Curved Roof

[andyanderson91]

I plan to build a curved Dutch hip style roof.

For the roof decking, if the radius (using a circle for it's gradual sweep) is large enough? Say a 264' radius as I've roughly calculated with the equation of a circle set to the actual width or run of the roof, and the rise superimposed to copy other designs. Question, will typically 1/2" plywood bend that little bit without any thought given by the building inspector? How much rise/bend along an 8' sheet of plywood if gradual as seen in a typical circle's radius, is too much for the fasteners to handle or the plywood itself?

I'm aware it will have to be engineered regardless since it will not be a prescriptive roof design. Thought for a moment if it were a gable roof and the meat in the rafter left after cutting a curve, met the prescriptive requirement, would the plywood bend cancel out the ability to build that curved rafter prescriptive?

 

[chenxin2894]

A curved Dutch hip roof has a radius of 264', and 1/2" plywood may struggle to bend to such a mild curve without additional support or lamination. Building inspectors normally adhere to engineering standards, and a structural engineer should review the design to guarantee safety and code compliance. Prescriptive requirements may not be applicable to such unusual designs.

 

[Paul Sweet]

If the plywood is run perpendicular to the rafters like it should be, it would have to bend less than 1/8" across the 4 ft. width. A 264' radius would be almost indistinguishable from a straight line. The rafters would probably bow more than this.

 

[andyanderson91]

Thanks for your response. My rafter run to the end of the tail is 174". The rafter LL if a straight line is drawn from the head cut of the upper down to lower common's tail is 221.4". There are two sets of commons rafters with two slightly different pitches. It's a 696.3" radius curve (my bad I mistakenly gave the wrong number...) just looked back at my notes to confirm that.

Threw together some notes to recall the rough concept in the photo attached. Not sure if my scribbles make any sense... I came up with a 1.5" curve per 4' sheet of decking. Total of about 5-1/2" of curve in the total roof ridge to tail.

Is that a subtle curve?

I also will making my hip Jacks graduate up about 4" over 6'6" on top of a curved cant strip nailed to the top plate to get my 4x6 rafters to gradually swoop up to my oversized 4x10 hip beam.

Hard to see in this photo but I'll basically be copying what I saw in this Korean framing book written in korean. Not trying to re invent the wheel but I am excited to try something other than natural, irregular hips and other hex or octogan bay roof shapes.

I should mention I am actively trying to avoid multiple plys of decking. I'd prefer to stick with typical 1/2" one layer application for labor and material efficiency (already going to take a minute longer to play around with this unorthodox roof). I'm willing to smooth the curves to suit the plywood. I know I can test this out myself and see how well a sheet bends or stays, but my real question is if anyone has experience with standard 1/2" on curved rafters and about how subtle was the curve? Did it last? Did anyone use partially screws mixed in to keep the plywood shape?

 

[andyanderson91]

A curved Dutch hip roof has a radius of 264', and 1/2" plywood may struggle to bend to such a mild curve without additional support or lamination. Building inspectors normally adhere to engineering standards, and a structural engineer should review the design to guarantee safety and code compliance. Prescriptive requirements may not be applicable to such unusual designs.

I asked a few engineers around here about the curve, and they basically said to send over a design and they would review it but also they don't know the APA's standard in this regard. Prescriptive does seem out of the question I agree. I'm looking for a starting point to direct my designer so the engineer doesn't get out of hand. Wondering if anyone here has first hand experience with subtle curves.

 

[bill1952]

I wish you had even a crude sketch. Googled "curved Dutch hip style roof" and now assume the roofing is concave. I did that on a garage - transitioned from a 8:12 to a 4:12 portion over a shop extension. Just lined up rafters and them cut 8' 2x12s to set on top of 8:12 and in line 4:12. Very easy. No hip to deal with but could have.

 

[andyanderson91]

I wish you had even a crude sketch. Googled "curved Dutch hip style roof" and now assume the roofing is concave. I did that on a garage - transitioned from a 8:12 to a 4:12 portion over a shop extension. Just lined up rafters and them cut 8' 2x12s to set on top of 8:12 and in line 4:12. Very easy. No hip to deal with but could have.

When you say set on top of the 8:12 main roof, did you scab on curved sleepers/cleats that transitioned onto the lower 4:12 roof? If not then I'm not sure I know what you mean. I do have this sketch from a while back. Also couple pics if sketch doesn't make sense.

 

[Yankee Chronicler]

None of those three photos appears to show a roof such as you describe.

Your rough sketch isn't to scale and doesn't identify the size, the species of the rafters, or the spans. Once you start cutting away, the spans will have to be based on the minimum depth of the rafters after the cuts, unless you engage a licensed structural engineer to calculate out the maximum bending moment at the mid-point of each rafter and the capacity of each cut rafter based on the depth at that point.

My gut feeling is that 1/2-inch plywood probably can bend 1/5 inches over 4 feet, but I wouldn't be comfortable using CDX. CDX allows -- and usually has -- numerous unpatched knot holes in the 'D' side, which is installed toward the inside. That will be the tension side of your curve, and voids in the tension face of the sheathing is not what you want. I think the least you should be looking at is B-C with exterior glue, or maybe C-C plugged with exterior glue.

 

[Yankee Chronicler]

And, yes -- to achieve the bend the plywood should be screwed, not nailed.

 

[andyanderson91]

None of those three photos appears to show a roof such as you describe.

Your rough sketch isn't to scale and doesn't identify the size, the species of the rafters, or the spans. Once you start cutting away, the spans will have to be based on the minimum depth of the rafters after the cuts, unless you engage a licensed structural engineer to calculate out the maximum bending moment at the mid-point of each rafter and the capacity of each cut rafter based on the depth at that point.

My gut feeling is that 1/2-inch plywood probably can bend 1/5 inches over 4 feet, but I wouldn't be comfortable using CDX. CDX allows -- and usually has -- numerous unpatched knot holes in the 'D' side, which is installed toward the inside. That will be the tension side of your curve, and voids in the tension face of the sheathing is not what you want. I think the least you should be looking at is B-C with exterior glue, or maybe C-C plugged with exterior glue.

I probably didn't do justice in describing the intent, my apologies. So the sketch actually is to scale, which is a foot per square (also a 1/4" per foot I suppose). All I have are the exterior elevations. The common rafter span is half the building depth of 26' so span is 13', plus there will be a 4 foot overhang which sort of factors into the overall curve cut into the rafters. I planned to break the span up with a mid span purlin and have two different rafter pitches lap the purlin so the curve cut into the two rafters is less. I would cut the 5-1/2 curve out of oversized stock but that would be tough to do with engineered lumber which doesn't work for the exposed tail look in the photos.

I completely agree with you on the CDX, I've never felt it was even as nice as osb to work with. So B-C plywood is good? Is that the same as ACX grade or is it better?? I've never used b-c to deck a roof before but it sounds like it's real smooth and good for under the overhangs appearance wise as well?

 

[Yankee Chronicler]

ACX is better than BCX. The letters refer to the grade of face plies, with A being best and D being worst., The X means exterior glue.

If you're going to be bending the plywood, you want the best face possible on the tension (convex) side. B would be the least I'd entertain, and A would obviously be better.

 

[bill1952]

A very rough - true - napkin sketch at the breakfast table of what I tried to describe. Not sure it's of any use since seeing your later sketch and photos.

I've required bending lots of ply (and gwb) for my work (theatres) and your radius is hardly a bend. APA says 1/2" parallel with grain bends to 6' radius dry. Read their paper.

 

[andyanderson91]

A very rough - true - napkin sketch at the breakfast table of what I tried to describe. Not sure it's of any use since seeing your later sketch and photos.

I've required bending lots of ply (and gwb) for my work (theatres) and your radius is hardly a bend. APA says 1/2" parallel with grain bends to 6' radius dry. Read their paper.

I see your roof now with the pitch change at the wall lapping rafters or ledgered I suppose? It looks like a 4' ply sheet bends about 3" on your 8:12 to to 4:12 transition. That's about an 8' radius so according to your APA bulletin you attached you didn't have to use hot water to bend it right? Sounds like my current designs curves are subtle enough that the plywood won't even be a consideration short of using BC or AC grade to keep it free of knots and defects. I like the idea of using nice plywood for the exposed stained look I'm after under the eaves and gable up top. How tight a radius have you used?

The rafter species will be Doug fir #1, since that's traditionally what was used. It's pretty spendy so I hoped not to cut too much meat out of the rafter to get that continuous curve through the whole roof. Hence the whole curve amount discussion, more to consider than meets the eye.

Thanks for the APA bulletin that helps a lot showing the limits. This helped me realize I didn't take into account the curves radius projects past the mid point since I have a 4' OH planned. I'll need to math it out again to be sure rafter sizing is of the minimum effective depth required for the spans.

Thanks for your advice and the napkin drawing, these curves are nothin'

 

[ICE]

 

[bill1952]

I think you just have to work it out in drawing. I wouldn't go to the APA minimums if I could help it, but doesn't seem like you're near them. I also wouldn't bother with better than CDX unless it's for appearance. Maybe just do first course? Actually, I try to use sawn lumber if bottom side is visible, not ply at all. My last build, a detached garage/barn, was all rough sawn pine, every stitch of it including eaves that are exposed.

 

[andyanderson91]

I think you just have to work it out in drawing. I wouldn't go to the APA minimums if I could help it, but doesn't seem like you're near them. I also wouldn't bother with better than CDX unless it's for appearance. Maybe just do first course? Actually, I try to use sawn lumber if bottom side is visible, not ply at all. My last build, a detached garage/barn, was all rough sawn pine, every stitch of it including eaves that are exposed.

Do you reduce your tails like a 1/4" to drop the decking in the eaves? I have some of this left over from soffiting, it's 1x6 t&g DF pre stained. Should one reduce the tail to accept the 3/4 board plus another 1/2" of plywood to give the roofers a chance to not poke nails through below? I like the color it's pretty much what I would use. That sounds like it would look nice under the eaves and gable ends. Did you change direction with it from eaves up your gables rounding the corner? If you did, 24" oc lookouts with a sub barge board nailed to them??

 

[bill1952]

I sheathed entire roof in rough sawn so not much help. (It was noticeably less expensive than sheathing.) I don't know if I'd shave 1/4" off rafter tails or the boards, or go to 3/4" sheathing or shim the rafters 1/4". They all have plusses and minuses.

I did not change directions on gable. I could stand on the rough sawn extending 16", so the 24" on center look outs and the 2x6 barge rafter are nearly decorative, but yes, barge is end nailed to lookouts.

I'll try to remember to take and post a photo tomorrow.