A circular saw will enter the lumber at an angle when cutting out stair stringers, and this causes the guard to hang up, exerting an uneven force on the saw that throws off your cut.

Lay out and cut the stringers. (See How to Build Deck Stairs for more information.) Make the bottom tread cut 1-1/2 in.

When the blocking is attached between stair stringers, it has no effect on the tread dimension. This greatly simplify matters, but it’s not a cure all for all types of stairs.