How to Make a Stair Tread Jig

Making the Jig

• 1

  Cut two pieces of 1/4-inch plywood into rectangles measuring 12 inches by 30 inches with a table saw or circular saw. Mark a radius of 4 inches with a pencil compass in a corner of each board. With a jigsaw, cut out the scribed mark, which should be the shape of a quarter circle. This will be the upper outside corner of the jig.

• 2

  Place a mark 5 inches from the lower outside corner on the lower edges of the boards. Mark a radius of 2 inches with a pencil compass from this mark. Extend the top of this mark, which should resemble a half circle, parallel to the bottom edge of each plywood board, using a straightedge.

• 3

  Cut this marked outline with a jigsaw, leaving a tab where the lower outside corner of the jig will be. This bottom notch or cutout enables you to perfectly measure the tread space even if the skirt -- the boards above the stairs on the wall -- or the tread surface is uneven or irregularly shaped.

Using the Jig

• 4

  Overlap the two cut plywood pieces in the middle so the bottom notches are above the stair tread, the bottom tabs press against the spot where the tread meets the skirt, and the outside edge of each piece presses vertically against the skirt boards. The jig at this stage sits exactly where the front of the completed riser will be installed.

• 5

  Place two F clamps loosely on the top overlapping edges of the jig pieces. Expand the jig outward against the skirt boards again and tighten the clamps firmly.

• 6

  Pull the jig out from the skirt boards. Scribe your riser stock to match the outline of the jig and cut the stock, leaving a trace of the scribe marks; it should pop in place exactly.

• 7

  Rotate the jig 90 degrees to measure step treads. Lay the jig horizontally from skirt board to skirt board exactly where the top of the completed tread will be installed. Expand the two sides to touch the skirt boards, tighten the F clamps, remove the jig and cut your tread stock to match the jig outline as you did your riser stock.