tile vent for low pitch roof

[sm_jamieson]

Turns out BCO is being fussy about the roof. So, I need to compy completely with the regs, fitting instructions etc. Roof pitch is around 11 degrees, BCO has OK'd interlocking forticrete centurion tiles for this, which are spec'd down to 10 degress in a monopitch roof. But the orignal spec is for eaves and high level tile vents in the roof. Building regs say for pitches less than 15 degrees, a high level tile vent should be used providing equivalent of 5mm opening along the ridge. Its a monopitch roof but joining to neighbours monopitch roof with a wet-fixed ridge tile. So cannot have ridge ventilation easily, and abutment vents are suitable (not a lean-to). Now, all suitable vent tiles I can find are only spec'd to 20 degrees. So, how can I vent the roof ? Simon.

[Hugo Nebula]

On Mon, 18 May 2009 08:07:22 -0700 (PDT), a certain chimpanzee, sm_jamieson randomly hit a keyboard and produced:

Use breathable felt.

[sm_jamieson]

That's one option, although lots of people think it doesn't really work, and since the roof pitch is so low, I was going to rely on the underlay as a waterproofing backup, so preferred impermeable underlay. I suppose I could add some gable vents although a boiler terminal will be there, so this may not be possible.

However, I also need to vent an extractor fan through the roof, so still need to find suitable vent terminals for this (cowl vents ?). I could use a cowl vent to vent the roof I guess, since they are available for flat roofs, but a row of mushrooms may look a bit funny !

Cheers, Simon.

[Phil L]

Breathable felt does work - we used it recently, along with roof vents and the BCO pulled his face about it saying there was *too much* ventilation and it should have had one or the other, but not both.

Also you can use soffit vents, available in strip form.

[The Natural Philosopher]

use 20 degree cappings throughout, vented?

[leenowell]

How about cross battening and fit soffit/ facia vents along the 2 gable sides?

[leenowell]

Thinking about it again, why not make it a "warm roof" and fit the insulation on top of the rafters e.g. Superquilt. I think (but not

100% sure) that you don't need vents for this but I could be wrong...

[sm_jamieson]

Interesting ideas. Problem is, there is only 1 gable, i.e. the "side" of the roof is against an existing wall. Plus I think venting with the slope of the roof is recommended in order to get the airflow. Most of the roof only has 50mm airflow in it, only the top has an apex void (a bit like a loft conversion). Cheers, Simon.

[sm_jamieson]

Depth and height of the roof was an issue, so a insulated deck on top would not be possible. Believe me I've considered all the options. I'll probably have to use permeable underlay and add a small vent in the apex for good measure.

Cheers, Simon.

[leenowell]

I knew I read it somewhere...... Have a look at page 3.....

memory, super quilt/ Tri-iso are only about 30mm and require a

25mm gap on top so this should only raise your roof height by about 55mm. Is this too much? If so, are the rafters spec'd to minimise height e.g. reduce gaps between them/ double them up? May be able to gain some there?

[sm_jamieson]

I have now decided to increase the height of my ridge by about 225mm to get the maximum roof pitch / ceiling height I can. This means my ridge will no longer be shared with the neighbour - her roof will form an abutment onto 3 extra courses of brick above her existing monopitch ridge.

Therefore, I will have a vertical area of wall that could contain airbricks to vent the roof. Only problem is that if someone in the future chose to extend her side, these could be blocked up. Is it worth the risk just to get some vents in ? I could then save money on permeable underlay. Simon.

[Hugo Nebula]

On Wed, 20 May 2009 01:52:26 -0700 (PDT), a certain chimpanzee, sm [e-mail] randomly hit a keyboard and produced:

If you're now increasing the roof pitch, you could get the tile vents in that you first mentioned. This is by far the best solution. As you say, putting vent bricks through the party wall is fraught with potential problems.

If you no longer have to match ridges, then a warm roof also becomes an option, as does a breathable membrane with counterbattens over. Going back to your previous post, breathable membranes are just as moisture resistant as traditional felts; the only concerns are about whether they remove as much vapour from the roof void as traditional ventilation, and their long-term vapour permeability (will the pores still allow vapour through after 30-odd years of dust has collected?).

[sm_jamieson]

Do you always need counterbattens over the permeable membrane ? I gather that depends on the air tightness of the tiles. Do interlocking concrete tiles need the gap ? Thanks, Simon.

[sm_jamieson]

I just read that counterbattens are not usually required if the membrane is draped rather than pulled taught. Since it will be draped (10mm), I may not need counterbattens. Simon.

[sm_jamieson]

All this reading. Apparently you have to ventilate under the membrane (cold roof) or over the membrane (warm roof) anyway, so you would still need vents in the eave and ridge for a monopitch roof. I looked in part C and it had little information on roofs. If I vented in the gable, I would need 4 standard plastic air bricks, and I have a boiler terminal nearby, so this is getting a bit tricky. I could solve it with a parapet wall and abutment, but I really don't want that appearance. Simon.

[Hugo Nebula]

On Thu, 21 May 2009 01:46:44 -0700 (PDT), a certain chimpanzee, sm [e-mail] randomly hit a keyboard and produced:

Just throwing an extra suggestion into the mix.

Q1 - Not always, but if headroom inside is important, it allows you to fully fill the depth of the joists with insulation rather than leaving a void over and having to insulate under the rafters.

Q2 - The BS does say that concrete interlocking tiles should be vented above the membrane, but it's something that rarely gets pulled up on site.

[sm_jamieson]

The roof is a vaulted roof with a small roof void near the apex. The design requires several 225mm purlins due to the low pitch, and the gap between these purlins will be fully filled with glass fibre insulation. The BCO also wanted 20mm celotex under the entire roof structure to reduce cold bridging. The rafters above the purlins are left empty to allow ventilation. The roof was to be vented at the eaves and by tile ventilators. It is only since I found the tile vents are only spec'd for 20 degrees or more, else they may let in water, that I have had to rethink.

But its a bit of a risk if you've done the roof without and you do get pulled. I guess then, put in the counterbattens (are they just 25 x 38 roofing battens ?), and why not eaves vents, but not bother at the ridge. But then, the eave vents might just get the BCO thinking along those lines. Simon.

[<me9]

What size of counterbatten is necesary? 25mm seems to be normally quoted, but somewhere I saw 6mm. I wiould think 6mm would be enough to clear any water as compared with the usual sag in the sarking.

[sm_jamieson]

I'm looking into the forticrete mono pitch dry ridge system. Just waiting for them to tell me it cannot be used a low pitch ;->

Simon.

[Hugo Nebula]

On Thu, 21 May 2009 03:04:04 -0700 (PDT), a certain chimpanzee, sm [e-mail] randomly hit a keyboard and produced:

What gap between the purlins? By 'purlins', I assume you mean the timbers that run parallel with your wall plate and have rafters over them at right angles? If you are, and you say you need several, that suggests a roof spanning at least 6-8m from wall plate to ridge! If you're not, I've not a clue how you're roof is constructed.

If you can afford to lower your ceiling by 225mm, then headroom was never an issue. So why couldn't you have lowered your wallplate to get you the pitch you needed?

I'm confused. And also a little bit sad for becoming involved in all of this on a day off :-(