Curved Rafters

[Steve]

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

[bridger]

very little with plywood.

tightbond 2 is plenty strong and waterproof enough as long as it won't be getting rained on directly.

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?

[Lew Hodgett]

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

[Duane Bozarth]

...

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.

[Steve]

Well spotted that the rafters are to be bowed and not form an arch!

At 8ft diameter not 10ft though!

The rafter has to carry just about its own weight, nothing more, its an open frame garden structure (a pergola).

Thanks for the advice

Steve

[Steve]

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

[Steve]

Me too but in the UK its as rare as hens teeth and probably just a bit cheaper than solid gold!

Best Regards

Steve

[Lew Hodgett]

Is that the case with all hardwoods in the UK or just white oak?

Lew

[Duane Bozarth]

....

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... :) )

[bridger]

nothing over them? what will protect them from water?

[Steve]

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

[Steve]

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

[bridger]

definitely do not laminate them from narrow strips of plywood, then.

[Lew Hodgett]

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.

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

[Steve]

Not even marine ply?

[bridger]

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.

[Steve]

I'd better try and find some board then!

Thanks

Steve

[Lawrence Wasserman]

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.

[Duane Bozarth]

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"...

[Larry Jaques]

Yabbut he wants consistent curvature, not blatant warpage, LW. Y'know, curves, not pretzels.

- Woodworkers of the world, Repent! Repeat after me: "Forgive Me Father, For I Have Stained and Polyed."

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