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#1
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Curved Rafters
I intend to make 3 curved rafters for a garden application. Each rafter
needs to be 7ft(L) x4inch(H) x 1-1/2inch(W) finished. The radius of the curve will be 10ft. Material will be 1/4 inch external grade ply laminated to make up the width, glued and screwed over a former. How much "spring back" should I allow for? I had Resourcinol in my head but a previous post suggests this is OTT for this sort of application (probably mega expensive too), suggestions welcome. I can probably buy these, but then thats just another learning opportunity lost. Best Regards Steve |
#2
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On Sat, 16 Jul 2005 08:09:10 +0100, "Steve"
wrote: I intend to make 3 curved rafters for a garden application. Each rafter needs to be 7ft(L) x4inch(H) x 1-1/2inch(W) finished. The radius of the curve will be 10ft. Material will be 1/4 inch external grade ply laminated to make up the width, glued and screwed over a former. How much "spring back" should I allow for? very little with plywood. I had Resourcinol in my head but a previous post suggests this is OTT for this sort of application (probably mega expensive too), suggestions welcome. tightbond 2 is plenty strong and waterproof enough as long as it won't be getting rained on directly. I can probably buy these, I've never seen them for sale. there's a reason for that, too. curved (bowed) rafters like I think you're talking about take weird side loads. are you sure you have your engineering in order? but then thats just another learning opportunity lost. Best Regards Steve |
#3
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Steve wrote:
I intend to make 3 curved rafters for a garden application. Each rafter needs to be 7ft(L) x4inch(H) x 1-1/2inch(W) finished. The radius of the curve will be 10ft. Material will be 1/4 inch external grade ply laminated to make up the width, glued and screwed over a former. How much "spring back" should I allow for? I had Resourcinol in my head but a previous post suggests this is OTT for this sort of application (probably mega expensive too), suggestions welcome. I can probably buy these, but then thats just another learning opportunity lost. Unless these are exposed to the weather, resorcinol should not be required. Understand the new TiteBond III is pretty good for wet, not underwater, applications, but no personal experience. Personally, I would use 1/4" solid material such as quarter sawn white oak rather than plywood for laminating stock. Don't have a clue about spring back, but close spacing of clamps will help minimize it. Lew |
#4
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#5
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wrote in message ... On Sat, 16 Jul 2005 08:09:10 +0100, "Steve" wrote: I intend to make 3 curved rafters for a garden application. Each rafter needs to be 7ft(L) x4inch(H) x 1-1/2inch(W) finished. The radius of the curve will be 10ft. Well spotted that the rafters are to be bowed and not form an arch! I can probably buy these, At 8ft diameter not 10ft though! I've never seen them for sale. there's a reason for that, too. curved (bowed) rafters like I think you're talking about take weird side loads. are you sure you have your engineering in order? The rafter has to carry just about its own weight, nothing more, its an open frame garden structure (a pergola). but then thats just another learning opportunity lost. Best Regards Steve Thanks for the advice Steve |
#6
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"Duane Bozarth" wrote in message ... s wrote: On Sat, 16 Jul 2005 08:09:10 +0100, "Steve" wrote: I intend to make 3 curved rafters for a garden application. Each rafter needs to be 7ft(L) x4inch(H) x 1-1/2inch(W) finished. The radius of the curve will be 10ft. ... If I didn't miss by a mile, I get that the total drop (assuming the 7' is the chord length, not the arc) is just under 8". If so, why not simply scribe the rafter on tuba-12 stock and cut them to shape? You may be just under 4" depth, but not by much or you could cheat just a little on the radius if the depth is absolutely critical. The length of the arc is 7ft, it's the thinnest dimension I need to bow. I intend to make the former from a couple of 3/4 inch ply sheets. Thanks Steve |
#7
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"Lew Hodgett" wrote in message ink.net... Steve wrote: I intend to make 3 curved rafters for a garden application. Each rafter needs to be 7ft(L) x4inch(H) x 1-1/2inch(W) finished. The radius of the curve will be 10ft. Personally, I would use 1/4" solid material such as quarter sawn white oak rather than plywood for laminating stock. Me too but in the UK its as rare as hens teeth and probably just a bit cheaper than solid gold! Best Regards Steve |
#8
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Steve wrote:
Me too but in the UK its as rare as hens teeth and probably just a bit cheaper than solid gold! Is that the case with all hardwoods in the UK or just white oak? Lew |
#9
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Steve wrote:
..... The length of the arc is 7ft, it's the thinnest dimension I need to bow. I intend to make the former from a couple of 3/4 inch ply sheets. Ah...I thought you were wanting a sloping roof pitch. My idea doesn't work then, granted. (Easier if it were what you wanted though... ) |
#10
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On Sat, 16 Jul 2005 21:56:00 +0000 (UTC), "Steve"
wrote: wrote in message .. . On Sat, 16 Jul 2005 08:09:10 +0100, "Steve" wrote: I intend to make 3 curved rafters for a garden application. Each rafter needs to be 7ft(L) x4inch(H) x 1-1/2inch(W) finished. The radius of the curve will be 10ft. Well spotted that the rafters are to be bowed and not form an arch! I can probably buy these, At 8ft diameter not 10ft though! I've never seen them for sale. there's a reason for that, too. curved (bowed) rafters like I think you're talking about take weird side loads. are you sure you have your engineering in order? The rafter has to carry just about its own weight, nothing more, its an open frame garden structure (a pergola). nothing over them? what will protect them from water? but then thats just another learning opportunity lost. Best Regards Steve Thanks for the advice Steve |
#11
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"Lew Hodgett" wrote in message nk.net... Steve wrote: Me too but in the UK its as rare as hens teeth and probably just a bit cheaper than solid gold! Is that the case with all hardwoods in the UK or just white oak? Lew The only hardwood thats appears prevalent (and some of that is not best grade) is mahogany. Hardwood is only available from specialist suppliers Best Regards Steve |
#12
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wrote in message ... On Sat, 16 Jul 2005 21:56:00 +0000 (UTC), "Steve" wrote: wrote in message . .. On Sat, 16 Jul 2005 08:09:10 +0100, "Steve" wrote: I intend to make 3 curved rafters for a garden application. Each rafter needs to be 7ft(L) x4inch(H) x 1-1/2inch(W) finished. The radius of the curve will be 10ft. nothing over them? what will protect them from water? The rest of the frame is conventional timber section which is "tanalised" - pressure treated with preservative. I was planning to coat these rafters with preservative spring & fall like I do with the garden shed, furniture etc. Thanks Steve |
#13
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On Sun, 17 Jul 2005 08:01:19 +0100, "Steve"
wrote: wrote in message .. . On Sat, 16 Jul 2005 21:56:00 +0000 (UTC), "Steve" wrote: wrote in message ... On Sat, 16 Jul 2005 08:09:10 +0100, "Steve" wrote: I intend to make 3 curved rafters for a garden application. Each rafter needs to be 7ft(L) x4inch(H) x 1-1/2inch(W) finished. The radius of the curve will be 10ft. nothing over them? what will protect them from water? The rest of the frame is conventional timber section which is "tanalised" - pressure treated with preservative. I was planning to coat these rafters with preservative spring & fall like I do with the garden shed, furniture etc. Thanks Steve definitely do not laminate them from narrow strips of plywood, then. |
#14
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Steve wrote:
The only hardwood thats appears prevalent (and some of that is not best grade) is mahogany. If it is not Honduras, it's not mahogany, IMHO. There is a bunch of crap imported from some place in Africa that tries to pass itself off as mahogany, but I don't use it. Hardwood is only available from specialist suppliers I would expect nothing less. BTW, based on your description of the application, trying to use laminated plywood will be a total waste of time and money IMHO, but it is your time and money. Lew |
#15
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wrote in message ... On Sun, 17 Jul 2005 08:01:19 +0100, "Steve" wrote: wrote in message . .. On Sat, 16 Jul 2005 21:56:00 +0000 (UTC), "Steve" wrote: wrote in message m... On Sat, 16 Jul 2005 08:09:10 +0100, "Steve" wrote: I intend to make 3 curved rafters for a garden application. Each rafter needs to be 7ft(L) x4inch(H) x 1-1/2inch(W) finished. The radius of the curve will be 10ft. nothing over them? what will protect them from water? The rest of the frame is conventional timber section which is "tanalised" - pressure treated with preservative. I was planning to coat these rafters with preservative spring & fall like I do with the garden shed, furniture etc. definitely do not laminate them from narrow strips of plywood, then. Not even marine ply? |
#16
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On Sun, 17 Jul 2005 17:35:50 +0100, "Steve"
wrote: nothing over them? what will protect them from water? The rest of the frame is conventional timber section which is "tanalised" - pressure treated with preservative. I was planning to coat these rafters with preservative spring & fall like I do with the garden shed, furniture etc. definitely do not laminate them from narrow strips of plywood, then. Not even marine ply? nope. marine ply has glue that will be up to the task, but the transverse plies still present end grain to the weather, from top and bottom. water will soak these plies. expansion and contraction will tear the beam apart after a short while. |
#17
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wrote in message news On Sun, 17 Jul 2005 17:35:50 +0100, "Steve" wrote: nothing over them? what will protect them from water? The rest of the frame is conventional timber section which is "tanalised" - pressure treated with preservative. I was planning to coat these rafters with preservative spring & fall like I do with the garden shed, furniture etc. definitely do not laminate them from narrow strips of plywood, then. Not even marine ply? nope. marine ply has glue that will be up to the task, but the transverse plies still present end grain to the weather, from top and bottom. water will soak these plies. expansion and contraction will tear the beam apart after a short while. I'd better try and find some board then! Thanks Steve |
#18
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#19
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Lawrence Wasserman wrote:
If I've said it once I've said it a thousand times: You can find these ready-made at the regular lumber section in Home Depot. OP is in UK, I think...although I know they have some "super stores", in my limited times over the pond I never had the opportunity to investigate what would be their Borg--although by now, maybe they've been exported there as well??? It's been some time since I was last there... I've posted it before, but my favorite related wreck-related experience was visiting a chap in his shop--what attracted me was the sign "Joinery for Purpose"... |
#21
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"Lawrence Wasserman" wrote in message ... If I've said it once I've said it a thousand times: You can find these ready-made at the regular lumber section in Home Depot. -- Larry Wasserman Baltimore, Maryland As someone once said, Baltimore ain't in Maryland. I'd be amazed if they sold rafters 7ft arc length on a 9ft 6" radius. I can get off the shelf 8ft radius in the uk. They are about $100 a pop in your money. Steve |
#22
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Here's a quick way to figure springback for a lamination:
springback = original_deflection / (number_of_laminations)^2 This is actually derived for a cantilever beam, so it's only an approximation for a radial bend. Robert Steve wrote: "Lawrence Wasserman" wrote in message ... If I've said it once I've said it a thousand times: You can find these ready-made at the regular lumber section in Home Depot. -- Larry Wasserman Baltimore, Maryland As someone once said, Baltimore ain't in Maryland. I'd be amazed if they sold rafters 7ft arc length on a 9ft 6" radius. I can get off the shelf 8ft radius in the uk. They are about $100 a pop in your money. Steve |
#23
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Here's a quick way to figure springback for a lamination:
springback = original_deflection / (number_of_laminations)^2 This is actually derived for a cantilever beam, so it's only an approximation for a radial bend. Robert Steve wrote: "Lawrence Wasserman" wrote in message ... If I've said it once I've said it a thousand times: You can find these ready-made at the regular lumber section in Home Depot. -- Larry Wasserman Baltimore, Maryland As someone once said, Baltimore ain't in Maryland. I'd be amazed if they sold rafters 7ft arc length on a 9ft 6" radius. I can get off the shelf 8ft radius in the uk. They are about $100 a pop in your money. Steve |
#24
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Steve wrote: ... nothing over them? what will protect them from water? The rest of the frame is conventional timber section which is "tanalised" - pressure treated with preservative. I was planning to coat these rafters with preservative spring & fall like I do with the garden shed, furniture etc. definitely do not laminate them from narrow strips of plywood, then. Not even marine ply? Marine ply is not made from rot-rsistant wood though the glue is _supposed_ to have a mildew retardant additive. Even if you can find Mahogany-faced marine plywood chances are good that the inner veneers are poplar. The most popular marine plywoods are Okoume, Sapele, and Doug Fir. The latter is probably more rot-restant than the other two. -- FF |
#25
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Lawrence Wasserman wrote: If I've said it once I've said it a thousand times: You can find these ready-made at the regular lumber section in Home Depot. I live not far from Balmore and have NEVER seen rafters curved with a 10 foot radius in any section at any Home Depot. -- FF |
#26
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On Sat, 16 Jul 2005 08:09:10 +0100, "Steve"
wrote: I intend to make 3 curved rafters for a garden application. Each rafter needs to be 7ft(L) x4inch(H) x 1-1/2inch(W) finished. The radius of the curve will be 10ft. Material will be 1/4 inch external grade ply laminated to make up the width, glued and screwed over a former. How much "spring back" should I allow for? I had Resourcinol in my head but a previous post suggests this is OTT for this sort of application (probably mega expensive too), suggestions welcome. I can probably buy these, but then thats just another learning opportunity lost. Odds are, you've already done this, but I did this exact thing last week, and just happened to spot the thread today. The contractor I now work for does very high-end historical remodeling/renovation, and we had to fabricate a new roof for a garage extension to match the existing structure- which in this case meant curved rafters with a curved hip. To get it in, we got a bunch of 2" x 12" douglas fir, and cut the rafters to shape with a bandsaw on-site. The first rafter was rough sawn and then shaped with a rasp until it fit the curve of the existing soffit perfectly, and then the subsequent rafters were cut and then attached to the first (master) rafter with a C-clamp and sanded to match the master with a belt sander. To get the hip, we rough cut a hip rafter, tacked it into place, and then ran a string line across the regular rafters every 4" to get reference marks to use for plotting the extended curve. Looks really nice, and it may or may not help you, depending on how quickly you've gone after this project. FWIW, there was no springback involved, as the new roof was sheathed with 1" x 4" cedar planks rather than plywood to match the original construction. |
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