MortarThePoint Posted January 16, 2021 Share Posted January 16, 2021 I've been looking at the NHBC guidance for all chimneys and hoped someone could help. The page shows three drawings of DPCs in chimneys, two of which are on pitched sections of roof. I am understanding this as two alternatives, not that there should be a damp tray at the top junction and at the bottom junction. The one with the tray at the bottom 'junction' is on the eave of the roof. Can you use the other approach at the eave instead? Does anyone have a preference for which of the two approaches is adopted if it is a case of just one? "In areas of severe and very severe exposure, the following details should be used. In lower exposure zones, the tray upturn may be on the outside of the flue liner. All other details are the same." We're near Cambridge which is a 'Sheltered' exposure zone I believe. Generally I have found understanding damp proofing one of the hardest aspects to get right as some of the best practice / requirements are not so clearly defined. Link to comment Share on other sites More sharing options...
MortarThePoint Posted January 31, 2021 Author Share Posted January 31, 2021 The LABC guidance looks to have an aluminium cavity tray lower down: Lots of places I look, except the NHBC guidance, has 2 levels of trays: LABC: We've got a chimney at the wall edge and one at the junction between a flat room and a pitched section that comes down from the flat roof. Link to comment Share on other sites More sharing options...
MortarThePoint Posted March 21, 2021 Author Share Posted March 21, 2021 For the chimney on the side wall we had a low level aluminium tray below wall plate and then a Code 4 lead tray higher up to coincide with the back gutter. The chimney is continuing upwards another ~2.5m tomorrow and will hopefully get finished. This takes my focus to the topmost DPC at which I appear to have some options (extract from technical manual below). Fabricating a lead tray is probably not possible now if it needs an upstand as time is too short. The mortar being used for the flaunching will be very stiff (cement 1:3 sharp sand). This chimney has a cavity wall (diagram below). I'd happily incorporate two courses of solid blue engineering bricks just below the corbels, but I don't see how that would help stop damp going down the cavity. What about slate incorporated into the flaunching? Link to comment Share on other sites More sharing options...
MortarThePoint Posted March 21, 2021 Author Share Posted March 21, 2021 @Declan52 sorry to call you out, but do you have any thoughts on this as it's stressing me out. Link to comment Share on other sites More sharing options...
Declan52 Posted March 21, 2021 Share Posted March 21, 2021 Only ever used a single large lead tray made to suit whatever the size of the chimney plus enough to turn up on the other 3 sides. A hole gets cut in the tray to allow the flue to come on through then it gets sealed around with sealant and then built on till it's finished height. This would have been done where the chimney breaks through the roof. Trays then get cut into the brickwork above the tray. Never seen a chimney with 3 lead trays before. At the very top I have seen everything from concrete paving flag to slate to dpc used then a strong motar mix to haunch the flue. Link to comment Share on other sites More sharing options...
PeterW Posted March 21, 2021 Share Posted March 21, 2021 1 hour ago, MortarThePoint said: Fabricating a lead tray is probably not possible now if it needs an upstand as time is too short Why..?? What is the chimney section ..?? As in how square is it ..?? Link to comment Share on other sites More sharing options...
MortarThePoint Posted March 21, 2021 Author Share Posted March 21, 2021 1 hour ago, Declan52 said: At the very top I have seen everything from concrete paving flag to slate to dpc used then a strong motar mix to haunch the flue. Plastic dpc? Link to comment Share on other sites More sharing options...
Declan52 Posted March 21, 2021 Share Posted March 21, 2021 27 minutes ago, MortarThePoint said: Plastic dpc? Yep. I've seen a lot of strange things. Link to comment Share on other sites More sharing options...
MortarThePoint Posted March 21, 2021 Author Share Posted March 21, 2021 I not keen on using plastic. It would probably be fine die to Isokern but doesn't feel right. 2 hours ago, PeterW said: Why..?? What is the chimney section ..?? As in how square is it ..?? I don't have the lead for tomorrow. I could pick it up but I can't weld it. It's 1200x1050 with a cavity Link to comment Share on other sites More sharing options...
PeterW Posted March 21, 2021 Share Posted March 21, 2021 That just got a single flue coming up the middle of it ..? Could do it with 1200mm Code 4 but a 3m roll weighs 87kg or so... would want to make a former for it using 3x2 and some OSB but you could drop the dressed edges on it in an hour. Don’t need to weld the corners, tuck them back and will look ok from the ground. Need the isokern location spot on, find the centre of the circle, scribe the pot diameter then cut an inch inside the line. Gentle heat from a blow torch and warm it and dress it up to the line - flat piece of 25mm ply with the flange size you need helps here and dress it up the inside of the ply. Any reason it has to be done tomorrow ..?? And how are you supporting the lead as it will sag and potentially pool water if there is nothing solid for it to sit on. Link to comment Share on other sites More sharing options...
epsilonGreedy Posted March 21, 2021 Share Posted March 21, 2021 @MortarThePointI have been cogitating the same aspects of chimney construction as you, though in my case I am just debating the upper or lower position for the tray. My conclusion is that because in my case the chimney sits on both the outer facing bricks and inner wall block wall it is more important to protect against moisture descending into the inner blocks with potential for damp in bedrooms. In this situation tying the tray into the top of the back gutter flashing seems optimal. My roof has a 30 degree pitch so that won't leave many bricks exposed on the downhill site of the chimney before the eave flashing offers protection. My chimney is 550mm wide (2 1/2 bricks) in the direction of the roof slope which equates to about 5 courses of bricks exposed between the tray and the eave side flashing assuming 150mm high eave side flashing and 225 high flashing on the back gutter side. Link to comment Share on other sites More sharing options...
epsilonGreedy Posted March 21, 2021 Share Posted March 21, 2021 The following YouTube video demonstrates a technique for forming weep holes to allow the tray to shed moisture. Starts at 3 minutes 15. The video producer runs a business that fabricates lead chimney trays. An interesting feature of his trays is that the flu liners below and above the tray are separated by a circular upstand formed in the tray. Link to comment Share on other sites More sharing options...
MortarThePoint Posted March 21, 2021 Author Share Posted March 21, 2021 @epsilonGreedy You have to tuck the upstand inside the flue liner in severe weather areas, elsewhere you can have it on the outside. @PeterW needs to be tomorrow, last two days for the bricky now and he's kept me waiting a month for them. The hard bit is making the circular upstand. If it wasn't for that it would be easy. Other trays already made so it's only the very top one that's a concern. Link to comment Share on other sites More sharing options...
PeterW Posted March 21, 2021 Share Posted March 21, 2021 8 minutes ago, MortarThePoint said: @epsilonGreedy You have to tuck the upstand inside the flue liner in severe weather areas, elsewhere you can have it on the outside. @PeterW needs to be tomorrow, last two days for the bricky now and he's kept me waiting a month for them. The hard bit is making the circular upstand. If it wasn't for that it would be easy. Other trays already made so it's only the very top one that's a concern. seriously they aren’t difficult. Just needs to be knocked up the inside of the pot - 90 degrees is ideal but even 45 catches the water running down the pot and kicks it out the pot. Link to comment Share on other sites More sharing options...
MortarThePoint Posted March 21, 2021 Author Share Posted March 21, 2021 2 minutes ago, PeterW said: seriously they aren’t difficult. Just needs to be knocked up the inside of the pot - 90 degrees is ideal but even 45 catches the water running down the pot and kicks it out the pot. But the circular upstand needs to be welded on and I don't have oxyacetylene torch etc Link to comment Share on other sites More sharing options...
PeterW Posted March 21, 2021 Share Posted March 21, 2021 No it doesn’t ..! And you definitely don’t want Oxy unless you want to create puddles of lead..! These used be done with paraffin torches - all you are looking to do is anneal/soften the lead so you can bend it into the pot. Link to comment Share on other sites More sharing options...
MortarThePoint Posted March 21, 2021 Author Share Posted March 21, 2021 3 minutes ago, PeterW said: No it doesn’t ..! And you definitely don’t want Oxy unless you want to create puddles of lead..! These used be done with paraffin torches - all you are looking to do is anneal/soften the lead so you can bend it into the pot. I don't understand, the upstand is about 100mm high Link to comment Share on other sites More sharing options...
Declan52 Posted March 21, 2021 Share Posted March 21, 2021 You cut in into wedges from the centre of the hole and soften it and bend it up. It won't look as tidy as that welded one but no one will ever see it. Link to comment Share on other sites More sharing options...
PeterW Posted March 21, 2021 Share Posted March 21, 2021 Doesn’t need to be. It needs to be enough to catch water running down the inside of the pot and divert it to the weeps. The biggest thing is making sure you put a proper base for the lead to sit on otherwise it puddles down the sides and eventually leaks. Link to comment Share on other sites More sharing options...
MortarThePoint Posted March 21, 2021 Author Share Posted March 21, 2021 6 minutes ago, Declan52 said: You cut in into wedges from the centre of the hole and soften it and bend it up. It won't look as tidy as that welded one but no one will ever see it. That sounds simple enough Link to comment Share on other sites More sharing options...
MortarThePoint Posted March 21, 2021 Author Share Posted March 21, 2021 A bit like this ? Link to comment Share on other sites More sharing options...
PeterW Posted March 21, 2021 Share Posted March 21, 2021 Yes but don’t cut to the edges..! Only cut to about an inch from the pot. What are you supporting the tray with ..?? Link to comment Share on other sites More sharing options...
MortarThePoint Posted March 21, 2021 Author Share Posted March 21, 2021 Just now, PeterW said: Yes but don’t cut to the edges..! Only cut to about an inch from the pot. What are you supporting the tray with ..?? It won't fold up without all the little cuts. I won't cut the brim of the hat of course as the chimney isn't round. Around the liner is Leca fill which should support the lead. I could lay slate across the cavity to support the lead. I could also use slate on top of the Leca. Nearest supplier of Code 4 lead at this width that I have found is ~40 miles away unfortunately. Really, I'd rather not have the lead and use a couple of layers of lead and a stiff M12 type mix. Nobody has indicated that I can do that though. Link to comment Share on other sites More sharing options...
Declan52 Posted March 21, 2021 Share Posted March 21, 2021 6 minutes ago, MortarThePoint said: A bit like this ? Yes. It's just tiny cuts and put a heat gun on it and tap tap away until your there. You don't want to cut it too much so baby steps and take your time. Set the flue on the tray and draw around it and mark another circle line 25mm in from the edge. This is your limit. Don't cut past this. Cut the wedges and heat it up and tap it till it's where you want it. 1 Link to comment Share on other sites More sharing options...
MortarThePoint Posted March 21, 2021 Author Share Posted March 21, 2021 9 minutes ago, MortarThePoint said: Nearest supplier of Code 4 lead at this width that I have found is ~40 miles away unfortunately. Really, I'd rather not have the lead and use a couple of layers of lead and a stiff M12 type mix. Nobody has indicated that I can do that though. That made no sense, sorry. Really, I'd rather not use lead and use a couple of layers of slate and a stiff M12 type mix. Link to comment Share on other sites More sharing options...
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