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

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

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

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

"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

"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

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

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... :) )

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

"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

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

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.

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

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?

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.

wrote in message
...
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

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

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"...

On Mon, 18 Jul 2005 14:05:05 GMT, the opaque
(Lawrence Wasserman) clearly 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.

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."
-
http://diversify.com Comprehensive Website Design

"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

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

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

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

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

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.