Curved Stair Railings 101
From contributor F:
I would say that if you have no stair building experience (very complex work) you are probably taking on too much by starting out with the most difficult task in that line of finish work. Also, if you have no bent lamination experience you are in for a stressful wrestling match at glue up time.
All bent laminations have some spring-back, but this is usually not a problem since the work can usually be forced back into the shape that the forms or whatever holding devices had, and the work will then be restrained by fasteners (hand-rail supports, newel posts, ballisters, and etc.)
I donít know of any instructions or the like. I helped an english furniture maker do one once and we bolted shop made clamping blocks all along the curved walls of this particular curved stairway and also we were using shop made wood laminates, not the commercial ones that nest together.
From contributor R:
Thereís nothing like starting at the hard part. As for spring-back- when you set your brackets on the treads or sub-treads, set the middle one first, then move the upper and lower ones tighter 1/16" for each tread as you move up and down, this allows for most of the spring-back.
You're going to need boko clamps, 1 to each bracket and 2 or 3 in-between brackets. Be sure to mark position on brackets. Apply glue to laminations- sandwich together, wipe excess glue, and tape around about every two feet to hold together. Then it gets intense. You will need 3-4 people to work it, and you will only have a few minutes.
Also as you get to each end the rail will tend to twist, you will need to manually twist with a large clamp attached and pop a kicker under to hold the clamp. Be sure to wax the bending mould before hand and do a test run, before the real thing. Coffman puts out a stair manual that covers a little bit on bending, but not completely. Leave it in place clamped up as long as you can - 3-4 days best. Then the clean up and sanding is not a fun part.
From contributor F:
To contributor R: Can you explain what a bending mold is?
From contributor R:
To contributor F: It's a female version of the side profile of the rail, extends about 3/8 above and below and about 3/8 thick at these points. Itís generally in poplar, although you can also get it in plastic. Sandwich the bending rail between or use in short pieces where you are putting clamps. It protects the rail from clamp marks and gives a squared (flat) clamping area.
From contributor H:
I agree with the others. There is nothing like starting at the finish. We bend our rail on the forms used to build the stringer. But they can also be bent using the blocking clamped to the stair on site. This is often a messy thing to do in the clientís home. One small tip about using the bending moulding is that we have found that when using the bending moulding, whether wood or plastic, it's better to use small pieces only in the area that is being clamped.
We clamp at every riser line. This allows you to actually see what is happening to the rail as it is clamped and twisted, and then spring clamp the areas between. Also be prepared for the rail to get some long grain checking as the wood lets loose. The tighter the radius the more you might get. This won't always happen, but when it does it doesn't mean the rail is ruined. Just fill and sand.
Spring-back is not a big problem on large curves. They pull back when installed to the posts and balustrade. Don't rush to unclamp everything. We leave our curved rail clamped for three full days before releasing. The glue may look and feel dry on the outside over night but inside itís still soft. Don't rush and you won't have to do it again.
From contributor D:
I would think twice about this kind of work, the reason being that there are a lot of factors that you have to consider in this line of work; all other trim work is a walk in the park compared to this work. The spring-back for me is a secondary issue in comparison to the twisting of the rail, as you clamp the rail into place you have to be very careful so the rail rides perpendicular to the treads otherwise the bottom of the rail will becomes your side of the rail, and if you have a over the post system then you are in real trouble.
Another thing you have to watch for is the way you do your cuts for the connections (gussneck, volute, etc). Itís better to cut twice then do the job all over again. I only do staircases and most of the trim guys that I run into only mess with straight rail jobs, and only a low percentage get involved with curved railing.
From contributor C:
I use west system epoxy. It costs about five times as much as yellow glue but you have all the time you want to glue up. Another advantage is that it webs any cracks that you will all most always get. I glue up 3 X 3's and it gives me some wiggle room to clean up some errors. I have quit using anything but mahogany because of its ease of profiling.
I clamp every three inches with cauls that extend below and above the rail. I have made the cauls out of Brazilian cherry because they are so hard. I usually cut the strips thin enough that it takes about 15 to get the thickness in the finish rail. Since I make my own volutes and other parts it gives me a wider number of options as to how I want to design the first three treads.
I like the stacked commode look and my clients like the one of a kind. I make springing volutes and it gives the balustrade a very unique look. I also build them on the job, I have a mobile shop that makes this work for me. It took me a while to get here, but every job I do is a labor of love and I pursue my craft as an art form. I hope this is a benefit to you guys that are just getting started.
The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).
Comment from contributor O:
Comment from contributor B:
Bending mold is a must. Itís the only way to get the clamps to work like they should. I have found that making square horseshoe shaped pieces nailed in about every foot make for a great bending rail bracket. They also help when the rail not only bends but twists because you can clamp them vertically and horizontally.
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