Warm roof - which vapour control layer and which PIR?

[BotusBuild]

The time is approaching for the roof to go on at Botusbuild towers 🙂 

It's a warm roof construction, essentially a flat roof (single pitch at about 9 degrees). From the web joists (400mm c/c) it will be constructed as follows:

1. 18 or 21mm OSB or ply

2. vapour control layer

3. 150mm PIR

4. 18mm tongue and groove OSB (GRP installer recommendation)

5. 2 layers 600 weight GRP

6. In time, a green roof will be installed

 

Questions:

1. Any views on the choice of first layer board?

2. Recommendation on the vapour control layer to install

3. So many to choose from - which PIR should I use?

[jamiehamy]

We have similar build up. 

 

PIR - we used tissue faced Kingspan TR27. 

VCL - We used both Alutrix 600 and Elotene DSN. Note - they both require a primer and the primers for each are different. These are 'self adhesive' and pretty easy to apply, although application of the primer is horrible. 

We have 18m T&B OSB3  (600x2400)as the base (first layer) - T&G much better because the joins can be mid span. 21mm will be very heavy and overkill. 

 

We didn't other with boards on top of the PIR, we contact adhered the EPDM to the tissue faced boards. Worked fine but can see the joins slightly. 

[Iceverge]

10 minutes ago, jamiehamy said:

We didn't other with boards on top of the PIR, we contact adhered the EPDM to the tissue faced boards. Worked fine but can see the joins slightly. 

 

I prefer the idea of not trapping timber between two impermeable materials. A recipe for failure in my opinion. The tissue pir is better.

 

Do you think the boards shrank?

 

 

51 minutes ago, BotusBuild said:

single pitch at about 9 degrees

 

 

You could consider a hybird roof easily at that pitchI would say.

[BotusBuild]

3 hours ago, Iceverge said:

a hybird roof

A what? Can you provide an example

[BotusBuild]

3 hours ago, jamiehamy said:

We used both Alutrix 600 and Elotene DSN.

Why both?

[Dave Jones]

another vote for alutrix. be sure to bring it down onto the internal walls.

Edited April 13, 2023 by Dave Jones

[jamiehamy]

15 hours ago, BotusBuild said:

Why both?

Different builds! I think Alutrix was cheaper on the first one, that was all. 

 

 

[jamiehamy]

19 hours ago, Iceverge said:

 

I prefer the idea of not trapping timber between two impermeable materials. A recipe for failure in my opinion. The tissue pir is better.

 

Do you think the boards shrank?

 

Doing it again, I would use thermally broken fasteners on each corner as well as the adhesive. I suspect where it has happened maybe the board is very slightly warped and the edge is a mm up from the adjoining one. 

[Iceverge]

i'm not really a fan of PIR. 

 

Rockwool have a warm roof system or I can't see any reason EPS couldn't be used. Has anyone done this?

 

 

[Dave Jones]

14 hours ago, Iceverge said:

i'm not really a fan of PIR. 

 

Rockwool have a warm roof system or I can't see any reason EPS couldn't be used. Has anyone done this?

 

 

 

depth

[BotusBuild]

On 13/04/2023 at 09:23, Dave Jones said:

be sure to bring it down onto the internal walls.

Can you show a diagram of this so I totally understand what you mean?

Thanks

[Dave Jones]

9 hours ago, BotusBuild said:

Can you show a diagram of this so I totally understand what you mean?

Thanks

 

 

[steveoelliott]

I have asked this question before but never got an answer but I am intrigued...

 

If a VCL is installed correctly, would it effectively mask a minor leak in the roof membrane owing to the fact water would presumably be directed down the internal wall cavity.

Edited April 18, 2023 by steveoelliott

[saveasteading]

Yes. Any water getting through the outer covering will reach the vcl and run down to the eaves, where the detailing should allow for it to drip outside, not into the cavity.

[SteamyTea]

On 13/04/2023 at 17:45, Iceverge said:

i'm not really a fan of PIR. 

 

Rockwool have a warm roof system or I can't see any reason EPS couldn't be used. Has anyone done this?

 

 

Not if the area is being coated with glass mat and polyester resin. Just the styrene fumes can cause a significant problem.

[Iceverge]

6 minutes ago, SteamyTea said:

Not if the area is being coated with glass mat and polyester resin. Just the styrene fumes can cause a significant problem.

 

How about the bonded rubber type? Isn't there normally some kind of timber product between the insulation and membrane anyhou? 

[SteamyTea]

5 minutes ago, Iceverge said:

 

How about the bonded rubber type? Isn't there normally some kind of timber product between the insulation and membrane anyhou? 

Rubber should be alright, does depend on what adhesive is used.

A styrene molecule (C₈H₈) is around 160 nm, so can easily pass though most timber boards. 

My polycarbonate lenses in my glasses disintegrated after a few days in my factory. Had to go back to glass lenses.

 

[steveoelliott]

2 hours ago, saveasteading said:

Yes. Any water getting through the outer covering will reach the vcl and run down to the eaves, where the detailing should allow for it to drip outside, not into the cavity.

Based on this, one might never know about a leaking roof (unless severe).

[Radian]

44 minutes ago, SteamyTea said:

My polycarbonate lenses in my glasses disintegrated after a few days in my factory. Had to go back to glass lenses.

 

I'll try to remember that next time I attend a riot. 

[saveasteading]

29 minutes ago, steveoelliott said:

Based on this, one might never know about a leaking roof (unless severe).

So that is a good thing, that the  sign of the issue isn't a wet ceiling.

It isn't difficult to do properly, and simple laps are sufficient to make the water run away properly.

BUT as we know on BH some simple things are beyond the understanding of some people calling themselves builders. I get your concern though. It can't be good that a roof is leaking all the time, for ever really.

I think most drips will be small ones, and dry out in the vented space.

problems with tiles and slates are usually visible with occasional inspection.

screws on metal roofs can leak, but usually seal themselves just with gunk over time.

 

If there is a big leak, then it may be visible by the time it reaches the ground, and pretty well in line with the problem, which can then be searched for and sorted.

 

Is there any other concern?

[steveoelliott]

15 minutes ago, saveasteading said:

So that is a good thing, that the  sign of the issue isn't a wet ceiling.

It isn't difficult to do properly, and simple laps are sufficient to make the water run away properly.

BUT as we know on BH some simple things are beyond the understanding of some people calling themselves builders. I get your concern though. It can't be good that a roof is leaking all the time, for ever really.

I think most drips will be small ones, and dry out in the vented space.

problems with tiles and slates are usually visible with occasional inspection.

screws on metal roofs can leak, but usually seal themselves just with gunk over time.

 

If there is a big leak, then it may be visible by the time it reaches the ground, and pretty well in line with the problem, which can then be searched for and sorted.

 

Is there any other concern?

Thanks for the response... I was talking about the scenario of a flat roof actually not a pitched roof with sarking / felt on the underside of tiles.

[Iceverge]

When out house was built (cold loft) we got a few showers of rain before slating. 

 

It allowed me to spot a hole where the chippie had been playing Rambo with the Paslode and tried to get 50 nails into one spot in a 45mm rafter.  

 

A bit of tape and good as new. A sealed flat roof could have a drip internally for quite some time. Soaking into the ply whatever without ever having the opportunity to fix it during construction. 

[BotusBuild]

Back to the VCL. For the flat roof build up in the OP, if I've laid OSB over the top of the joists how the heck would I get VCL to go down, past the joists as suggested by Dave Jones. Doing that would be a ballache. Surely the VPC should go up and over the wall, with the PIR on top of it?

FYI, the exterior walls continue up above the joists to withhold the warm roof and then a green roof. The GRP will also go up and over the external walls.

I'll add a drawing later, and maybe a photo or two

[BotusBuild]

[Dave Jones]

is it a parapet roof ?