Roof ventilation ? vent it or membrane......

[bandekk]

ok, although I've posted some of this on another topic, this question better clarifies my query.

I have a 18 degree lean to roof on my extension which I am struggling to complete due to the ventilation requirements.

The construction at present consists of 7 inch joists from pitching plate to wall plate on internal wall. There will be 3 velux windows in the vaulted ceiling as well.

I had planned the following.

Install 100mm celotex in between joists, leave a 50mm air gap above, then put down tyvek supro membrane (tight or sagging?), battens, then tiles. BC also requested I put 30mm celotex on the inside of the room joists to insulate them.

After speaking to sandtoft who make the tiles, they have stated I can either vent the roof, with their very very expensive eaves and top abutment systems, or I can completely seal the roof. After visiting the roofers merchants today and getting some more advice, they said they hardly ever sell these venting systems.

So really I just need to know how to put this roof together. The joists are down, I have the insulation, battens, membrane and tiles ready to fit, but I cant start until I work out this ventilation problem, and whether I actually need to do this. My house has no facia or soffet boards on it. I believe the roof is a ‘warm’ roof. I also dont have any tolerance in place to fit counterbattens due to position of windows above the roof.

I have read numerous documents from Sandtoft, Tyvek, Clay roofing association etc. all seem to contradict each other.


Thanks in advance for any help.

 

[bandekk]

just a quick update, I've measured the distance for counterbattens and fitting these would leave me with a 60mm upstand on the lead flashing, 15mm below the min 75mm, maybe I can use 10mm counterbattens instead of 25mm???

 

[bandekk]

ok, I'm thinking aloud here as I try to resolve this problem. Even though the Tyvek documentation says to make the membrane taught, I could drape this by 20mm between the rafters which would allow water/vapour to run out, negating the need for counterbattens. Although how do I manage this above the velux windows area......and I would have to cut down my noggins between the rafters to allow water to run all the way down the roof.

 

[freddiemercurystwin]

So you have 7 inch (175mm) rafters? Fit 100mm insulation between the rafters and 30mm beneath. This leaves a 75mm gap between the outer face of the insulation and the outer faces of the rafters. Fit your breathable membrane to the outer faces of the rafters and allow it to sag 25mm. Job done.

All as shown in figure 1c in here: http://www.insulation.kingspan.com/uk/pdf/k7.pdf

You can use nails instead of the small battens used to push the insulation up against.

 

[bandekk]

yes thats what I thought, but then I would have to cut down all the noggins as they are level with the joists at present ?

any what happens with water collecting above the velux opening area, does this just eventually find its way down either side of it?

 

[freddiemercurystwin]

Why have you got noggins in your roof? If they are 175mm like your rafters then yes you'll need to trim them down a bit. The membrane should be dressed into the velux's and and if installed as per the instructions any rainwater should find its way down each side of the Velux. If the the roof is tiled correctly there shouldn't be any water on the membrane. Its really there as a backup if a tile should slip or fail at some point in the future.

 

[bandekk]

great, thanks, I'll get my saw.....

 

[bandekk]

now what do I do with the underlay support trays I had planned on fitting then under the membrane leading into the gutter, can this still be done, to a fashion......ta

 

[freddiemercurystwin]

You're referring to something like this presumably? http://www.cavitytrays.co.uk/pdf/109-Hardedge-Strip-1200.pdf

Yes you'll still need them fitted, the membrane is dressed over the edge of the support trays, so that both are overhanging the gutter.

 

[bandekk]

yes, similar to those.

Just one other query, as I have no soffit or fascia board, do the last row of tiles just rest / overhang on the outer wall, using tilt fillet where necessary to maintain the correct tile angle, and presumably to maintain a gap underneath where water etc. can run under into the gutter as well. Cant seem to find any good diagrams for eves without fascia anywhere. Does the gap underneath need any vent?

thanks

 

[freddiemercurystwin]

You'll still need some kind of member for the underlay support trays to rest on which in turn will support the edges of the eaves tiles between the rafters. You could fit something between the rafters instead of a continuous length to the ends of the rafters if you want to try and mask it from view (ie behind the gutter). Tilting fillets as well.

No need for any additional ventilation, your breathable membrane will do all the ventilating.

 

[bandekk]

so even though the outside wall will support the tray at its 'back', it still needs supporting at its 'front', between the rafters ? and this could just be a batten or similar between the rafters ? all this whilst still trying to maintain a 25mm drape.

As the tiles will be resting on the membrane, then undertray, then outside wall, the gap will be minimal between the tiles and wall, will this still allow vapour out ok ? Should any of this be bedded down?

just been reading this....

http://www.roofconsult.co.uk/articles/tiling/tips60.htm

and this is where I though about the ventilator strips as these are the tiles I am using

http://www.clayslates.com/tiles/tec...ad-details/2020CottageEaves.html?ContentID=11

once again, thanks.

 

[freddiemercurystwin]

The walls should not be supporting the tiles or undertray or anything else. The battens support the tiles. A batten or similar can be fitted to support the ends of the tiles and the leading edge of the undertray. You obviously cannot achieve a 25mm drape immediately over the undertray.

As previously mentioned, you have a breathable membrane. This eradicates the need for any other ventilation devices to be installed at the eaves or ridge or verge or anywhere else.

You can use some kind of comb filler to stop birds or other suchlike from entering the void if you wish.