I am looking at putting a 4 / 12 slope roof on a building that is basic a half round, 28 ft. diameter. there are structural milled posts every 4 ft. around the perimeter. the existing roof was sloped inverted to the front and into a drain, built up rolled ashpalt, w/ many layers of patch tar, poor flashing at the perimeter. my roof will slope to the outside of the building, like a ‘grain bin’. there is 2 structural walls, 3’6″ apart in the center, reverse sloped, I intend to build. a ‘pony wall’ on them for support of the rafters and join the rafters at the center point 1/2″ plywood sheeting, 15 lb. felt, 3 tab ashpalt shingles . Metal has been suggested, but I can’t think of a good way to make the ‘ curve ‘ of the roof’ Any suggestions or help will be appreciated.
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Gotta sketch or pic you can post?
REMEMBER THE BOSTON BATWANGER.
Round as in this?http://domania.us/FramerT/fun%20house/11190341570001.image.jpg
I have to draw up a '3 D' pic for the person in charge of the 'purse strings' , ( ever draw a isometric round object?) est. to replace the built up roofing was $8.000. SOOOOOO, we want to price out and try to reverse the slope. The shape is a 28 ft. diameter cut in half, building also has 2 - 4' 'wing walls, open air, which I may bring rafters to, to help stabilize the rest. I think I have it figured out, but the devil is is the details, i.e: blocking for sheeting, fascia, shingles, & how I finish the gable end to look right. This building is a washroom in a Prov. Park. and..... if it turnes out good , 4 more just like it and 2 similar. O.. I work for the park so time is not an issue, $$$$$$ is the final factor. ( and another park has the same problems .... road trip!!!!!
IF IT WAS EASY, EVERYONE COULD DO IT !
What's "reverse the slope"?
cheers
***I'm a contractor - but I'm trying to go straight!***
Edited 6/26/2005 12:29 pm ET by piko
From what I can gather you are building a half of a conical section for a porch on a park building. It has post every four feet around the circumference of the radius. The wing walls I can't envision. This sounds like a really cool project,I would start out by laying out either a scaled down version or a full scale section of the roof, basically a cut through the roof from peak to eave. I prefer full scale but I don't know what kind of work surface you have. I would snap the common rafter out full scale. This will help determine if the seat of the rafters, all of them, will be able to bear on headers between your existing posts. If so the radius fascia will be cosmetic, and probably bent with Azek skinned over a simple frame. If it is determined that the existing header will work, this will also help calculate where the seat cut the the rafter will bear on the header.The horizontal blocking here I think is key to your curved surface area. If you can sheet it curved and give the roofer a good curved plane to work from, I think the roof will have much better chance of success. Refer again to your section, and lay it out horizontally on 24'' centers starting at the rafter seat and working your way towards the peak. I would snap two lines representing 2X lumber for clarity. Now you should have a triangle with horizontal lines through it. You can measure along the line representing the bottom of the blocking at each of the blocking lines. This will represent the long point bevel (4/12)of the blocking that will describe the curve of the roof as you travel toward the peak. Use a trammel set up, simply a stick with the radiuses notched into it, to mark out a sufficient number of oversized (perimeter wise) blocks to be cut and fit in after the rafters are installed. Use of a centering head on a combination square will help mark the cuts. These cuts can be calculated but right now I'm thinking measure rafter to rafter at the blocking line representing a chord on the radius at that point, and transfer the chord to the block. Use the centering head to point through the marks and describe the cut line. The centering head always points to the center of the circle.When building I would try to describe the perimeter and at least an edge of each rafter line to refer to while building. This would be an ideal scenario but I don't know what kind of surface area you have: concrete, decking, dirt. If it is level even on dirt you could rig something: temporary plywood. You could even rig a trammel the length of your run swung off a center point. This would show each rafter when swung into place and show the circumferance anywhere along the edge, but I don't know if you are going to be able to access the true center with enough room for a pivot point. You will have to decide if you think its worth it. This is something not a lot of us, including myself, work on everyday. So proper planning, leaving no question unanswered will be the key to success. The things that are clear are clear, but its those unaddressed issues (I call it the ostrich syndrome, to be avoided at all costs) that might seem small (because we haven't taken the time to look at them) that can really jump up and bite us in the arse.I also think a laser would be very helpful on this project. Self plumbing and leveling, like a PLS 5.As far as the roofer, I am not one but I had a landlord when I was younger who worked copper everyday and I helped him many times on the weekend, with a good surface, a template can be made for the copper sections. Thanks I hope this helps
Edited 6/26/2005 12:40 pm ET by quicksilver
Thanks all, this is a re- roof of an existing building, the existing roof sloped to the front of the building, into a eavestrough., previously into a drain and down to drain in the center service room ( really stupid in a cold , snow area w/ trees all around.) any way, gonna try it, hero or goat ? wish me luck!
IF IT WAS EASY, EVERYONE COULD DO IT.
Three-tabs would work for some or most of the roof. Try laying out a trammel (string will do) on a flat area that's the radius of the roof, and lay a shingle at the outside end. Then cut the tabs to an arc of that radius, and the ends of the shingle to the angle formed by the trammel. Now, when you get towards the focal point of the roof, is there enough of the tabs left for the correct exposure ('weather')? . If so, or it looks like it COULD work, go ahead with the 3-tabs. But of course, nothing will line up so it will look like heck.
I've seen a roof that was cone-shaped (which is what I think you're trying to describe) which had standing seam metal. How they did that I couldn't tell you other than to talk to roofing manu's or sheet metal shops in the area. You may get someone who'll form it for you.
You didn't say what the structure is...if it's a residence you could use cedar shingles or shakes. They all would have to be cut to taper towards the peak.
AHHH wait a minute, you've got a roof formed of several flat sections approx 4' wide, but tapering to 0? (I know, it's 2:25 a.m., and I'm not too fast ) if that's the case you only taper where the panels meet, but you'll have to have ridge pieces there. SO ignore all that trammel stuff, lay 3-tabs on each flat panel but again just cut the ends where they are at the ridges formed where the beams are, right? and do the normal cutting the tabs into 3 to make these ridges.
Of course you could also go with torch-on such that there are a series of pie-shaped pieces cut to form either a proper cone (as l first thought) or the series-of-flat-rooves l've just described.
***I'm a contractor - but I'm trying to go straight!***
Edited 6/26/2005 5:16 am ET by piko
Edited 6/26/2005 5:29 am ET by piko
Artworks, I worked on a job a bunch of years ago as a kid on an old Victorian house with a round porch on a round tower that had burnt out in a fire. The tower was like n upside down ice cream cone, I think it was 14 foot diameter. The porch roof was flatter (3:12 pitch?) and it came out from the tower. The new owner was a half asped electrical engineer. He got his hands on some flat pieces of transite - about 3/16 inch thick that he cut up to use as roofing on the porch. He set up different patterns for the layers as he went higher toward the "peak". They were the same pattern (essentially rectangles with the outside bottom corners cut off about an inch up and an inch in), but got narrower, and narrower. I can't help you with the mathmatics of it because he used his computer at work, but over 12 feet or so, I think they went from about 12 inches wide to about 8. It looked pretty nice once the pieces were nailed on and the roof got a coat of paint. Good luck. Don
I framed a half round roof for a sleeping porch on my own house. The roof connects to the main house with two normal valleys, goes out square for 8' and is round on the outside with a 16' diameter. The roof is 12 in 12.
There is NO framing on the round section. I took 2x6's ripped from 5 1/2 to 0 with a slight bevel (2 degrees?). These triangle were lagged through to a round header and nailed to each other (not many nails) along their length. There is a catwalk and 6 ties at the ceiling height. That is it. The roof has a slight belly which was present at the time of construction but which has not worsened. The whole structure seems to be self-supporting. I can't say that I would ever try this with a paying customer or that a building inspector would go along with this but it has held up for almost twenty years and looks great from underneath.
This roof is shingled with 3 tab shingles. The shingles are 3 tab at the bottom, then 2 tab and single tabs at the top. You will figure this all out as you go along. The slickest look would be cedar shingles or a standing seam roof.
Are we talking about a roof that is like a quarter of a sphere. or a half of a cone, inverted?
The walls under it are curved, right?
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Yes, the walls are curved, the building is a 28' dia. basic circle. cut in half. The front is flat. The existing roof is a 'funnel' to the front eavestrough. There is mens's washroom to the left. women's to the right with a 3' 6" service room in the center. ther are concrete block structural walls between both washrooms & the service room, I hope to support my 'pony wall ' which will partialy support my new rafter system. Oh yes, I will be the one shingling this baby also!I am thinking along the design lines of an agricultureal 'grain bin' to visualize. ( having assemble a few) There are structural milled posts along the perimeter wall every 4 ft. and this is what I want to support the bottom of my rafters . oh I wish I could 'charge' for my engineering time! Got to start drawing something up today, better have a good eraser!
IF IT WAS EASY, EVERYONE COULD DO IT!
I'd be very inclined to take a page from blue-eyed-devil's "book," and assemble this on the ground against a "jig" to match the existing walls & structure.
That, if only to make the sheathing simpler, if nothing else (making the cut "against" the new flat front will be much easier not agaisnt the existing structure, I'm thinking).
I'd be sore tempted to use a board sheathing, so it could be "faired" around the curve--but that's because you may run into some BI resistance to two layers of 10mm ply as much as anything else. (I'd lay out plywood at an angle, and reverse the second layer, too; if I could.)Occupational hazard of my occupation not being around (sorry Bubba)
Capn, I'm not sure what he's shootiing for.
These pics might be helpful.
The first pic show the plywood pre cut to fit the eave. The second shows the ply being layed on the cone.
blue
Capn, I'm not sure what he's shootiing for.
If I get the info, this is a half "hat." The front wall is straight, and 28' long with two wing walls. The back wall is a half-circle with a 14' radius. The exisiting condition is a bowl draining to an interior roof drain about in the midpoint of the flat front wall.
Since thats a crummy way to drain a park building, OP wants to reverse the 3 or 4 in 12 roof pitch so it drains to the edge of the round wall.
Since that means a boatload of radial rafters, with not very much rise at all, your previously published methods for assembling such structures on the ground leapt to my mind. Being able to run the sheathing wild, and trim it before placing has more than a small amount of appeal to me. Particularly given that this effective pitch of the radial rafters is off a square (and possibly not even in a CM <g> calculator).
Now that I think about it, it might need to be a quarter circle, at 14' x 14' to make an easier lift. Given how shallow this assembly is/will be, it's very tempting to me to think that it almost needs to be lofted like a boat hull, rather than framed.Occupational hazard of my occupation not being around (sorry Bubba)
hey Blue, how did you figure out those curves in the ply beforehand ?
Joe, figuring those curves is easy. I just hired a guy that knows things like that!
Curves and circles isn't my strong suit, but the formula isn't that hard. If someone doesn't post it, I'll ask Steve or Frank and post it for you. At this point I'm sure I could figure it out, but I'd rather defer to someone that actually knows how to do it instead of guessing how to do it.
blue
Good! I'd sure like to know how others' would set about it.
Thanks all, great ideas, the last post got the idea for what I'm shooting for. I have drawn up a plan and will frame & sheet the roof like 'pizza ' slices ( i'e : agricultural metal grain bin.) , 15 - 14 ft. rafters starting at 4' o/c and joining at the other end. plywood will be lenghwise up the roof, cut into triangles, blocking every 2 ft. maybe a metal hub at the top to help join the rafters. every section should be the same , a few details to iron out, will do a full scale pattern to 'nail down' the angles ect at this lenght and slope, compound angles should not cause me too many problems.
IF IT WAS EASY, EVERYONE COULD DO IT!