Bent Cedar Rafters

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Forum topic by stop23 posted 09-16-2015 09:28 PM 933 views 0 times favorited 12 replies Add to Favorites Watch
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4 posts in 405 days

09-16-2015 09:28 PM

Topic tags/keywords: bent lamination cedar exterior

I have a plan to build a small pergola over a curved seat wall. I want the pergola to curve with the contour of the wall. I am planning build or buy bent laminated rafters. I am having trouble finding a local contractor who has done this before and the ones willing to try are offering up high bids. I found an out of state outfit that is a little bit more reasonable, but the logistics and uncertainty make me nervous. So I’m leaning toward making them myself, but want to make sure I know what I’m in for…

There are 5 rafters. Each bent around concentric radii ranging from 6 to 13 ft. I am planning to resaw 5/4×8” rough cedar boards to make 3/8” plies and laminate 5 plies.

1st question is about materials. I found a mill to do the resawing for me and have a price on ”#2 or better” cedar. Is #2 cedar adequate for this? What grade should I look for? I priced CVG heartwood cedar, but way to $$$, unless absolutely necessary…in which case I want to make sure that’s spec’d in the bid I have. If #2 is not a good choice, what grade would be better? I’m a bit confused as to the different grading scales (numbers or letters). The yards I’ve asked only grade cedar with numbers.

2nd question is about the glue up. I’m planning to glue them up in my unfinished basement, so I have enough space and environmental control when doing the glue-up. I’ve planned out the forms and expect to spend about $400 on clamps and materials. Will I need more than 1 clamp every 12”? I’ve read a lot about glue, and slow cure epoxy sounds like the best bet for my application, but i’d be interested to hear other opinions.

Thanks for your help.

12 replies so far

View Nicky's profile


695 posts in 3513 days

#1 posted 09-16-2015 10:24 PM

For you first question you can look at for some info. #2 could have some knots that could cause some issues in the bends. #2 may have small knots, but they should be tight and not loose.

Your gluing strategy is fine. More clamps are always better. Dry clamp a test on the smallest radii beam you’ll laminate and insure you don’t have any gaps. If you do, buy more clamps.

Spending time selecting good stock will be worth your time.

-- Nicky

View mahdee's profile


3462 posts in 1189 days

#2 posted 09-17-2015 01:09 AM

I am not sure if you are aware that cedar prices have skyrocketed lately. 4 1×10’s cost me 82 bucks last Friday.


View Jim Finn's profile

Jim Finn

2390 posts in 2343 days

#3 posted 09-17-2015 01:33 AM

3/8” cedar will not bend very well. I do not think you could do a six foot radius bend with it.

-- "You may have your PHD but I have my GED and my DD 214"

View stop23's profile


4 posts in 405 days

#4 posted 09-17-2015 03:05 AM

Thanks for the link Nicky. Unfortunately many of the good independent lumber yards in my area have closed. The yard I used for my cedar railing was 30 miles away. The #2 they delivered was noticeably better than the lumber stocked at the big box. It had some tight knots and much was mixed or flat grain. Appearance wise I don’t mind the small tight knots since it’s rough sawn anyway. But how will flat grain or knots affect the bend?

Jim, Do you have experience bending cedar. I don’t but from what I’ve read 3/8” is on the conservative side for the radius (it’s a 110 degree arc). Most of what I’ve read claims to do this with 1/2 or 3/4.

Mr jinx, the quote I got early this summer was $2.30/lf for 5/4×8 S1S2E #2 BTR. $8.50/lf for CVG all old growth heartwood. I don’t mind paying a premium to get it right, but the top grade puts this project out of reach. I’m not sure where pricing lands in between but it would be nice to pick the pieces from stock rather than special order. I just haven’t found a place that stocks anything other than these two.

View jerryminer's profile


497 posts in 863 days

#5 posted 09-17-2015 04:35 AM

I’ve done bent laminations with cedar and I agree with Jim Finn: 3/8 will not play well with a 6’ radius—-but to know for sure, try a test piece. 1/4 would work much better, IMHO.

Also: I only bend clear stock (CVG). #2 will have knots, which will tend to kink and break the laminate. You might get away with it in the interior, but I would not use it for the exterior faces. If you insist on #2, you will need thinner laminates.

View shipwright's profile


7094 posts in 2219 days

#6 posted 09-17-2015 03:45 PM

If you are using epoxy, don’t finish the glue faces dead smooth and don’t over clamp. A rough-ish surface and moderate clamping pressure should prevent dry joints. You need to have epoxy remaining between the pieces. Over clamping can drive it all out resulting in failure.
In extreme epoxy laminations a strip of thin ‘glass cloth can be added between layers to help with this.

-- Paul M ..............If God wanted us to have fiberglass boats he would have given us fibreglass trees.

View Nicky's profile


695 posts in 3513 days

#7 posted 09-18-2015 03:08 AM

shipwright, the glass cloth is a really good idea. I’m replacing a pvc pipe system for a grapevine with a wooden structure. like stop23, I’ll be bending some beams.

stop23 – the closest big box is 45 miles, my closest lumberyard is 110 miles. I’ve driven out of state to get lumber, its painful.

-- Nicky

View stop23's profile


4 posts in 405 days

#8 posted 09-18-2015 02:16 PM

I do like the idea of mesh between the layers. I imagine the clamping pressure is a bit by feel and experience…since this is my first go at it, the mesh sounds like a good crutch. I was also planning to hit the glued surfaces with a 50 grit belt at slow speed just to rough them up enough to give the glue something to bite.

I may have half the stock resawn to 3/8. If it doesn’t bend on the 6’ form I’ll use it for the larger radii (largest is about 13’. I don’t think that should be a problem at all.

Nicky – I’m not complaining about the 30 miles…just that I’m a hobbyist, so getting there during business hours is inconvenient and challenging. There are closer yards, but I haven’t had good experiences with their product or customer service.

View FellingStudio's profile


93 posts in 1104 days

#9 posted 09-18-2015 02:44 PM

So, I recently completed a similar project … a trellis. You can check out photos and some of the details if you go to my website.

My experience -

  • I used Titebond III (a perfectly good water resistant glue … you aren’t building a boat, so fiber mesh and epoxy aren’t necessary).
  • 3/8” laminations are fine for a 10’ radius. You will almost certainly want thinner laminations for your smaller radius parts.
  • I used cedar 2×6 material that I cherry picked from the local lumber yard. It was a little bit better material than you might find at the big boxes, but still had plenty of knots. Locate knots on the inside of the lamination, and they won’t be an issue. You might need to put a clamp directly over a knot to keep the thing from breaking and creating a big void.

Good luck. Sounds like a fun project.

-- Jesse Felling -

View McFly's profile


181 posts in 449 days

#10 posted 10-10-2015 12:25 PM

We work with this fairly often in our spiral staircase shop. My admittedly novice $0.02 is this;
For a 10’ radius, 3/8” thickness may be fine, but for anything smaller than say, 7’ you might want to mill down to 1/4” and use more lams to obtain your final dimension. Also, you’ll want clamps every 6-8” to hold your radius true all the way across the smaller curve.

I will also second the TB I I I suggestion. We use it all the time for exterior projects and have had zero issues with de-lamination to date so long as you don’t over-crank your clamps and squeeze out too much glue

View stop23's profile


4 posts in 405 days

#11 posted 03-11-2016 09:29 PM

Well I didn’t get to this project before the weather got bad last fall, so I’m starting it back up again. Thanks to the input here, I’m planning to use #2/BTR Select from a small yard. I’ll start with a test run and mill enough for one rafter. 2×8 cedar stock will be resawn twice giving me slightly under 3/8 plies. After refiguring the plan a bit, the tightest bend is a little less than a 9’ radius. If the plies don’t bend well in a dry run I’ll use them for the bigger arc and mill thinner for the small radius.

I’ve read more about glue over the winter than I can possibly comprehend. Pros and cons to all of them. I think I’ll avoid the PUs because of the mess and staining. Seems like some prefer TB III, but I’ll skip that too because of the number of people who have had trouble with adhesion to cedar and I like the longer open time of the other options.

I was planning to use West, but read in a number of other posts about Urea Resins like Unibond 800 and DAP Weldwood. I’m leaning toward the Unibond mostly because it’s less expensive and seems to be well liked for similar applications. Will the Unibond hold up outdoors i.e. Heat/cold, wet, UV?

Any other advice before I start?

View runswithscissors's profile


2127 posts in 1446 days

#12 posted 03-11-2016 11:09 PM

Urea resin glues used to be the standard for waterproof glue ups. Resorcinol was supposed to be better, but was pricey, and required perfectly fitting joints to work reliably. Actually, U R glues also need close fitting joints. In other words, there is little to no gap filling ability. But it shouldn’t be hard to get good close fits with a lamination. U R is about the cheapest glue you can get, very strong, and waterproof unless you decide to boil your laminations.

As to whether wood should be smooth or rough for gluing, there is a school of thought that says smooth surfaces glue better than rough. I haven’t tested these competing ideas in any scientific way, however.

If you do use epoxy, be aware that the stuff is worse than honey for being runny. Over clamping can force the glue right out of the joint. I always use a filler fiber with epoxy, such as wood flour from a sander, for gluing wood. I think it makes a stronger joint, and somewhat ameliorates the runny behavior. Be sure to wear disposable gloves working with epoxy. Disposable clothes would be good too. And shoes. If you get it on your hands (you will), wash your hands in vinegar rather than acetone. Cheaper, and not so toxic. In fact, you can mix the left over vinegar with olive oil and have it on your salad.

I agree that thinner lams are better than thicker. Spring back over time will be less, for one thing.

-- I admit to being an adrenaline junky; fortunately, I'm very easily frightened

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