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Arched Rafter for Pergola

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Forum topic by ellerbrj posted 03-23-2018 12:48 PM 517 views 0 times favorited 13 replies Add to Favorites Watch
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ellerbrj

4 posts in 181 days


03-23-2018 12:48 PM

Hello All,

Looking for some advise. I need to create a arched 2×6x14’ rafter with a 24’ radius for a pergola I am building. I have done the math and will need a board that is 18.5 inches wide. Was thinking about using a combination of 1-2×8 and 2-2×6’s doweled and edge glued together? The only weight on these will be some additional 2×4 purlins. Is this doable and the right approach?

Thanks in advance for any input!
Jon


13 replies so far

View Redoak49's profile

Redoak49

3450 posts in 2105 days


#1 posted 03-23-2018 01:24 PM

I think I would build it up with 3/4” boards glued together. This way you would not have any joints going all the way through.

View jimintx's profile

jimintx

820 posts in 1701 days


#2 posted 03-23-2018 01:43 PM

Redoan’s suggested technique sounds good to me. You’d end up with a laminated beam, which is now a common construction element.

Part of the how-to on this will be to select the correct, or best, type of wood for such an exposed exterior beam. A outdoor, pergola beam sounds like it might be cedar, but maybe in your area, or for your intended service, there are other choices.

My immediate thought for an exposed beam tends to use pressure treated pine, the most common construction material used in my region. But I don’t know about the glue up method, if there is one, for using such pressure treated boards.

I am thinking a lamination approach would include use of 1” dimensional lumber. You’d have to decide if the boards needed to be surface planed prior to glue up.

Perhaps this could also be done with scarfs in the 2” material you have in mind, to give the joints more inherit strength.

.

-- Jim, Houston, TX

View bondogaposis's profile

bondogaposis

4926 posts in 2468 days


#3 posted 03-23-2018 01:56 PM

If you do it the way you propose, you will have the round overs on the edges showing in your lamination. If that doesn’t bother you, then it will work.

-- Bondo Gaposis

View John Smith's profile

John Smith

1308 posts in 279 days


#4 posted 03-23-2018 02:17 PM

Jon – can you post some examples of what you want to do ?
photos and drawings go a long way for the gallery to provide accurate information.
as well as what part of the country you are in.

.

-- some people are like a Slinky - - - pretty much good for nothing. But still make you smile when you push them down a flight of stairs.

View JBrow's profile

JBrow

1366 posts in 1037 days


#5 posted 03-23-2018 03:08 PM

ellerbrj,

For what it is worth; noting that I am not a structural engineer nor in the construction business:

Three sticks edge glued would yield the 18-1/2” wide 2X but with 2 glue seams. Eliminating one glue joint would reduce structural dependence on the glue. This could be achieved with a 2×12 x 16’ + 2×8 x 16’ (11.25” + 7.5” = 18.75”).

Glue selection is important in structural applications. Typical PVA woodworking glue is not suitable since it is a non-structural structural glue. The USDA offers info concerning adhesives. The text surrounding Table 10-2 on page 10-10 discusses adhesives and their properties…

https://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr190/chapter_10.pdf

Dowels would help with keeping the faces of the lumber aligned during glue-up. However I doubt the dowels would improve the strength the of the joint to any significant degree.

Edge gluing long lumber with a tight fitting joint along the entire length would be challenging for me. Any gaps in the joint could weaken the glue bond. A half-lap joint cut long the long mating edges of boards would provide more glue surface and could thus be stronger than a dowel reinforced butt joint. A rabbeting bit in a router could cut the half-lap. The half lap could eliminate the need for dowels, if dowels are used primarily for face alignment.

However, the rabbeting method would require two-way clamping pressure. The half lap joint would also require slightly wider lumber. In your example, it could require 2 – 2×8 + 1 – 2×6. In my two board example, it would require 1 – 2×12 + 1 – 2×10. The glued-up rafter would probably have to be ripped to final width.

The glued-up rafter could the mechanically reinforced with long timber screws. Timber screws would reduce reliance on glue only. Also, the timber screws could also be used in lieu of clamps. They can be removed when cutting the arch and then reinstalled.

If this was a Pergola for my backyard, I would use full length lumber for the rafter and avoid any splices. Butt splices would rely on glue alone for strength. But if a splice to lengthen the rafter is used, a structural joint such as Stopped-Splayed Wedged Scarf Joint, along with glue, may be worth considering. Here is a video showing its construction in large format timbers.

https://www.youtube.com/watch?v=6j-U1P49r8U

View Fresch's profile

Fresch

302 posts in 2037 days


#6 posted 03-23-2018 03:16 PM

How do you plan on finishing it?
3/4 ply wrapped with 1x?
Interlock your 2xs with 1/2” dado joints?

View ellerbrj's profile

ellerbrj

4 posts in 181 days


#7 posted 03-23-2018 03:16 PM

Thanks to all so far. The boards will be cedar located in northwest Ohio. He is a rough image and I don’t mind if the dowel plugs show

View ellerbrj's profile

ellerbrj

4 posts in 181 days


#8 posted 03-23-2018 04:00 PM

View Loren's profile

Loren

10477 posts in 3765 days


#9 posted 03-23-2018 04:53 PM

That should be fine. If you’re clever about using
the stock you may be able to saw out the
bottom of the curve and glue the waste to
the top.

View BobAnderton's profile

BobAnderton

283 posts in 2907 days


#10 posted 03-23-2018 04:56 PM

Looks like most of the 2×8 will be wasted. You could cut the two triangles that come out of the 2×8 out of about 1/3 of a board.

The is a very gradual bend. Any interest in creating it by gluing up multiple strips in a bent form?

-- Bob Anderton - Austin, TX - Nova 3000 lathe, Alaskan Mark III mill, Husqavarna Saw

View jerryminer's profile

jerryminer

942 posts in 1558 days


#11 posted 03-23-2018 05:13 PM

I think your proposed method will work. Use a structural glue (epoxy, resorcinol, Urea-formaldehyde.)

You can save a little on lumber with a layout like this:

As noted above, the round-over edges will show. I would rip off the factory edges both to eliminate the round-overs and to get fresh edges for a better glue bond. If this leads to a too-narrow blank, You can add another piece as needed.

-- Jerry, making sawdust professionally since 1976

View jimintx's profile

jimintx

820 posts in 1701 days


#12 posted 03-23-2018 05:53 PM

Seems like there is some really great advice being given.
I hope we get to see the actual execution and the final beam and then the overall project.
I suspect your pergola will be a unique and very nice one.

-- Jim, Houston, TX

View ellerbrj's profile

ellerbrj

4 posts in 181 days


#13 posted 03-23-2018 06:56 PM

Wow. Thanks for all the input and suggestions. I especially like the creativity with eliminating waste and to jerry for the drawing example with dimensions

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