dnb Posted August 27, 2020 Share Posted August 27, 2020 I am a little puzzled by part of one of the drawings submitted for building control. Nobody has argued with it so I am not concerned about mistakes but I obviously don't get everything it is trying to tell me. I can't post the actual drawing until tonight but here is a sketch to illustrate. The architect drawing looks like it has a support tray over the top of the 25mm over fascia vent, but shows the membrane as in my sketch, unsupported by the tray. So something doesn't look right. The trays i have seen can't go under the vent because it will close the vent off and the membrane probably shouldn't go over the vent because it will close off the bit of roof that needs ventilation - my limited experience suggests that roof membrane would make a fine alternative to sail cloth! Am I missing a product that is designed to do this? There isn't much experience locally of SIPS roofs and the architect is on holiday, but it seems a problem that must have a fairly standard solution so I turn to the best supply of building experience I know about. My desire is to get any water on the membrane to drain in the gutter rather than down the fascia - especially not the inside edge of the fascia! Link to comment Share on other sites More sharing options...
bassanclan Posted August 27, 2020 Share Posted August 27, 2020 The roofing membrane should definitely direct water out of the building, upload the actual drawing and we will see. Link to comment Share on other sites More sharing options...
Temp Posted August 27, 2020 Share Posted August 27, 2020 If the membrane is vapour permeable there is no need to ventilate UNDER it. However where slates or interlocking flat tiles are used its advisable to ventilate between the membrane and slates. This isn't necessary when textured hand made clay tiles are used because they don't lay flat and there are lots of gaps. So your sketch looks right. The vents are for the gap between membrane and slate and stop birds getting up there. They should also allow any rain that get blown under the slates to run down and escape. Link to comment Share on other sites More sharing options...
Carrerahill Posted August 27, 2020 Share Posted August 27, 2020 (edited) Post the real drawing so we can see where the lines representing things actually go. Edited August 27, 2020 by Carrerahill Link to comment Share on other sites More sharing options...
Temp Posted August 27, 2020 Share Posted August 27, 2020 This page has various drawings. This one shows over fascia vents above a vapour permeable membrane. Others show ventilation required with "conventional underlay" etc https://www.fastrackcad.com/CADFrame.asp?Company_id=107&LevelEC=1&Level0=Cembrit&Level1=02%24FIBRE+CEMENT+SLATE+APPLICATIONS Link to comment Share on other sites More sharing options...
dnb Posted August 27, 2020 Author Share Posted August 27, 2020 23 minutes ago, Temp said: This one shows over fascia vents above a vapour permeable membra This is more or less what I think the architect meant to draw. I am content the ventilation aspects of the roof itself are all correct. It is getting the detail at the eaves correct that concerns me. It looks like the drawing Temp posted has a support tray under the vent or a combined part. This is probably what I need to find and what is not obvious on the original drawing. I promise to post the original drawing when in get home. Can't do it while at work. Link to comment Share on other sites More sharing options...
ProDave Posted August 27, 2020 Share Posted August 27, 2020 That Support tray is what I used, I got one called Easy Tray I think from Travis Perkins, very cheap. The standard easy to get ventilator stocled by the builders merchants, would not work, I used an "OV10" vent strip which I got mail order from one of the building plastics suppliers. Link to comment Share on other sites More sharing options...
dnb Posted August 27, 2020 Author Share Posted August 27, 2020 That looks like the part I need. I have a box of OV25s so it might just work. Link to comment Share on other sites More sharing options...
Temp Posted August 27, 2020 Share Posted August 27, 2020 Toolstation and Screwfix also sell membrane support trays .. https://www.toolstation.com/eaves-felt-support-tray/p43994 https://www.screwfix.com/p/felt-support-tray-1-5m-5-pack/36622?_requestid=448357 Link to comment Share on other sites More sharing options...
dnb Posted August 27, 2020 Author Share Posted August 27, 2020 (edited) 1 hour ago, Temp said: Toolstation and Screwfix Why would I want to pay their prices? Found trays much cheaper than that, amazingly without needing to mail order!! Looks like there are a few subtly different profiles of tray with different joggles and leg lengths from the drawings. Hopefully I have got one that works for my purpose. Cutting 20 of them would be a pain. Here is the original drawing showing the source of the confusion: Note the tray appears over the vent and does not appear to support the membrane. There appears to be no tray under the vent. Edited August 27, 2020 by dnb Adding picture before I forget. Link to comment Share on other sites More sharing options...
Russell griffiths Posted August 27, 2020 Share Posted August 27, 2020 Be careful with cheap I brought about 25 cheapies and they where very thin and brittle, they split when nailed and if you tried to bend them a bit to alter the run into the gutter they split on the fold, the ones from the local merchants where so much thicker. I chucked them in the bin in the end. Link to comment Share on other sites More sharing options...
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