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Warm flat roof edging and external wall insulation


hotnuts21
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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

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7 hours ago, hotnuts21 said:

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

So I boarded around 150mm past the edge of the external wall. Then attached a wooden edging, they call it a ladder. Used 2 by 4 I think. Used this to have something to fasten facia to and also as edge support for insulation which would otherwise squash when walked on. We also had wood fibre insulation and also had one roof butting up to a 2nd floor wall. What I did was stop wood fibre around 200m short and then did last 200mm with PIR so it wouldnt track water up. I then fastens osb to this and then ran the epdm from flat roof up up onto osb. Then we had the vapour layer over woodfibre which also went over epdm. Batton, counter batton and then wood cladding. Cladding stops about 150mm short of epdm roof

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This is great do you have any photos or diagrams of the buildup as you did it by any chance?  I only need to overhang one side due to neighbours, but it's perpendicular to the joists which makes the ladder a bit more awkward.

 

Love the idea of using pir below the WF so obvious but didn't think of it, even though I'm using XPS below the DPC floor in the same way

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1 hour ago, Dave Jones said:

unless its gong to be walked on you dont need the top OSB, EDPM will glue to glass fibre backed insulation no problem.

 

What make of vapour seal are you using ?

I didn't use a top osb, did use tissue backed high compression pir for flat roofs. It still squashes a bit if you tread on a corner, hence putting the timber frame all round

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14 hours ago, hotnuts21 said:

This is great do you have any photos or diagrams of the buildup as you did it by any chance?  I only need to overhang one side due to neighbours, but it's perpendicular to the joists which makes the ladder a bit more awkward.

 

Love the idea of using pir below the WF so obvious but didn't think of it, even though I'm using XPS below the DPC floor in the same way

 

20200804_205050.jpg

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Cheers @redtop those photos are really handy.

 

To answer your questions @Dave Jones 
1,I will be using Alutrix 600 vapour barrier and bring it up above the insulation on the sides to it meets the EPDM where I can.   

2,The insulation I am using is celotex xr4150 which according to the celotext tech dept requires 18mm min board above and below the insulation otherwise they wont support/guarantee any issues (not that they do anyway). Plus we want to put sedum/greenroof trays on the roof in the future so it makes sense to have it boarded on top.

3, We also have a rooflight so I was going to use the same 150x40 round the edge of that and bring the VB upto and over the top of that.  The velux rooflight has its own upstand which essentially sits on top of the deck, and has its own VB that hangs down, which I can then tape to where it meets the alutrix where its come up and over.

 

The only thing I cant get my head around is if i should put the vertical pir onto the bottom board deck before or after the VB is laid, eg, do I run it up under the celotex and make that part of the flat roof. Im also unsure about the front of the house where the flat roof meets the second story but the wall below continues. Maybe a external corner patch there.

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the alutrix needs to be under the insulation though and sealed to the walls inside the house. No punching holes through it for downlighters etc!!!

 

It's bloody good stuff and will waterproof the roof on its own for something like 6 weeks.

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My understanding was that it went under the insulation then up the outside of the wall and upstands to just above insulation/top deck level so it joins up with the epdm essentially encapsulating the insulation (in a warm roof) so no moisture gets in from below or above.  But yes I agree I hear its great stuff, once applied to the bottom deck, I think its 5 weeks guaranteed water/weatherproof giving you time to get the insulation and top deck on (essentially a two weekend job).

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It needs to go under the insulation as a dewpoint will form inside the insulation and this is where the mould will come.

 

You are sealing it from the house so nothing can get into the insulation as it cannot breath.

 

You will have a mouldy roof unless you do the VCL properly. Sorry not good news.

 

Here is a good vid explaining it, he has many examples of what you currently have full of mould.

 

 

 

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9 hours ago, Dave Jones said:

looks like your missing a vapour barrier as it should have been brought up around the skylight upstands (they need to be insulated as well) ?

Half right, and I half screwed that bit up. Vapour Barrier in, but didn't bring it up roof lights so ended up insulating from inside. Not ideal...

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2 hours ago, Dave Jones said:

and sealed to the walls inside the house

David I think we are both on the same page, maybe just not explaining ourselves clearly ? 

Your quote above threw me, as the vapour barrier doesnt go inside the existing house, it goes on top of the lower osb under the insulation which is technically outside the house when your building it ? and then up the outside of the wall at the side/back.   Ive watched all Steves (london roofer) videos so I hope im on the right track as I plan to install it exactly as he suggests ?

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38 minutes ago, redtop said:

Half right, and I half screwed that bit up. Vapour Barrier in, but didn't bring it up roof lights so ended up insulating from inside. Not ideal...

Were you able to lap the vapour barriers together, from the bit you insulated inside?

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10 hours ago, hotnuts21 said:

David I think we are both on the same page, maybe just not explaining ourselves clearly ? 

Your quote above threw me, as the vapour barrier doesnt go inside the existing house, it goes on top of the lower osb under the insulation which is technically outside the house when your building it ? and then up the outside of the wall at the side/back.   Ive watched all Steves (london roofer) videos so I hope im on the right track as I plan to install it exactly as he suggests ?

 

 

essentially what you are trying to achieve with the alutrix  is to seal the inside of the house from the roof makeup which is why you want it lapping down the walls on the inside.

 

I cant see how you can do this putting it outside as you cant lap it down all the inside walls.

 

With regard to the rooflights, insulate the upstands and bring the alutrix up the inside of them and lap it over the lip the light will sit on.

 

If you get any air movement into the insulation from inside the house you are going to end up ripping it all out as it will  rot 100% as once the moisture is in the makeup it cant get out.

 

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3 hours ago, Dave Jones said:

which is why you want it lapping down the walls on the inside

HI Dave, can you point to any info on lapping it down into the walls, im geniunely interested how this works, as its not something ive come across and cant get my head around how you go down below the deck.

 

This video at about 2mins in talks about lapping it up the wall not down, into the roof structure.

 

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  • 1 month later...

Insulation Deck Flat Roof Insulation

All edges should be supported - add noggings where necessary.

Stagger fixings where boards are butted.

Boards should be embedded in vapour resistant mastic to provide a vapour control layer in conjunction with foil facing.

Mastic should be laid wide enough to facilitate 2 panel edges and be continuous around all edges. 

OSB Insulation Deck should be staggered and butted.

Each edge should have a minimum bearing of 20mm on joist...

 

osb pir board: https://insulationgo.co.uk/129mm-xtratherm-flat-roof-thermal-ply-pir-9mm-osb-deck-eco-insulation-rigid-vcl-decking-td4000-tr31-board-126mm/

ply pir board: https://www.roofingoutlet.co.uk/collections/flat-roof-insulation-boards/products/warmline-insulated-decking-board-126mm

ply pir board: https://insulationgo.co.uk/126mm-xtratherm-frtp-flat-roof-board-ply-pir-6mm-wbp-plywood-insulation-rigid-vcl-decking-td4000-tr31/

ply pir board: https://www.insulationshop.co/126mm_eco_therm_eco_deck.html

 

Warm roof insulation fixings explained:

 

nails https://insulationgo.co.uk/195mm-headed-helical-flat-roof-nails-warm-roof-fixings-pack-of-25/

roof decking: https://insulationgo.co.uk/blog/osb-deck-ply-board-sip-pir-insulated-kingspan-tr31-celotex-td4000-recticel-plylok-xtratherm-fr-tp-99-109-119-129-mm-8-4-2400-1200/

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