Scottish roof (ply under tiles) in England...help please!

[Allthegearnoidea]

Evening all,

 

I would be grateful for a bit of help if anyone has any knowledge in this area!  The structural engineer has specified a 12mm plywood skin on our single storey rear extension with mono-pitch roof.  I'm planning to use BMI Redland Mini-mockbond tiles, as they're closest to the tiles on the main house and can handle the low pitch (17.5°).  

 

I'm at the point of building up the facia / soffit, but with the vertical battens necessary over the ply skin, I'm struggling to visualise the facia / guttering arrangement.  My CAD mockup shows the water is going to overshoot the gutter, but if I use a taller facia board I'll be blocking the breathable membrane, which needs to sit either on top of the ply or on top of the vertical battens (it will probably be Tyvek Supro if BC are happy with it, as the data sheet shows it's used for this sort of build up).

 

 

Any of you knowledgeable folk know what to do here?

[Allthegearnoidea]

Perhaps cut a proper tilting fillet and mount the gutter to that where it overlaps the vertical battens?

[Nickfromwales]

The tile course can be set back a little, and the ply + membrane ‘fly’ over and into the gutter. 30mm or so should suffice.

A vent strip makes things look nice and stops you seeing up the ventilation void. 

 

Rain only really tends to overshoot if it’s very heavy rainfall, otherwise it holds on to the surface of the tile and pretty much falls (near) straight down.

[saveasteading]

2 hours ago, Nickfromwales said:

Rain only really tends to overshoot if it’s very heavy

And the gutter will probably ( can be) be bigger than shown. That looks small and far too shallow..... gutters seldom go in level or to a steady fall, so any dip or rise severely reduces capacity. 

[Russell griffiths]

TBH that drawing is all a bit wrong  

delete out the two battens at the bottom they won’t be there

lift the facia up to 25mm below the bottom tile

add an over facia vent on top of the facia to make up the 25mm height. 

[ProDave]

44 minutes ago, Russell griffiths said:

lift the facia up to 25mm below the bottom tile

add an over facia vent on top of the facia to make up the 25mm height. 

And the membrane comes up over the fascia but under the fascia vent.  You either need a strip of OSB or a proper eaves tray to support the sloping bit of membrane so there is not a sag to fill up with water.

[Russell griffiths]

4 hours ago, ProDave said:

And the membrane comes up over the fascia but under the fascia vent.  You either need a strip of OSB or a proper eaves tray to support the sloping bit of membrane so there is not a sag to fill up with water.

Well yea, loads of bits I missed out, but it should get him going in the right direction. 

[Allthegearnoidea]

Thanks folks, I really appreciate the input.  Latest still doesn't feel completely right, but I'm getting there perhaps?

 

 

I actually bought an over-soffit ventilator to get the measurements, I find it quite flimsy for supporting the tiles - they obviously do their job but I'm unimpressed. I don't think there is actually any point to ventilating anything here as far as I understand - the ventilation needs to go on below the ply in the 50mm void between ply and insulation which I will provide with a soffit vent strip at the eaves and a Glidevale Monovent at the top end.  In which case might I just as well cut a tilting fillet out of timber? 

 

I'm dead set on a wet verge, I can't stand the look of the modern dry verge facias you see, but I'm not sure I've got the cement board right.  I've drawn 12mm cement board, which I think exists, simply to match the ply.  I had in mind for the ply to stop on the inner leaf an inch or so back from the cavity.

 

Edited Wednesday at 21:56 by Allthegearnoidea

[Allthegearnoidea]

13 hours ago, saveasteading said:

And the gutter will probably ( can be) be bigger than shown. That looks small and far too shallow..... gutters seldom go in level or to a steady fall, so any dip or rise severely reduces capacity. 

 

It's normal 112mm guttering, this is used on the rest of the house and it seems odd that it can take the water off the main roof which is twice the size but couldn't handle this - also it might look a bit odd?

[saveasteading]

8 hours ago, Allthegearnoidea said:

roof which is twice the size but

It's the number of outlets that controls the capacity. The gutter can be infinitely long as long as there are enough outlets.

However, a long slope can pour dramatically into or over it if it is small.

[Allthegearnoidea]

Here's how it would look with a tilt fillet and smaller facia to match the main house, vent strip in soffit not shown.  I'm told I need to maintain the flow of air interrupted by a Velux window further up the roof.  Could I do this with vents in the ply above and below the window?  I'd rather avoid ventilating tiles where possible, I know I'll need at least one for a bathroom fan vent though.

 

[Russell griffiths]

Why not put the tilting filler under the ply

rip a 300-400 mm strip of ply and tilt that up behind the facia, on top of this goes an eaves tray then membrane on top of the eaves tray. 
I believe you can get eaves tray and vent strip in one now, but I’ve never used them. 

[saveasteading]

The accepted spelling is fascia,( right back to Roman times I've just seen)  with the other being a more recent spelling, mis-spelt often but not correct.

 

 

 

[ADLIan]

Don't forget you probably need a ventilated airspace (50mm?) below the ply deck/above any insulation and install eave to ridge vents

[JohnMo]

Or do a proper Scottish roof. With sarking boards and small gaps between them.  Then you can full fill insulation with a breather membrane. Do slate instead of tiles and don't need battens either.

 

Using ply isn't really Scottish roof. It's just ply on a roof.

[Allthegearnoidea]

13 hours ago, saveasteading said:

The accepted spelling is fascia,( right back to Roman times I've just seen)  with the other being a more recent spelling, mis-spelt often but not correct.

 

 

 

 

I always thought it was fascia, but I'm definitely guilty of forgetting the 's' - it's one of a few words I have to consciously think about! 🤣

 

 

13 hours ago, Russell griffiths said:

Why not put the tilting filler under the ply

rip a 300-400 mm strip of ply and tilt that up behind the facia, on top of this goes an eaves tray then membrane on top of the eaves tray. 
I believe you can get eaves tray and vent strip in one now, but I’ve never used them. 

 

I think because the ply is structural, it needs to be screwed directly to the rafters - I did ask the SE about a few options but he wasn't keen.  Massively wasteful too as all the joints need to be staggered.

 

 

7 hours ago, ADLIan said:

Don't forget you probably need a ventilated airspace (50mm?) below the ply deck/above any insulation and install eave to ridge vents

 

Yes, absolutely - that has been planned in.  There is a 50mm gap below the ply, then insulation.  At the eaves I'll add a vent strip in the soffit and at the top there is a Glidevale Monovent specified by the architect which takes up a load of space below the windows and ultimately forced me to use a lower pitch than I could have had otherwise.  I just need to work out the best approach to ventilating that space above and below the Velux windows.

 

 

3 hours ago, JohnMo said:

Or do a proper Scottish roof. With sarking boards and small gaps between them.  Then you can full fill insulation with a breather membrane. Do slate instead of tiles and don't need battens either.

 

Using ply isn't really Scottish roof. It's just ply on a roof.

 

Ah, I didn't know - I've just found a few helpful resources relating to roofing in Scotland.  I don't particularly want to change too much at the moment, I'm just trying to find a build-up that will actually work and satisfy building control.  I was hoping to use a roofing contractor for this aspect, but as soon as they hear about the ply they're not interested as it's unconventional.  Just received a labour-only quote from a good roofer I've used before for £6K - it sounds like a 'go away' price to me so I will probably end up doing this myself.