Unique "timber" stairway

I am in the process of building a stairway using a Kerf-Wedged Dovetail through Mortise and Tenon joint to tie the treads to the stringers. I am building this stairway of Butternut treads and Red Oak, full face stringers and Cherry wedges. There will only be end stringers and no risers. I've cut all the wood to outer dimensions and am ready to begin cutting the joints. Just curious to hear from someone who may have done this before.

My plan for the joint dimensions are a 1x8 by 1.5" long tenons cut into 2x10 treads. The wedges will be 3/16" wide at the butt with a 1/4" kerf to the edge of the tenon. The treads are housed into the stringer 1/2" at either end. The Oak stringers are 2x12" so i have 1.5" of tenon plus 1/2" of housed thread coming flush to the outer surface of the stringer which will all be exposed.

We recently built an open riser staircase out of white pine. The stringers were 4"x14" and the treads were 3"x12". Through mortise was 2"x10" and wedged with similar red oak shims in from the edges about 3" . The tenon was centered. Wasn't actually a dovetail joint, as we glued it also. Our concern was splitting of the treads so we weren't to aggressive pounding the wedges into the 1/8" kerf. We didn't house the joint because of shrinkage concerns. Biggest difficulty was the mortises. We bolted the stringers together with exposed sides out and then drilled from outsides to the center.
That's how we did it. Good luck on your project.
Mark Surnoskie

Shaun, In your post, it appears that when your stair tread tenon goes through your mortice at your stringer it will be flush with the outside surface. If planed, sanded, and finished, you will end up with a furnature or cabinet grade end product. For a more rustic look, extend the tenons through the outer face of the stringer 1/4" to 5/16" and carefully chamfer the ends of the tenons. Assemble your stairs, drive in the
wedges and chamfur to match. Another option is, extend the tenons aprox. 1-1/2 through the stringer and make small mortices through the tenon, make small square wedges with a slight taper and drive them down through the top of the tenon which will pull the stair tread tight to stringer. Two wedges spaced about 4"to6" apart also looks good. If you wse this method make sure you use a good wood glue where the stair tread fits into the stringer dato, and DON'T over tighten the wedges or you will split out your tenon. Also build the stairs on a flat surface or in the stair opening for a perfect fit, or your stairs will be racked or twisted. Good luck and let us know the end result, John...

Thanks for sharing your comments guys.

Mark, I don't quite understand the bolting description. Was this for added reinforcement? Did you find your wedges to actually work and spread the tenon out being 3" from the edge of the tenon? that seems like a lot. I planned my wedges to be only about 3/4" from the edges.

John, I did give the more rustic approach some thought, but was talked out of it, By Jack Sobon of all people.... he live near by, so i bug him for advice now and then. I've already committed to the "furniture look" of flush tenons as I've already cut the tread stock to length. thanks for the ideas anyway.

will yet ya, know how it turns out. maybe will post a pic if i don't screw it up....

humm, i think i get it: Mark, you bolted the stringer together in order to drill the mortises? I plan to drill one corner or each mortise to get a router started and router out the housings and mortises then use a corner chisel to square out the corners. I'm doing the tenons and wedge kerfs in the treads all on a table saw. I'll use a generous diameter stop drill at the end of each kerf to avoid splitting as it will be hidden in the housing anyway. I think I will also glue all the joints to avoid a creeky stairway.
And then hope for the best.....