Roofing questions for the brave!

[ProDave]

24 minutes ago, pocster said:

Yeah this where I get confused 

I'm using roof guard which is a breathable membrane . Will sag 10mm between trusses to allow water run off . Insulation will be between trusses not above ceiling . So I'm guessing I still stop the membrane 30mm short of apex ; perhaps it's irrelevant for a breathable membrane ?

 

cheers 

So the membrane will sag onto, or squash the insulation?

 

I really don't like this droopy system. The Scottish system of using a solid sarking board (OSB or ply) is so much better, and I believe although not required in England, you are allowed to use that method.

[Tennentslager]

6 minutes ago, ProDave said:

So the membrane will sag onto, or squash the insulation?

 

I really don't like this droopy system. The Scottish system of using a solid sarking board (OSB or ply) is so much better, and I believe although not required in England, you are allowed to use that method.

+1

Much better and for only a few dollars more you get improved racking strength and it's easy to walk on!

[Temp]

As I understand it the reason the membrane is deliberately allowed to droop is to prevent water from pooling above tile battens causing them to rot. If the batten presses the membrane against something like a sarking board or insulation then dirt washed down the membrane can accumulate and prevent water draining away.

If the tile battens are raised up on counter battens then it's not an issue.

[Pocster]

Also if there's OSB and taught sarking you'll need cross battens again to avoid pooling of water.

 

I could OSB it out and have the insulation tight (from the underside) to the OSB.

I intend to have the insulation about 10mm or so from the edge of the trusses to allow the membrane to sag; so water can always run off in the event of tile failure or extreme weather. Then on the roof underside I'll insulate (PIR) over the solid insulation again and the trusses .

[ProDave]

29 minutes ago, pocster said:

Also if there's OSB and taught sarking you'll need cross battens again to avoid pooling of water.

 

I could OSB it out and have the insulation tight (from the underside) to the OSB.

I intend to have the insulation about 10mm or so from the edge of the trusses to allow the membrane to sag; so water can always run off in the event of tile failure or extreme weather. Then on the roof underside I'll insulate (PIR) over the solid insulation again and the trusses .

With a sarked roof, you fit a non tenting membrane direct to the sarking board, counter battens that run down following the line of each rafter, then your normal tile battens across.

 

The insulation can then go right up to the OSB inside between the rafters.  Altogether a much better roof construction. Less to go wrong (e.g with a non sarked roof,  insulation pushes membrane up against tile batten, water pools and eventually leaks through membrane into insulation)

 

[Stones]

1 hour ago, ProDave said:

So the membrane will sag onto, or squash the insulation?

 

I really don't like this droopy system. The Scottish system of using a solid sarking board (OSB or ply) is so much better, and I believe although not required in England, you are allowed to use that method.

 

+ another 1

 

I wouldn't consider a roof without sarking. 

 

Our last few houses had slate, nailed directly into whitewood board sarking. Very robust.

[ProDave]

@Stones makes a valid point. If using slate rather than tiles, we use a thinker planked sarking board and the slates nail direct to that no need for battens.

[Jeremy Harris]

Interesting variations.  We have 18mm OSB sarking, 50 x 25 counter battens lined up with the underlying rafters, membrane, then 50 x 25 battens for the slates.  The counter battens allow the membrane to droop enough to form a drainage path under the battens.

 

There's no doubt that sarking makes for a much more robust roof.  In our case it's primarily adding all the racking resistance to the ridge beam hung rafters, but I like the fact that the insulation is retained by the sarking, too. 

[Oz07]

Think you are getting confused here OP.

 

Sound to me like you're putting on a warm roof if insulation is to be between rafters.

 

You will have to leave a 50mm gap not 10mm gap from insulation to the outside face of the rafter. Membrane to droop approx 10mm as you say.

 

You leave the membrane short at the ridge by 30mm because you have a ventilated, dry fix ridge. The air comes into the roof at fascia level and exits at ridge level.

[Oz07]

sorry I'm wrong that is also cold roof what I described but utilising roofspace

[Pocster]

7 hours ago, Oz07 said:

Think you are getting confused here OP.

 

Sound to me like you're putting on a warm roof if insulation is to be between rafters.

 

You will have to leave a 50mm gap not 10mm gap from insulation to the outside face of the rafter. Membrane to droop approx 10mm as you say.

 

You leave the membrane short at the ridge by 30mm because you have a ventilated, dry fix ridge. The air comes into the roof at fascia level and exits at ridge level.

Why 50 mm gap from felt to insulation??

[Oz07]

Because the big bad BCO says so! I dunno it's just standard

[Stones]

To ensure insulation doesn't touch the felt, and to ensure sufficient cross ventilation so that any warm moist air escaping from the house is vented out rather than getting trapped causing the to of to sweat.

 

50mm may well be more than is actually required to do the job, but ensures there is sufficient working gap taking uneven insulation surfaces or sagging felt into account.

[PeterW]

All depends on the membrane too. Cromar for example allow the membrane to touch the insulation. 

 

Really need to find the manufacturers declaration of conformity to get the details on how it is designed to operate. 

[Pocster]

Quick thought .

for the felt on the front nin bit . Do I just run a straight piece across it ? Or a cut a triangle bit to suit . Worried about potential water getting under the overlap . Can draw a pic if that helps explain 

 

[PeterW]

Straight and wrap it over - no issues with multiple layers and as long as it's all bottom up layering with the recommended horizontal overlaps then you will be fine. 

 

It does mean you use more membrane but it's not expensive - my roof is ~ 190sqm but with 6 valleys and three dormers I've ended up with using 270sqm of membrane.  In some areas it's 4 layers thick as the valleys have the overlap from each side plus two layers of their own but I'm not complaining. 

[Pocster]

11 minutes ago, PeterW said:

Straight and wrap it over - no issues with multiple layers and as long as it's all bottom up layering with the recommended horizontal overlaps then you will be fine. 

 

It does mean you use more membrane but it's not expensive - my roof is ~ 190sqm but with 6 valleys and three dormers I've ended up with using 270sqm of membrane.  In some areas it's 4 layers thick as the valleys have the overlap from each side plus two layers of their own but I'm not complaining. 

Ok;

 

Well I drew a nice pic to explain my dilemma ! - which is now wrong !; please see attached pic 1.

[Pocster]

BUT! :-)

 

I'm guessing you are suggesting this?

I don't like those overlapping edges though .......

 

I'm guessing also that the front hip detail does to require 30mm ventilation gap to the ridge??

 

It's highly possible I'm over thinking this. Which is better than not giving it enough thought and getting it wrong! 

 

 

Edited August 13, 2017 by pocster

[PeterW]

So felt runs horizontally from the bottom upwards. Go from right to left in that picture, wrap over the hip rafter and cut off the excess after the first jack rafter. Go back to the last rafter before the hip and start again, wrapping over the hip roof onto the back. Stop, cut and back to the last jack before the hip and wrap onto the back. 

 

Fit your bottom three battens to gauge and start again from right to left,  next layer up overlapping the first by the manufacturers recommended allowance. 

 

On the ridge I would either cut a piece to cover or wrap the last run from the back over the front. 

[PeterW]

Check the MIs carefully..!!

 

Cromar Vent 3 for example does not need any ventilation. You only leave the 30mm if you have a vented ridge and soffit vents. Breathable membrane in most cases does not need any gaps. 

[Pocster]

11 minutes ago, PeterW said:

Check the MIs carefully..!!

 

Cromar Vent 3 for example does not need any ventilation. You only leave the 30mm if you have a vented ridge and soffit vents. Breathable membrane in most cases does not need any gaps. 

Yeah the "to leave 30mm or not leave 30mm' at the ridge seems a bit confusing. My architect is going to decide on that. I have a dry fit vented ridge. But I also (now!) have insulation between trusses (so warm roof). But the membrane is breathable! . So it's ermm, a bit contradictionary - architect can choose.

 

Sorry Peter you're getting technical on me now :-) "jack rafter." ????

 

I'm still somewhat confused :-)

 

BUT!. I will felt and batton 1 straight side; do the hip detail (felt and batton ) then upload a photo.

 

What I don't want to do is be taking tiles off/ walking on tiles or going on the roof again!.

 

Appreciated!

 

p.s Just googled 'jack rafter'. The rafters in-between the hip rafters???

Edited August 13, 2017 by pocster

[PeterW]

2 minutes ago, pocster said:

My architect is going to decide on that.

 

I couldn't think of a worse person to make that decision .....

 

What membrane are you using ..???

 

And a jack rafter is one that runs from the wall plate up to the hip rafter and doesn't go to the ridge...!

[Pocster]

Just now, PeterW said:

 

I couldn't think of a worse person to make that decision .....

 

What membrane are you using ..???

 

And a jack rafter is one that runs from the wall plate up to the hip rafter and doesn't go to the ridge...!

"I couldn't think of a worse person to make that decision ....."

LOL I assumed (!!!!) he'd make a better decision than me! 

 

Membrane is (detail sent to architect also) 

 

http://www.proctorgroup.com/images/downloads/Condensation-Control/Roofshield/Proctors - Roofshield Brochure.pdf

 

The expensive roof shield!. 

[PeterW]

Ok so he's gone Rolls Royce but hey ...

 

This is what you need to read, specifically the section on roof construction. 

 

If it's a warm roof to the ridge i.e. Insulation all the way within the rafters, then no ventilation is needed. Cold roof i.e. Insulation at ceiling level, then you need ventilation. 

 

http://www.proctorgroup.com/images/downloads/Condensation-Control/Roofshield/BBA/Roofshield - Warm Roof BBA (96-3220 i8).pdf

 

 

[Oz07]

To be fair this scenario is still cold roof as far as venting is concerned isn't it. I.e. The insulation will be between and below rafters meaning you will need ventilation to stop condensation on timbers. A true warm roof would have insulation above rafters and no condensation risk.

 

thats my understanding anyway...