A fast, accurate way to make mortised stairs

A fast, accurate way to make mortised stairs

The typical approach to stair-building is to cut bird’s-mouth notches in a pair of stringers, then affix the treads to the tops of the bird’s mouths. A far-stronger method is to cut mortises in the sides of the stringers to accommodate the treads. The drawings below illustrate the jig I use to lay out and cut mortised stringers quickly.

First, on a piece of 1/2-in. plywood, lay out the stair’s rise and run. As shown in drawing #1, the plywood should be big enough to work as both a router template and as a layout tool for the next mortise.

Next, cut a slot in the plywood deep enough to accommodate the depth of the treads. The slot’s width should equal the thickness of the treads, plus a tiny bit of wiggle room. Align the plywood jig on the stringer, and affix a pair of guide cleats to each side of the jig. The cleats register the jig on the stringer and give you purchase to clamp the jig.

Clamp the jig to the stringer, and use a router with a bearing-guided bit to cut a 3/4-in.-deep mortise (drawing 2). Make more than one pass to cut the full depth. Before moving the jig, draw a line across its top edge to mark the edge of the next mortise. Cleats on both sides allow you to flip the jig and rout the other stringer.

Cut all the treads to equal lengths (drawing 3). Set one end of the first tread in the first mortise of one stringer. Bring the other stringer to it, and set the tread in its first mortise. Continue from one end to the other, getting all the treads started in their mortises. Pipe clamps can come in handy at this point for drawing the opposing stringers toward one another. The fits should not be so snug that you have to pound things together, and not be so loose that the assembly will rack.

You can nail the stringers to the treads, but I prefer screws (drawing 4). They offer a little insurance against the stringers’ warping and pulling the nails out of the end grain. If the stairs are going to be used outdoors, be sure the screws are corrosion resistant.

If open risers such as these don’t suit your needs, nail a riser to the back of each tread. Or nail a 1/4-in. plywood panel to the back of the stringers. The panel has an auto-return feature: things kicked off the steps come sliding out at the bottom.

 

 

 

 

 

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