Cedar shingles

[NBG]

Hi everyone,

first, I apologise in advance for the weird formulations I'll use. I'm supposed to use pretty specific word that I know only in french, but that's definitively too specific for my dictionary.

Anyway... I got a contract for two "carport" (don't know if you guys use that word, in belgium we use it to name a building without walls desinged to shelter a car. so it's basicly a roof on 4 or more poles), and I'm supposed to cover them with Cedar Shingles. I've done a bit of research on the net, and I found various advice about the size of the part of the shingle I can let visible, and the one wich will be covered by the next row. My shnigles will be 40 cm long (15.9 inches), and the slopes of my roof will be only 25%. I see here and there that the "standard" measures for the visible part is 12.5 cm (5 inches). But I found one site where they say that for such a roof's slope, I should use 4 inches.

And this sucks a lot, ecause my price estimate has been accepted has it was, and it's a bit late to say that I was wrong and that it will cost several hundred bucks more. The surface of the two roofs is 90 square meters (675 square foot), so this will really make a difference.

So... any advice? Is it realy important this 4 or 5 inches? Is it for the sealing (in this case it's not that important as this is not a real house) or is it to hold against the wind?

Thanks in advance,

[sbarton22]

First, yes, carport is the correct word. Second, I am an architect, so hopefully my advice helps.

Next, what is the assembly of the roof you are making? I assume you have some sort of rafter or joist. Then, are you using some kind of sheet good (plywood?) or are you using battons spaced apart?

On top of that, are you using some sort of underlay? Felt paper or some sort? This is key. This is the actual waterproofing layer. The shingles are to shed water as best as they can as a first line of defense, but their real job is to take on weather....protect against snow, hail, rain, sun, etc.

Usual installation calls for overlapping the underlayment over the part of the shingle to be covered by the next row. If you are using a sheet good, you can put a layer down underneath everything. That will give you an extra line of defense.

25% slope is known in the US as a 4:12 slope (for every 4" up, you have 12" run). This is the borderline on how you can use cedar shingles. If you can cheat that to 26+% (or just a hair more than 4:12), then, depending on the quality of the shingle, you can expose up to 7". Part of the exposure issue is to prevent capillary action (that is when water gets sucked uphill by negative pressure from within the crack). When it is steeper, capillary action is fought harder by gravity. The second issue is wind. Wind can blow water under the shingle when the slope is lower. Also, there is wind load. The flatter the roof, there is a tendency for the wind to get underneath the shingle and blow it up. Again, the steeper the roof, the more of that force is taken by the face of the shingle and not the end.

So, at the end of the day, your carport is open on 4 sides. Rain, wind and snow will get in, but you don't want any leaks, which will ultimately cause mold, decay, and failure. I would raise the peak higher than 4:12 and expose a little more shingle. They definately say to never expose more than 7.5 inches on an 18" shingle (standard in the US), so I would guess not to expose more than 7" on your 16" shingles.

I hope this helps.

[NBG]

The idea was simply to put rafter, then battons, the directly the cedar shingles without underlay sheet. I know this is not how your suposed to do for a real house, but as it is not a house, and that the esthetic of the roof seen from below is an issue, I guessed that it would be enough. Actualy, I thougth that the roof would leak in case of realy heavy rainfall, but as there are no wall to the structure... It's not a problem. Now, if you are saying that this will leak in case of any kind of rainfall... I'll probably have to rethink my design.

Regarding the slope, if you say that making a 26 or 27% slope will really help, that's not a problem, and I'll do that. BEsides, i'll keep the 5 inches exposure I used to make my estimate, hoping this will counteract the fact that I'm a bit too flat.

thanks a lot for you help, it helped more than a bit.

First, yes, carport is the correct word. Second, I am an architect, so hopefully my advice helps.

Next, what is the assembly of the roof you are making? I assume you have some sort of rafter or joist. Then, are you using some kind of sheet good (plywood?) or are you using battons spaced apart?

On top of that, are you using some sort of underlay? Felt paper or some sort? This is key. This is the actual waterproofing layer. The shingles are to shed water as best as they can as a first line of defense, but their real job is to take on weather....protect against snow, hail, rain, sun, etc.

Usual installation calls for overlapping the underlayment over the part of the shingle to be covered by the next row. If you are using a sheet good, you can put a layer down underneath everything. That will give you an extra line of defense.

25% slope is known in the US as a 4:12 slope (for every 4" up, you have 12" run). This is the borderline on how you can use cedar shingles. If you can cheat that to 26+% (or just a hair more than 4:12), then, depending on the quality of the shingle, you can expose up to 7". Part of the exposure issue is to prevent capillary action (that is when water gets sucked uphill by negative pressure from within the crack). When it is steeper, capillary action is fought harder by gravity. The second issue is wind. Wind can blow water under the shingle when the slope is lower. Also, there is wind load. The flatter the roof, there is a tendency for the wind to get underneath the shingle and blow it up. Again, the steeper the roof, the more of that force is taken by the face of the shingle and not the end.

So, at the end of the day, your carport is open on 4 sides. Rain, wind and snow will get in, but you don't want any leaks, which will ultimately cause mold, decay, and failure. I would raise the peak higher than 4:12 and expose a little more shingle. They definately say to never expose more than 7.5 inches on an 18" shingle (standard in the US), so I would guess not to expose more than 7" on your 16" shingles.

I hope this helps.