capping, so will be nicely ventilated- but that's above the breather. Link to comment Share on other sites More sharing options...
PeterW Posted June 15, 2016 Author Share Posted June 15, 2016 I "think" it the ventilation between tiles/steel and the membrane itself here - hence the need for airflow above the membrane. Link to comment Share on other sites More sharing options...
ProDave Posted June 15, 2016 Share Posted June 15, 2016 3 hours ago, PeterW said: I plan on using a dry vented ridge and the instructions on that are pretty clear about how the venting works however its the eaves like you that I'm looking at as suggested approach is OSB / eaves protector / membrane over the top, but where does the ventilator go..? It's that strip above the eaves protector and the membranne This is the detail for my sun room. The box profile sheeting (mocked up with the spirit level) won't quite touch the ventilator strip. The eaves filler for the box profile roofing will fix (probably with glue) to the top of the ventilator strip. I'm not yet sure how you ventilate the ridge of a box profile roof. Link to comment Share on other sites More sharing options...
PeterW Posted June 15, 2016 Author Share Posted June 15, 2016 So where do you cut off the membrane ..??! Link to comment Share on other sites More sharing options...
Temp Posted June 15, 2016 Share Posted June 15, 2016 (edited) Some membranes are approved for use in contact with sarking/insulation but not all. Check with the maker. I believe Kingspan do one that can be used in contact. The membrane should drape into the gutter so any rain blown under the tiles runs into the gutter. For this reason it is usually UV rated. Sometimes a plastic tray is used to support the edge of the membrane. The plastic tray goes under the membrane so again water runs down from the membrane onto the plastic tray and then into the gutter. Edited June 15, 2016 by Temp Link to comment Share on other sites More sharing options...
PeterW Posted June 15, 2016 Author Share Posted June 15, 2016 Yep - but if you use over fascia ventilation and these trays, where do they go..?? Link to comment Share on other sites More sharing options...
ProDave Posted June 15, 2016 Share Posted June 15, 2016 That picture above seems to show what you do if you have the felt above the counter battens, Then the vent strip can go under the support tray. That won't work if you have the membrane on the sarking and then the counter battens, which is why i have the vent strips on top of the support tray. The bottom of the membrane gets sandwiched between the tray and the vent strip and then the flapping edge I cut off with a knife. Thinking more, that picture is reallty a detail for a cold roof, where you want to ventilate the whole loft space and it doesn't matter where the air gets in, as long as it gets into the cold roof space. I am talking of a warm roof where I only want to ventilate the small gap between the membrane and the under side of the tiles or box section roofing, so there is nowhere else for the ventilator to go but above the membrane. Link to comment Share on other sites More sharing options...
warby Posted June 15, 2016 Share Posted June 15, 2016 These two references may help: http://www.ebuild.co.uk/topic/15353-ventilation-of-warm-roof/ http://www.ebuild.co.uk/topic/17493-warm-tiled-roof-eaves-vents-or-not/page__st__20 Link to comment Share on other sites More sharing options...
ProDave Posted June 15, 2016 Share Posted June 15, 2016 13 hours ago, warby said: These two references may help: http://www.ebuild.co.uk/topic/15353-ventilation-of-warm-roof/ http://www.ebuild.co.uk/topic/17493-warm-tiled-roof-eaves-vents-or-not/page__st__20 Interesting. One of those was my thread where I arrived at the detail I now have. The other concludes you don't need to ventilate a warm roof and I needn't have bothered. Link to comment Share on other sites More sharing options...
Crofter Posted June 16, 2016 Share Posted June 16, 2016 This is a really useful thread for me, thanks all. Having recently discovered that I need felt trays, I hadn't thought of shallowing up the angle at the eaves, but I guess this does make sense, to help close up what is otherwise a 3" gap between the fascia and the steel sheeting that I am using (25mm battens, 50mm counterbattens). I still need to get my head around some details here and ensure that the steel is adequately supported, though. Link to comment Share on other sites More sharing options...
TerryE Posted June 17, 2016 Share Posted June 17, 2016 We use a concertina black aluminium roll the same as the one you showed above. MBC advised that the roof needed ventilation between the slates and the felt, so we went with that. Because we were slating rather than tiling, we needed a small kick at the eaves line. So we used a 3" arras rail with the long side to the sarking (we have a 45° pitch) with the felt on top of it and ventilation strip pinned on top of that. It was about 2cm high and slotted. The counter battens stopped just above the arras rail. Worked well. The slater had no problems . Link to comment Share on other sites More sharing options...
Crofter Posted June 17, 2016 Share Posted June 17, 2016 How does this look for my eaves detail? (sorry for my poor CAD skills!) I guess I'll need some sort of beastie mesh between the underside of the steel and the felt tray (this is a 50mm gap created by the horizontal battens). Main concern is that the very edge gets a bit messy with the tray riding up over the edge of the fascia. I could drop the height of the gutter and keep the tray at the same angle as the roof, which was my original plan, but this leaves a bigger gap which might look a bit odd, and may be harder to find suitable vent strip or bug mesh for. Link to comment Share on other sites More sharing options...
PeterW Posted June 17, 2016 Author Share Posted June 17, 2016 18 minutes ago, Crofter said: I guess I'll need some sort of beastie mesh between the underside of the steel and the felt tray (this is a 50mm gap created by the horizontal battens). Use my [Patented] Stainless Steel pan scourer method ! And if you want to stop them moving, just retain with a quick squirt of gun foam and they don't move. @JSHarris used it to block the bottom of his cladding and I've used it on a cement sheet roof to stop squirrels Link to comment Share on other sites More sharing options...
Crofter Posted June 17, 2016 Share Posted June 17, 2016 Yes, I fancy the idea of that! Especially for fitting into the box profile, should be about the right size/shape. My decision is whether to kick up the angle using the felt tray or not. It'll be easier not to, but leave a more obvious gap- or at least put the final horizontal batten on show. So the actual gap would be the 25mm beneath that batten, plus the box profile would need infilled above it. The fascia will be deep enough to accommodate a lowered gutter so either option is viable. Link to comment Share on other sites More sharing options...
Tennentslager Posted June 17, 2016 Share Posted June 17, 2016 http://www.cladco.co.uk/products/fillers-and-tapes Does this help? Looks like they have an angled flashing... 1 Link to comment Share on other sites More sharing options...
Crofter Posted June 17, 2016 Share Posted June 17, 2016 Interesting, thanks. One problem I'm finding with using steel roofing is that the normal application is quite different from a house- i.e. an open underside, without sarking needing drained, and installation on purlins rather than rafters. Link to comment Share on other sites More sharing options...
Crofter Posted June 19, 2016 Share Posted June 19, 2016 Still wrestling with this one. Membrane due to go on tomorrow so I kind of need a decision! My plan detailed above will not work- the 50x50 horizontal batten would close off the drainage gap under the roof sheeting completely. So I would have to move that batten 'upslope' but this weakens the support for the roof sheets right at the eaves, where support is critical to resist wind loads. New plan is simpler- stick the felt trays down directly onto the OSB, with all the battens on top. I have not shown the membrane for clarity but it would go on top of the osb and overlap onto the start of the tray (googling suggests min 125mm overlap). The downside of this approach is a slightly more open eaves detailing and I wonder what it will look like. I would use a 25mm vent strip (or a DIY version) beneath the 50x50 batten, which would be hidden from view by the gutter; an eaves infill strip would go on top of that batten to close up the gaps along the bottom of the box profile. Link to comment Share on other sites More sharing options...
ProDave Posted June 19, 2016 Share Posted June 19, 2016 What is wrong with the way I am doing it, in post #6? the bottom horizontal batten is only about 6" up roof so I don't think it will leave much flapping in the wind. Your roof is going to be so high compared to the gutter, I would be worrided the water might miss the gutter. Link to comment Share on other sites More sharing options...
Crofter Posted June 19, 2016 Share Posted June 19, 2016 Have to admit I'm struggling a bit to interpret your picture, Dave. Perhaps there is some foreshortening going on and the tray is a bit hard to see. Studying it a bit harder, I wonder if the tray is quite flexible and you have battened it down almost to the bottom of the roof, then kicked it up to a shallow (near flat) angle to sit on top of the fascia board? Link to comment Share on other sites More sharing options...
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