The carpenter I work with intends to build a set of finish stairs for his house. The question is about the housed stringer. He doesn’t see the need to rout the housed stringer with a widening groove to then set the treads and risers with wedges. He wants to simply rout the stringer to fit the treads and risers exactly ala a cased bookshelf. I think the grooves provide ease of installation and means a quarantee of a tight fit on the visible side of the stringer. The article we are looking at (FH March 98 #114) does not speak specifically as to why one tapers the groove and uses wedges and so neither has a solid argument to convince the other. Is there a specific reason for the taper? Also, he thinks the wedges are to straighten out any cup in the treads and risers but he is glueing up the treads from cherry stock and planing them flat and using plywood for the risers and thus has nothing to straighten out. Thanks, Tim Hogan Madison WI.
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One reason for tapering the vertical slots is so you can cut a dado in the face of the riser to engage the back edge of the tread. With the tapered slot, you can push the riser up into place behind the tread and then swing it forward. Capturing the back edge of the tread like that should increase the stiffness of the back half of the tread. If you don't want the risers to extend down past the edges of the treads, you can rabbet the bottom of the back edge of the treads and use a narrower dado in the risers.
Another possibility would be to undercut the back edge of the riser slots and the bottom edge of the tread slots, like sliding half dovetails. Then cut matching undercuts into the backs of the risers and the bottoms of the treads, and cut the wedges with a trapezoidal cross section. That way, hammering the wedges in would force the tread and risers tighter _into_ the slots, as well as tighter against the sides of the slots.
Tapered wedges exert tremendous pressure on the treads and not only result in perfect fits with a minimum of effort, but prevent the treads from squeaking.
Trying to glue treads in like cabinet bottoms will result, I think, in a big mess.
Clampman
I second the big mess Result!
Plus the squeak thing.
Tell him if it was such a great idea they wouldn't be doing it the other way.
TDo not try this at home!
I am a trained professional!
Jim: Without a doubt...treads and risers mortised and wedged into a housed stringer is the strongest and neatest installation method I know of.
The attached picture is one in my shop being assembled using this method.
You are the best, Stan. I love seeing your work. Our local stairbuilder here has been doing something for the last few years, that is perhaps worth looking into. Using two routers and one jig setup, he first hogs out the housing in the stringer with a spiral bit, the takes the second router, outfitted with a dovetail bit with a minimal (about 4 degree) dovetail angle, and runs around the entire edge of the housing. He claims the slight undercut guarantees him snug fits every time. There is no corresponding dovetail cut on the tread ends, riser ends, or wedges.
Gene: Thanks for the compliment but I have to disagee with you about being the best..ok? Far from it. I have seen where they dovetail the mortise ...but I have never had a problem with the treads and risers snugging up real tight.
Question Stan?
When do you rout the dado in the side rails before you bend them or after and how about the stair tread do you radius to match bend side rails?
Fred: I mortise the stringers after they are glued up. I radius the end of the tread to match the bottom of the mortise.