GraemeHM Posted February 3, 2021 Share Posted February 3, 2021 Hi, We are getting conflicting advice so any thoughts that might help us get through this very much appreciated. We are building a 4m deep x11m long single story extension on the back of our house, mono-pitched roof joined to the exiting house. Due to the lay of the land and upper story windows we are limited for height at the back/house side (3m) and still want to achieve a fall above 5 degrees. There are two questions this raises. 1. One installer recommended a vented or cold roof zinc installation (something to do with moisture trapped during installation of the zinc) where as our design/architect guy recommended a warm roof i.e. unvented. Has anyone got views on which is better and/or any advantages of each? 2. If we were to go down the vented route. I have been unable to see how the back/house side of the venting would work and upstand for that matter. I have seen venting details on roof ridges. Has anyone got any experience of how that might work? Again any help much appreciated. Thanks Graeme Extension 26-01-21 - Drive side view.tif Link to comment Share on other sites More sharing options...
Russell griffiths Posted February 3, 2021 Share Posted February 3, 2021 I have a cold roof ventilated at the eaves and abutment wall, a warm roof will be 175mm approx higher than a cold roof as the insulation goes on the top, my ceiling height is 75mm lower than a warm roof as I have insulation under my roof rafters. So you need to draw draw it out and work out the dimensions you have to play with. cold roof ventilation detail at abutment. Link to comment Share on other sites More sharing options...
Russdl Posted February 3, 2021 Share Posted February 3, 2021 We have a zinc clad 5° warm roof for comparrison. Link to comment Share on other sites More sharing options...
GraemeHM Posted February 3, 2021 Author Share Posted February 3, 2021 Thank you so much. Not sure I completely understand the diagram..sorry, but let me see if I can get this right.....I see the flashing bit. Does that sit on top of the standing seam? with vented filler in between seams. Then beneath the zinc is vapour control on a bed of ply? Batten on top of the rafter where the bottom arrow shows airflow coming up? I've attached a pdf diagram what one installer has recommended.Zink roof detail.pdf Roughly how high/tall does that upstand/abutment flashing have to be? There is a complication. This in an old cob barn than many many years ago had a second story added. There is a line of tiles that are also provide for a window sill and a separation between two different thicknesses of wall. So we'll have to come under that, but not sure how the flashing can be worked in with the tile. And what BC might have to say about it. 2 x pictures included. Cheers Graeme Link to comment Share on other sites More sharing options...
GraemeHM Posted February 3, 2021 Author Share Posted February 3, 2021 19 minutes ago, Russdl said: We have a zinc clad 5° warm roof for comparrison. Thanks for the input. I must admit I like the idea of a warm roof, i.e. no cold bridging, but then as Russel said he insulated under the rafters. I presume you are happy and have had no problems... may I ask why you went warm roof? Link to comment Share on other sites More sharing options...
Russell griffiths Posted February 3, 2021 Share Posted February 3, 2021 Flashing sits on top and goes up the wall 150mm, I think this height has something to do with rain bounce, any lower and rain can bounce above this height and get behind the flashing, I believe this is a very old regulation and might be a bit more relaxed now, check with building control. Inbetween the pans and the upstands of the sheets is a foam filler, it has sticky tape on one side, it prevents insects getting in and rain being blown up, but allows airflow. I have seen other makes that consisted of a perforated metal trim that you stuck down with sealant and the flashing went over the top. Link to comment Share on other sites More sharing options...
Russdl Posted February 3, 2021 Share Posted February 3, 2021 @GraemeHM Our main roof is warm, giving us fully useable and open loft space. The single storey element in the image I posted was made warm roof mainly because it was easy to do with MBC. The drawing I attached was for Tata Steel which we didn't use in the end. Our zinc detail is much like that posted by @Russell griffiths with a similar sized upstand. The VMZinc mage you attached is how our roofs are built up and clad. Zero problems so far. Link to comment Share on other sites More sharing options...
Temp Posted February 3, 2021 Share Posted February 3, 2021 (edited) Google for roof abutment vent. If your zinc is going to have a profile then the supplier may also have a matching abutment vent. On regular pitch roofs the abutment vents sometimes just lift the flashing up a bit so air can get out from under it. On flat or shallow roof pitches there is an upstand that lets air go up between it and the wall then down and out from under the top flashing. If you are pushed for height under a window you will have to find a shallow solution (reduced upstand). Image borrowed from here: http://www.glidevale.com/article/70/faq-how-do-i-ventilate-a-cold-flat-roof-and-which-product-do-i-use Edited February 3, 2021 by Temp Link to comment Share on other sites More sharing options...
Temp Posted February 3, 2021 Share Posted February 3, 2021 (edited) Article on cold and warm zinc roof make up.. https://www.sigzincandcopper.co.uk/standing-seam-zinc-roofing-design-part-2-three-typical-buildups/ Summary of main differences: Cold roof has insulation between or below rafters. Needs height above the rafters for the 50mm ventilation gap and vent upstand (100-150mm?). Warm roof has insulation above the rafters (sometimes with 2/3rds above and 1/3rd between) so need height for that and a flashing. Edited February 3, 2021 by Temp Link to comment Share on other sites More sharing options...
SimonD Posted February 3, 2021 Share Posted February 3, 2021 (edited) 13 hours ago, GraemeHM said: Hi, We are getting conflicting advice so any thoughts that might help us get through this very much appreciated. We are building a 4m deep x11m long single story extension on the back of our house, mono-pitched roof joined to the exiting house. Due to the lay of the land and upper story windows we are limited for height at the back/house side (3m) and still want to achieve a fall above 5 degrees. There are two questions this raises. 1. One installer recommended a vented or cold roof zinc installation (something to do with moisture trapped during installation of the zinc) where as our design/architect guy recommended a warm roof i.e. unvented. Has anyone got views on which is better and/or any advantages of each? 2. If we were to go down the vented route. I have been unable to see how the back/house side of the venting would work and upstand for that matter. I have seen venting details on roof ridges. Has anyone got any experience of how that might work? Again any help much appreciated. Thanks Graeme Extension 26-01-21 - Drive side view.tif 125.16 kB · 5 downloads I got a strange feeling when I saw this thread as I was sure there's something wrong about warm roof build up and zinc standing seam. So I dug out my UK Guide to Good to Practice published by the Federation of Traditional Metal Roofing Contractors. Low and behold it recommends against using an unventilated warm roof buildup - or shall we say it diplomatically suggets to approach such a design with caution, especially with materials like zinc. So definitely listen to your roofing contractor, not your designer. The recommended design for a warm roof actually includes 50mm ventilation below the plywood/osb deck supporting the standing seam. Instead of writing and quoting loads, I've attached pictures of the pages so you can see for yourself. If you need any further details, let me know. HTH. Edited February 3, 2021 by SimonD Link to comment Share on other sites More sharing options...
Temp Posted February 4, 2021 Share Posted February 4, 2021 So I wonder why zinc covered OSB needs more ventilation than say GRP covered OSB? Link to comment Share on other sites More sharing options...
SimonD Posted February 4, 2021 Share Posted February 4, 2021 12 hours ago, Temp said: So I wonder why zinc covered OSB needs more ventilation than say GRP covered OSB? I can only guess it's down to risk of condensation behind the metal, but perhaps because of the riks of corrosion, there's a bit of a hedging against crap vapour barrier installation. Link to comment Share on other sites More sharing options...
Russell griffiths Posted February 4, 2021 Share Posted February 4, 2021 I wasn’t to keen on any of the designed systems so included the eye watering cost of one of those fancy membranes with the plastic fibres all over them to allow condensation to drain away. Link to comment Share on other sites More sharing options...
the_r_sole Posted February 4, 2021 Share Posted February 4, 2021 (edited) It's strange that the guide says warm roofs are risky if they are designed or installed incorrectly - surely that goes with any type of construction?! The condensation risk should be zero if all the insulation is on the outside? and the point in an unventilated warm roof is so that it doesn't need ventilation? the warm roof pictured has got more ventilation than the ventilated roof! Edited February 4, 2021 by the_r_sole Link to comment Share on other sites More sharing options...
SimonD Posted February 4, 2021 Share Posted February 4, 2021 3 minutes ago, the_r_sole said: the warm roof pictured has got more ventilation than the ventilated roof! It looks like it in the pic, but spec says 50mm just as with the cold roof Link to comment Share on other sites More sharing options...
SimonD Posted February 4, 2021 Share Posted February 4, 2021 14 minutes ago, Russell griffiths said: I wasn’t to keen on any of the designed systems so included the eye watering cost of one of those fancy membranes with the plastic fibres all over them to allow condensation to drain away. That's funny stuff, for a moment I thought it was just bitument felt with a fleece backing! Looks like your roof was the size of a football pitch ? Link to comment Share on other sites More sharing options...
the_r_sole Posted February 4, 2021 Share Posted February 4, 2021 2 minutes ago, SimonD said: It looks like it in the pic, but spec says 50mm just as with the cold roof but you also have a void between the rafters so you're ending up with loads of void - it kinda looks like a detail produced by someone who doesn't like warm roofs! incidentally, the technical standards in scotland encourage the use of warm flat roofs over cold roofs due the condensation risks! Link to comment Share on other sites More sharing options...
Russell griffiths Posted February 4, 2021 Share Posted February 4, 2021 Condensation is funny stuff. When I put mine on I had sheets just sat on the roof not fixed, you would think that the temperature was the same on the top of the sheet and the underneath, so how come when you pick a sheet up the underside is dripping in condensation, it can only be 1-2 degrees warmer on the underside. Link to comment Share on other sites More sharing options...
Mr Punter Posted February 4, 2021 Share Posted February 4, 2021 We have used the membrane with scouring pad / pubic hair topping like @Russell griffiths used. I think it means that you don't get a puddle of condensation directly on the zinc. I think the risk is reduced with some of the zincs with pre-patinated finish. Link to comment Share on other sites More sharing options...
SimonD Posted February 4, 2021 Share Posted February 4, 2021 2 hours ago, Mr Punter said: We have used the membrane with scouring pad / pubic hair topping like @Russell griffiths used. I think it means that you don't get a puddle of condensation directly on the zinc. I think the risk is reduced with some of the zincs with pre-patinated finish. I'm sure it does prevent pooling and build up of condensation against the zinc and as it lets it run off the roof it's got to be a good thing. The membrane I recieved with my coated steel has a fleecy backing to it but more normal fibre weave on the top. 2 hours ago, the_r_sole said: but you also have a void between the rafters so you're ending up with loads of void - it kinda looks like a detail produced by someone who doesn't like warm roofs! incidentally, the technical standards in scotland encourage the use of warm flat roofs over cold roofs due the condensation risks! Yeah, I agree it's really weird. There are certain types of warm roofs covered within the guide to good practice, but these tend to be warm roof systems, for example WarmFast, structural insulation board or composite metal faced insulated panels. In most cases however, there's a big emphasis on the number of fixings that penetrate the vapour barrier. As such in these cases they recommend the metal roofing contractor is used to fit the whole roof substrate. So the concern is definitely about moisture ingress as I quote from the manual on warm roofs: "The vapour barrier must be fully supported, self-adhesive, bitumen-backed aluminium foil sealed at the laps, to perimeter abutments and around penetrations. The vapour barrier must have the ability to self-seal when penetrated by through fixings.....Unless a Composite Panel System with cellular glass insulation is used, there is a risk of failure, most often caused by the vapour barrier not being laid and sealed correctly thus allowing eventual moisture ingress....no means of escape and therefore condenses and corrodes the underside of the metal or denigrates the timber.....This can particularly be the case with a structural warm roof installed by a generalbuilding contractor prior to the specialist metal roof installer's arrival." It also differentiates between vapour barrier and vapour control layer, how and where they should be used. Whoever wrote this doesn't have a lot of confidence in general building contractors either it seems! Under these circumstances it does seem sensible and easier to simply build in a 50mm ventilation space at least for this type of roof. 1 Link to comment Share on other sites More sharing options...
Kevin J Posted February 4, 2021 Share Posted February 4, 2021 On 03/02/2021 at 09:15, GraemeHM said: Hi, We are getting conflicting advice so any thoughts that might help us get through this very much appreciated. We are building a 4m deep x11m long single story extension on the back of our house, mono-pitched roof joined to the exiting house. Due to the lay of the land and upper story windows we are limited for height at the back/house side (3m) and still want to achieve a fall above 5 degrees. There are two questions this raises. 1. One installer recommended a vented or cold roof zinc installation (something to do with moisture trapped during installation of the zinc) where as our design/architect guy recommended a warm roof i.e. unvented. Has anyone got views on which is better and/or any advantages of each? 2. If we were to go down the vented route. I have been unable to see how the back/house side of the venting would work and upstand for that matter. I have seen venting details on roof ridges. Has anyone got any experience of how that might work? Again any help much appreciated. Thanks Graeme Extension 26-01-21 - Drive side view.tif 125.16 kB · 8 downloads Link to comment Share on other sites More sharing options...
Kevin J Posted February 4, 2021 Share Posted February 4, 2021 A simple ventilated abutment detail against the house is easy. However, continuous airflow from ridge to eaves is interrupted by the roof lights. architect guy is on it with warm roof build up provided the Vapour Control layer (VCL) is correctly specified and correctly installed. A solution could be Hard Metal Board over Alutrix VCL. You should belt and braces with a Structured Underlay such as ISO-Mat Metal. All at www.roofing-tools.com including the Zinc Link to comment Share on other sites More sharing options...
Kevin J Posted February 4, 2021 Share Posted February 4, 2021 9 hours ago, Mr Punter said: We have used the membrane with scouring pad / pubic hair topping like @Russell griffiths used. I think it means that you don't get a puddle of condensation directly on the zinc. I think the risk is reduced with some of the zincs with pre-patinated finish. ALWAYS ventilate Zinc, it cannot sit in condensates. If there is moisture and no Air the formation of Zinc Hydroxide, a White Powder will form and literally eat through the metal from the underside Link to comment Share on other sites More sharing options...
Kevin J Posted February 4, 2021 Share Posted February 4, 2021 9 hours ago, Mr Punter said: We have used the membrane with scouring pad / pubic hair topping like @Russell griffiths used. I think it means that you don't get a puddle of condensation directly on the zinc. I think the risk is reduced with some of the zincs with pre-patinated finish. Link to comment Share on other sites More sharing options...
Kevin J Posted February 4, 2021 Share Posted February 4, 2021 I'm fairly confident @Russell griffiths Used GreenCoat steel, not Zinc, as I supplied it. Long sheets in Snaplock if it's "hairy chest rug" or "pubes" as someone called it I can reveal it is now branded as ISO-Mat Metal Structured Underlay at www.roofing-tools.com Hi @Russell griffiths still hoping to get pics, best regards Link to comment Share on other sites More sharing options...
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