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HI all, I have used this forum for years but finally have to pose my own question rather than simply leech off others… I am replacing the existing felt flat roof on my 1970s kitchen extension with a warm roof build up of (existing) 18mm ply deck / Alutrix vapour barrier / 150mm PIR / 18mm ply / EPDM. I would like to add a flat roof light while I am at it which brings me to my question(s)… The roof dimensions (exterior) are 3.8m x 4.2m and the joists are 150 x 50mm at 40cm centres spanning 3m. I have currently narrowed down to two options which are as follows: 1. Install a 1m x 2m rooflight which would mean cutting 4 joists - I was planning to treble the joists at both ends and double the headers on the long sides (sistering joists and using hangers/fixings etc as outlined in building regs). 2. Install 2 x 1m x 1m roof lights which would mean cutting two joists for each - in this case I would only double the joists at either end but still double the headers. This design would leave two whole joists between the two roof lights. Option 1 is both cheaper and easier in that the EPDM could be a challenge with two roof lights to work around and clearly one window is cheaper than two! However, i feel that option 2 is better structurally but I would really appreciate some input as to the proposed design for option 1. One corner will also sit above a window lintel which is unavoidable in either lay out! Any advice would be greatly appreciated! Many thanks in advance, Will

Hi all, I was wandering if anyone could help me with suggestions as to how to marry up my new warm roof build up with my neighbour’s property? The issue is that my roof was installed about 50 years ago and since then my neighbour’s property has been extended meaning there is no access to the existing fascia etc. to make this even more fun, I am going to add a warm roof build up of 170-175mm (Velutrix, 150mm PIR, 18mm OSB, GRP over existing deck). This brings the new height to around halfway up their fascia board. I have my neighbour’s consent to take my roof to meet their wall and close the 40-100mm gap which currently exists but I would appreciate any thoughts as to how best to do this while allowing the GRP to run high enough… ie. Should this be behind their fascia board? I have added a picture for reference - the gap is below the gutter line although it does not really show very well. Any input would be greatly appreciated! Many thanks in advance…

Hi Can I put mineral wool between solid joists on a flat roof - see red on the graphic. TIA

Hi Guys, I have two questions. 1, all the images I have seen for warm flat roofs look like a sandwich with board, insulation, board. However im struggling to find any information on what to put around the outside and how I attach this to the flat roof, I could use OSB but then screw it into the top and bottom deck? Or do you run a batten on the inside of the sandwich, or not do anything. Im using 150mm insulation so was thinking of running a 150x40mm piece of timber along the edge of the roof to double up as something to attach the fascia etc too and enclose the roof and give me something to run the vapour barrier up around the insulation. 2, On one side of the flat roof it buts up against the second floor of my timber frame, however this is supposed to have woodfibre board insulation on the outside. Im not sure if I should run the roof upto the woodfibre, or install the roof under the woodfibre and bring it down on top, but then I worry about any water running onto it and keeping it damp. Hope that makes sense. Paul

Hello all! I'm looking to re-felt an existing flat roof on a town house in Crawley. It hasn't been touched for a good 30 years, although it has been repaired so is leak-free. I had a telephone chat with a duty surveyor for Sussex Building Control who seemed to suggest that a building control application isn't needed This is because the deck and below isn't being replaced, only the top felt, with additional warm roof being added on top of the existing structure. Would be good to know of anyone else's experiences! Thanks. Below is a rough outline of what we are planning. * Remove all existing roof covering to bare timber deck * Apply SA bitumen primer to whole of roof * Apply first layer of torch safe vapour control layer * Fix 50 mm timber kerb around whole of outside edge * Lay 50mm flat roof insulation boards * Create new timber outlets to downpipes * Apply torch on sanded underlay * Fix 100mm GRP drip trims to outside edge * Apply Technotorch mineral capsheet to finish

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

Hi all I am looking to have a warm flat roof done. I have a fair amount of Kingspan K103 remaining from doing my floors. My flat roof is being made so it is suitable for walking on (we will be putting a floating Trex decking system on top). I was looking to use the Kingspan K103 for the roof, I spoke to Kingspan today to ask if it would be okay to do so, they said they cant say whether or not it is because it has only been tested for floors. I would assume it would be okay as if its suitable for walking on via a floor then they same would apply on a roof. Unless there is an issue as this type of board is not foil faced, I don't have very much experience with insulation as this is my first big project so any pointers would be appreciated. I also haven't decided what would be the best roof finish before putting down the composite decking on top, every roofer seems to give conflicting advice. Thanks in advance

Hi all, I've had some pretty amazing replies to previous posts and thought I'd best carry on picking your brains. Im building a garden room under the permitted development route. It's 2 skinned using concrete block. I've kept the dpc as low as possible but worrying about keeping the finished height within the 2500mm. Ive got french doors to go in that are 2130 including the cill which doesn't leave much for the flat roof. The flat roof including 100mm insulation, 2 sheets of ply, timbers to span 3400 and internal skin plus the required fall for a rubber roof. The dpc is about 120mm above ground level. Any advice would be appreciated.

Now this may be a daft question and the answers may already be out there on the forum - I'm speedily working through all the posts and could have missed the relevant info. I've read all the roofing posts - currently on foundations at the minute then that sections done, but a thought keeps niggling at me about the warm roof VCL that I can't recall reading about. After reading @ProDave and others numerous plugs about sarking and warm roofs etc. I am sold that it is absolutely the way forward in my build. It seems a no brainer to me. So, as I understand it the roof makeup more or less is tiles, tile batten, counterbatten, breathable membrane, sarking (I'm sold on wood fibreboard), rafters, insulation fill between & below, VCL, plasterboard. Right as I understand it, the internal VCL stops warm moist air from the house permeating into the insulation where it could condense at dew point in the insulation. The external breathable membrane is there to stop moisture from rain etc. coming back into the structure but let moisture that may be in the insulation out - a one way system like a diode? If my thinking is correct about the above, my niggle is how do I continue the VCL down from under the rafters to the external wall of the rooms below to ensure a sealed box essentially for the whole house? Do I have to faff about bringing the VCL down between each joist at 600 centres and then taping/sealing around each one? A lot of the pictures I have seen show the VCL in the bedrooms running along the ceiling - but surely that creates two separate boxes - the roof and then the house. This would also mean every penetration through bedroom ceilings would need careful attention to ensure an air tight Seal? If I do then so be it? Just after a bit of clarity from those who have been there and have the t-shirt so to speak! TIA for any guidance. Jamie

Currently at structural engineering stage for building regs submission and looking at roof covering loading. The choice of waterproofing is subjective and well documented; single ply, bituminous, EDPM, GRP - seems that everyone has a preference or horror story. Anyway more fundamentally my design is showing a cold roof design for a new build flat roof extension (parapet roof), however having researched flat roofs the majority of advice is to go for a warm roof to eliminate interstitial condensation (eg Bauder, NFRC) . As I chose the timber frame provider to avoid this issue at the sole plate I'm thinking that I'm walking into this problem up above. Any experience or thoughts welcome.