Moonshine Posted February 13, 2020 Share Posted February 13, 2020 i am looking to construct a flat warm roof and now my attention has focused to how to get water off the roof(s) to down pipes. Attached is WIP roof plan, showing 1:40 falls, going to 300mm wide (is this enough?) box gutters that drain to down pipes. Also attached is the work in progress section E-E, which shows the roof make up but is going to be edited, as its currently 150mm PIR, but will go down to 100mm, and needs furrings shown for the 1:40 fall (where do the furrings go? above joists and below roof deck board?) I need to put the box gutter in, but not to sure how to detail this, and how deep do they need to be? Is it acceptable to have an area with thinner amount of PIR under, as below? 204_-_roof_plan.pdf 209_-_Section_E-E.pdf Link to comment Share on other sites More sharing options...
Dreadnaught Posted February 13, 2020 Share Posted February 13, 2020 (edited) A few observations from a layman: - Would a single downpipe be sufficient for a roof, especially your large roof section; or prone to a single blockage causing flooding? - Do you actually need the parapet to surround all of the roofs? For parts that may not be visible to you, you could leave out the parapet and instead have run-off directly from the roof into a gutter. I will have this on two full sides of my flat roof: a greatly reduced risk of a drain pipe blockage being acutely problematic. - [Minor] Doesn't coping have a drip feature on both edges? As to your Warm roof, to me that looks like a hybrid roof with (PIR) insulation above the structural joists (a warm roof) but also between the joists (a cold roof). If it were me I would be wanting a dew-point analysis (WUFI analysis) on that to insure against interstitial condensation, as you do not have any form of ventilation for the lower layer of insulation. And this is relevant for the section of you highlight in red. In that area, in the depths of winter, that part of the insulation between the joists immediately beneath the red section will be colder, with the concomitant risk of interstitial condensation forming particularly there. That would concern me. Edited to add: no service void for your ceiling? Edited February 13, 2020 by Dreadnaught Link to comment Share on other sites More sharing options...
Onoff Posted February 13, 2020 Share Posted February 13, 2020 This is a good read ref @Barney12''s gutter detail: Link to comment Share on other sites More sharing options...
Moonshine Posted February 18, 2020 Author Share Posted February 18, 2020 On 13/02/2020 at 11:59, Onoff said: This is a good read ref @Barney12''s gutter detail: Thanks, that is very nice solution, though i can't see how the insulation sites around that gutter former. @Barney12 can you advise? plus my design has a parapet wall Link to comment Share on other sites More sharing options...
Moonshine Posted February 18, 2020 Author Share Posted February 18, 2020 On 13/02/2020 at 11:22, Dreadnaught said: A few observations from a layman: - Would a single downpipe be sufficient for a roof, especially your large roof section; or prone to a single blockage causing flooding? - Do you actually need the parapet to surround all of the roofs? For parts that may not be visible to you, you could leave out the parapet and instead have run-off directly from the roof into a gutter. I will have this on two full sides of my flat roof: a greatly reduced risk of a drain pipe blockage being acutely problematic. - [Minor] Doesn't coping have a drip feature on both edges? As to your Warm roof, to me that looks like a hybrid roof with (PIR) insulation above the structural joists (a warm roof) but also between the joists (a cold roof). If it were me I would be wanting a dew-point analysis (WUFI analysis) on that to insure against interstitial condensation, as you do not have any form of ventilation for the lower layer of insulation. And this is relevant for the section of you highlight in red. In that area, in the depths of winter, that part of the insulation between the joists immediately beneath the red section will be colder, with the concomitant risk of interstitial condensation forming particularly there. That would concern me. Edited to add: no service void for your ceiling? Thanks some very apt points, a few responses I don't know for sure if that area (max 23.5 m2) needs two down pipes, i am hoping to use wide box gutters to negate the need for more down pipes. Where could i get a definitive answer? yeah the planning design is for a parapet wall. The warm roof / dew point is a very good issue to raise, and i have a bit of a calc of the two as attached and i think that i will stick with 150mm PIR above the roof deck and scale down the insulation in the cavity to 100mm. the condensation risk / poor insulation under the box gutter with 100mm PIR under is my main concern and i don't know how to get round the issue, and how it is done elsewhere. would the service void go beneath the joists? 25mm? U-Value Calculator - ChangePlan flat roof with 100mm PIR board.pdf U-Value Calculator - ChangePlan flat roof with 150mm PIR board.pdf Link to comment Share on other sites More sharing options...
Mr Punter Posted February 18, 2020 Share Posted February 18, 2020 If you have a parapet all round you must have 2 outlets. Why not scrap the box gutter and just direct the roof fall to the outlets? What is the proposed roof waterproofing? Link to comment Share on other sites More sharing options...
Barney12 Posted February 18, 2020 Share Posted February 18, 2020 4 hours ago, Moonshine said: Thanks, that is very nice solution, though i can't see how the insulation sites around that gutter former. @Barney12 can you advise? plus my design has a parapet wall Hi, the make up is as follows: 200mm celotex (between rafters) breather membrane 50mm batten OSB Roof Sheet Fibreglass The gutter is in front of this detail thus I have a circa 60mm gully without comprising the insulation. 1 Link to comment Share on other sites More sharing options...
Moonshine Posted February 18, 2020 Author Share Posted February 18, 2020 (edited) 5 hours ago, Mr Punter said: If you have a parapet all round you must have 2 outlets. Why not scrap the box gutter and just direct the roof fall to the outlets? What is the proposed roof waterproofing? is that two outlets per side of the ridge and 1:40 fall, and surely that can't be 2 outlets for each of the smaller sections. I have been looking at scupper drains that would not compromise the insulation, also could have the main scupper drain connected to a down pipe, and a couple of overflow scupper drains that weren't connected to down pipes, these would be emergency over flow, that would be stepped up 10-20mm so they only were in operation is the main drain was blocked. tbh i haven't looked at what i want to waterproof it with, what is typical for a flat roof? Edited February 18, 2020 by Moonshine Link to comment Share on other sites More sharing options...
Mr Punter Posted February 18, 2020 Share Posted February 18, 2020 Check with your warranty provider and building inspector for the required spec. Flat roofs are a big source of claims. There is no standard covering for flat roof. Can be felt, single ply (lots of types), liquid, fibreglass or GRP. It may be easiest to have thin deck on your joists, then firrings to fall direct to outlets (no box gutter), thin deck and insulation then deck to take the waterproofing. Link to comment Share on other sites More sharing options...
Tony C Posted February 18, 2020 Share Posted February 18, 2020 (edited) I have similar flat roof planned with parapet, Buildup : Roof deck plywood VLC adhered Tapered Kingspan insulation adhered IKO amourplan single ply PSG (fleece backed) adhered roof outlet https://www.wallbarn.com/protection-drainage/roof-drainage-outlets/through-wall-outlets/ The parapet needs to be at least 150mm to get a 20 year warranty from roofing manufacture. Single ply roofing will be more expensive but I am hoping it will last longer! Our architect warned that hybrid cold/warm roof is difficult, as it can cause condensation issues. They suggested this is avoided. Instead of adding warm roof on top, how about to add insulated plasterboard on ceiling internally? Edited February 18, 2020 by Tony C Link to comment Share on other sites More sharing options...
Onoff Posted February 18, 2020 Share Posted February 18, 2020 4 hours ago, Barney12 said: Hi, the make up is as follows: 200mm celotex (between rafters) breather membrane 50mm batten OSB Roof Sheet Fibreglass The gutter is in front of this detail thus I have a circa 60mm gully without comprising the insulation. That's the roof make up? Would that not lead to cold bridging through the joists or is there extra insulation somewhere? Everytime I look at my abortion of a dormer I think about incorporating sound of your details when I get around to redoing it! Link to comment Share on other sites More sharing options...
Russell griffiths Posted February 19, 2020 Share Posted February 19, 2020 I base a lot of my views on what I have seen fail over the years. I would say that most flat roofs fail because of inadequate fall and water gathering i would say that most gutters fail because the water cannot get out quickly enough i would design it to get that water into the gutter as fast as possible, then get it out of the gutter and down a pipe as fast as possible. There used to be rules regarding one gutter outlet every so many metres, now I see a whole house roof going down two outlets one front one back. Completely different to your situation, but I have a massive roof area all at a low pitch and my first thought was get that water off that roof get it in a gutter and get it on the floor quick smart. Link to comment Share on other sites More sharing options...
Russell griffiths Posted February 19, 2020 Share Posted February 19, 2020 I notice from your pdf that you have one roof ejecting water onto a lower roof, these are the areas you will need to look at carefully as the increased volume of water can overwhelm the lower gutter. Link to comment Share on other sites More sharing options...
Barney12 Posted February 19, 2020 Share Posted February 19, 2020 10 hours ago, Onoff said: That's the roof make up? Would that not lead to cold bridging through the joists or is there extra insulation somewhere? Everytime I look at my abortion of a dormer I think about incorporating sound of your details when I get around to redoing it! Sorry, I didn’t add the internal elements. From inside to out: Plasterboard Service batten 50mm celotex 200mm celotex (between 225mm rafters) breather membrane 50mm batten OSB Roof Sheet 1 Link to comment Share on other sites More sharing options...
Onoff Posted February 19, 2020 Share Posted February 19, 2020 9 minutes ago, Barney12 said: Sorry, I didn’t add the internal elements. From inside to out: Plasterboard Service batten 50mm celotex 200mm celotex (between 225mm rafters) breather membrane 50mm batten OSB Roof Sheet Gotcha, so 25mm air gap between top of Celotex and breather membrane. Link to comment Share on other sites More sharing options...
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