Laminating Curved Beams
Woodworkers approach the puzzle of creating structural porch beams that curve in the horizontal dimension. April 24, 2006
Question
I am rebuilding a wrap around porch on a 100 year old house. An old picture shows the corner being round. What is the best way to make the upper and lower beams for an 8 foot radius? Should I laminate quarter inch pieces of wood or cut grooves in 2 X 12s to allow them to bend?
Forum Responses
(Architectural Woodworking Forum)
From contributor F:
I think you will have to laminate them due to a strength issue. I would use one of the relatively new PUR adhesives called polyurethanes. Gorilla Glue is one brand, but for the quantity you would need, I would search the net for a wholesale supply.
From contributor E:
Why don't you use something besides wood? That would eliminate so many potential problems.
From the original questioner:
What could you use besides wood?
From contributor E:
Plastic, high density polyurethane is one option.
From contributor P:
Don't kerf bend the components. On an exterior job such as this, the amount of exposed joint work in kerfed work is asking for trouble. Also, kerf-bent stock will be a lot weaker, as you have to saw through more than half the thickness of the stock to make the bend. Steam bending is probably out as well, depending on the section required - it can be pretty dangerous, too!
The traditional and probably best way to do this is to make up your curved parts by "bricking," i.e. shorter pieces glued and cramped together in overlapping layers - like bricks in a wall. The ends of each piece need to be accurately mitered for a tight joint. Once assembled and glued, the whole piece can be band sawn to approximate curve and finished using either a shaper, curve jig and ring fence or a heavy duty router with a jig and follow template (bearing) bits. If you are into old tools, you could always try working the flat surfaces of the radii with a compass plane, although you'll need a big bench (look for a Record #020 or Stanley #20 on eBay).
I'd take a rain cheque on the PU glue, too. Gorilla glue is very expensive and works no better than an exterior grade PVA or aliphatic resin glue (you want one which is cross-linked) or a so-called plastic resin glue (urea formaldehyde glue) such as Aerolite 306. The gap-filing characteristics of PU glue aren't really a help at all, as the foam in the gaps has no structural strength. The only way to do it right is to have good tight joints.
From contributor B:
If the beams are simply there for nailing of the flooring and fascias, then you can use segmented construction or perhaps even the plastic material mentioned. However, if the beams are structural, then you need to laminate. I would not trust an overlapping segmented construction in this situation.
You'll need to build a form for bending and gluing the approximately 1/4" x full height x full length resawn strips. I would use marine epoxy (such as West Systems) for this. Again, if this is structural, you need to make sure it won't be falling apart when 10 people happen to stand on the porch. Also, if this is structural, you have to consider the local code issues. In CT I wouldn't take on the job, as I believe all structural built-up components need to be engineer stamped.
From contributor D:
I would only do this with bent lamination as contributor B suggests. 1/4" A/C exterior ply bent on a form, staggered ends, epoxy or urethane, with a few nails. This will give you great strength and rigidity to build from. When you get to railings, trims and the other curved elements, you may use some bricklay, some kerfed, and some sawn segments for curves. These things typically require every trick you can pull off.
From contributor F:
Yes, since you called it a "beam," I too assumed it was structural and recommended bent lamination. The PUR or poly glues are actually waterproof. Don't use the PVA that is supposedly so. I have done tests with poly and you can submerge the bent laminated joint in water. Marine epoxy and urea resin are waterproof.
From contributor C:
I am working with the questioner on rebuilding the Victorian wrap-around porch. The big question concerns the roof, as this is structural, and is cantilevered back into the 16 foot long straight sections of the roof. The columns at each end of the radius serve as the fulcrum points. The finished beam needs to be 10" high to mate with the one remaining original straight roof section. The questioner and I were planning to resaw 16 foot 2x10s, to make 1/4 inch thick laminations. These, then, were to be bent and glued up on a form. But we couldn't get good laminations with a 14" Delta bandsaw.
The suggestion of using 1/4" thick plywood sounds very promising, since we can't seem to cut the laminations ourselves. With this added background, can anyone else provide additional information? Your help is greatly appreciated!
From contributor F:
Like contributor D stated, you will need to precut your 1/4" ply laminates so that the joints will be staggered to provide the greatest strength possible to your beam. It is a lot of work, but the best gap free results I get with radiused laminations comes from clamping them up in a two piece form usually made from something inexpensive like segmented particleboard. I lay my forms out so that I can add 1/8" masonite or similar material as liners on both the concave and the convex halves of the form. These liners will bridge any minor flaws in the curved cauls and make it possible to have a gap free inside and outside first laminates. I also recommend a glueless trial clamp up, which can save a lot of misery later on. It helps you see a lot of problems you may encounter and allows you to have everything you will need on hand to have a successful actual glue up.
Be sure and wax areas that will be a problem if they adhere to the glue, or use plastic laminate to prevent adhesion where it is not wanted. After your dry clamp-up, you should have a sense of how much open time you will need from an adhesive.
From contributor E:
Ah, structural. Might be a no-go on the plastics.
From contributor N:
If you need to resaw planks for a glu-lam operation, many decent lumberyards can do that for you. Locally, we have several that will resaw and plane to order. It's not cheap, but relative to the labor you will expend otherwise, in my opinion, it's a no brainer.
From contributor C:
Thanks for the great input! I hadn't thought of that. That's what makes these forums so fantastic. An ordinary guy can get the brain power of the world's experts.
