This section is from the book "A Treatise On Architecture And Building Construction Vol3: Stair Building, Ornamental Ironwork, Roofing, Sheet-Metal Work, Electric-Light Wiring And Bellwork", by The Colliery Engineer Co.. Also available from Amazon: A Treatise On Architecture And Building Construction.
Where risers are curved, they should be made of solid wood, or of curved strips of seasoned wood glued solidly together. It is often difficult to get seasoned wood in pieces large enough to make a solid curved riser. In such cases it is desirable to bend pieces of 1-inch board by kerfing. Fig. 11 illustrates the method of finding the distance between kerfs for a curve of given radius. The method is as follows: Cut one kerf, as at a; then, marking the distanee a d equal to the radius a o, bend the board to position a b until the saw kerf is closed, as shown. Measure the distanee between d and b, or, in other words, the deflection of the board between a and b. This length d b is, therefore, the distance between kerfs, which, if continued, will shape the board to the curve a e. All the kerfs must be made with the same saw.