Mono-pitch roof design flaw.

banjolin

Hello,

3 years ago I built a large extension with a 15 deg. mono-pitch roof. This roof was meant to be a green roof and this was made clear by the architect at the time it was designed to the timber-frame company.

It is a fairly simple make-up: 400mm roof I-beams with an osb outer deck, an airtight inner membrane and the cavity filled with cellulose. Over the deck the timber-frame company design calls for a breather membrane and battens. Obviously this anticipates a traditional roof finish.

The green roof was not implemented at the time of the build due to cost, so the issue was not really given any real thought. A torch-on felt finish was used as a temporary solution.

My problem now is that I have realised that the torch-on is probably not breathable. Is that a problem??

And if it is a problem does that mean that I cannot implement a green roof, because that won't be breathable either?

It is a windy site and the green roof would add significant ballast to the structure. If I cannot go green what can now be done instead?

So to summarise:
1. I am I right in assuming torch-on is not breathable?
2. If it is not is that a problem?
3. Am I right in assuming a green roof will not be breathable.
4. What is the best alternative for 15deg pitch in an exposed site with the high side facing the prevailing wind i.e. the high side is south and the low side is north.

Any advice would be welcome.

Thanks.

BryanF

I got to through the first line and spotted something fundementally wrong with your proposal.. 15deg pitch for a green roof ! Wtf

sydthebeat

BryanF wrote: »

I got to through the first line and spotted something fundementally wrong with your proposal.. 15deg pitch for a green roof ! Wtf

ive seen sedum roofs at higher pitches than that

banjolin

sydthebeat wrote: »

ive seen sedum roofs at higher pitches than that

Yeah. I have seen several or I wouldn't even have considered it. Anyway, any advice on the breathability issue or alternate roof covering would be welcome.

BryanF

sydthebeat wrote: »

ive seen sedum roofs at higher pitches than that

I suppose in an Irish context of constant rain ok, but you won't be retaining much water on a 15deg roof. I've only ever specified a min fall with full tanked 20+ year guaranteed system

sydthebeat

BryanF wrote: »

I suppose in an Irish context of constant rain ok, but you won't be retaining much water on a 15deg roof. I've only ever specified a min fall with full tanked 20+ year guaranteed system

i trhink iot comes down to the aesthetic of the finish. im not a huge sedum fan as it looks quite "algae"ish IMO, i much prefer more leafy natural grasses, and those are restricted to low pitches

BryanF

banjolin wrote: »

Hello,

3 years ago I built a large extension with a 15 deg. mono-pitch roof. This roof was meant to be a green roof and this was made clear by the architect at the time it was designed to the timber-frame company.

It is a fairly simple make-up: 400mm roof I-beams with an osb outer deck, an airtight inner membrane and the cavity filled with cellulose. Over the deck the timber-frame company design calls for a breather membrane and battens. Obviously this anticipates a traditional roof finish.

The green roof was not implemented at the time of the build due to cost, so the issue was not really given any real thought. A torch-on felt finish was used as a temporary solution.

My problem now is that I have realised that the torch-on is probably not breathable. Is that a problem??

And if it is a problem does that mean that I cannot implement a green roof, because that won't be breathable either?

It is a windy site and the green roof would add significant ballast to the structure. If I cannot go green what can now be done instead?

So to summarise:
1. I am I right in assuming torch-on is not breathable?
2. If it is not is that a problem?
3. Am I right in assuming a green roof will not be breathable.
4. What is the best alternative for 15deg pitch in an exposed site with the high side facing the prevailing wind i.e. the high side is south and the low side is north.

Any advice would be welcome.

Thanks.

1. Yes
2. What is the current build up? Can I assume the felt on the ply? Can we assume the vapour barrier is fully and correctly taped and sealed to limit the passage of moisture into the roof structure?
3. Yes
4. The best alternative to what? can you confirm your priorities? Is this purely weathering, or would you consider aesthetics important, what was planning granted for? what about environmental costs or actual €€'s?

Zinc, copper, stainless steel, all super looking. Stainless steel for an extreme example, could be considered great from a longevity and recyclable perspective but cost wise and install wise, it could be a leap to far.

2 part bitumen may offer the best guarantee, it's used in many large projects.

Grp is popular but some complain about its potential uv break down

Slate will struggle at 15deg but you could use one of the above under it if planning has a bearing

BryanF

sydthebeat wrote: »

i trhink iot comes down to the aesthetic of the finish. im not a huge sedum fan as it looks quite "algae"ish IMO, i much prefer more leafy natural grasses, and those are restricted to low pitches

Like this?
http://www.amusingplanet.com/2010/09/grass-roofs-of-norway.html?m=1

sydthebeat

BryanF wrote: »

Like this?
http://www.amusingplanet.com/2010/09/grass-roofs-of-norway.html?m=1

hah some of those are crazy!

im not sure the grass roof and the vernacular style in those pics are a good balance

banjolin

Thanks for the feedback. Currently the torch-on felt is straight onto the ply. This is why I am concerned about breathability. Cost is an issue. More than aesthetics anyway. I like the look of the felt because it is a clean unfussy finish. See images.

It's well sealed from inside. Had airtightness test done. The extension is built passive, but because it is joined to a 200 yr old cottage it is not hermetically sealed. So really it is mostly about what might get in from outside. With the wind strength here you can't really afford to get it wrong.

Green roof is now off the table. Too flat for tiles. I have considered corrugated iron, as there are some farm buildings adjacent with it and it would look ok (IMHO). Concerned about lift in high wind.

I could remove the felt, batten over a breather membrane and redeck that and felt it, I suppose.

Just interested to see if there is anything out there that I have not come across that would address the breathability issue and not add significantly to the depth of the roof build-up.

PS How breathable is the OSB?

D_D

Not sure if you would like the aesthetic of them, but Roadstone do a roof tile which can be used in roofs down to 12.5 degrees...

https://www.roadstone.ie/products/centurion/

Issue will be where the tile meets the soffit, it will leave a small opening and may look odd.

banjolin

D_D wrote: »

Not sure if you would like the aesthetic of them, but Roadstone do a roof tile which can be used in roofs down to 12.5 degrees...

https://www.roadstone.ie/products/centurion/

Issue will be where the tile meets the soffit, it will leave a small opening and may look odd.

As you mention it they ain't pretty, and the way they fit that barge board along the side is just plain ugly. I am more concerned though about how to finish the high edge.

With the felt you just ran it over a fillet and over the pvc fascia, then dressed it with a board. Its a very clean finish albeit one that needs a bit of maintenance.

I'm guessing that I will have the same problem with corrugated.

BryanF

banjolin wrote: »

As you mention it they ain't pretty, and the way they fit that barge board along the side is just plain ugly. I am more concerned though about how to finish the high edge.

With the felt you just ran it over a fillet and over the pvc fascia, then dressed it with a board. Its a very clean finish albeit one that needs a bit of maintenance.

I'm guessing that I will have the same problem with corrugated.

can confirm whether the osb is required by the structural design for racking purposes?
What is on your planning ?
Will the planners accept corrugated? Black colour might work

banjolin

BryanF wrote: »

can confirm whether the osb is required by the structural design for racking purposes?
What is on your planning ?
Will the planners accept corrugated? Black colour might work

Personally I would say for racking, based on the overall construction method. The longest roof joists (ibeam) are well over 7m long. Possibly also to stop pumped cellulose insulation from bulging over the top of the joists as it might if there was just a membrane. There were spots inside where this was an issue.

Green roof for planning but I'm not too bothered. Will go for retention if I have to.

Corrugated roofs are clearly visible on other buildings from road (old stable and two span barn) so they are more likely to go for that than many of the alternatives. The extension was designed to look like a barn.

BryanF

banjolin wrote: »

Personally I would say for racking, based on the overall construction method. The longest roof joists (ibeam) are well over 7m long. Possibly also to stop pumped cellulose insulation from bulging over the top of the joists as it might if there was just a membrane. There were spots inside where this was an issue.

Green roof for planning but I'm not too bothered. Will go for retention if I have to.

Corrugated roofs are clearly visible on other buildings from road (old stable and two span barn) so they are more likely to go for that than many of the alternatives. The extension was designed to look like a barn.

Your engineer/architect need to approve the build up. You should ventilate that roof.

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Corrugated sounds like your best option. Both visually and financially. Very easy to achieve good ventilation with corrugated.

It can utilise the membrane and battening system as called for by the roof company at ease.

One tip though, the low side to the North will need a gutter. Design a metal folded gutter that has a high leading edge so it sits a bit higher than the edge of the corrugated to prevent full blown winds up into the corrugation. It also helps show nice clean lines where roof edge meets the vertical.