How to measure total stair rise with uneven floor?

Question

My house is antique and I would like the stairs rebuilt because they are too steep. I know the first thing to make the stringers is calculate the total rise. But my problem is the floor where the first stair on the first floor begins is about 1.25" lower than the floor directly under the landing of the second floor.

So, I can't just measure from the second floor to the floor beneath. Would I have to use a long level and run it from the floor at the first step to a point below the second floor landing, mark where it is level below the landing and then take that measurement from the level mark to the second floor landing?

I hope this makes sense, I am attaching a picture of the problem.

Answer 1

(For anyone looking at the history, I think I misunderstood the question to mean "what should I measure" and not "how should I measure".)

• Geometry

You can measure the exact length of the diagonal (a string from bottom front of first step to 2nd floor landing). Measure the angle of the string to the (assuming it is level) landing. Imagine the total rise and total run as the other 2 sides of a right triangle. Since you now know the 3 angles and the length of the hypotenuse, you can calculate the lengths of the rise and run. The fact that the intersection of the (virtual) rise and run may be above or (in your case) below the actual first floor is irrelevant.

• Measurement

If there is a constant slope (not necessarily even but all one direction) then measure 2nd floor landing to first floor. Then place a long straight piece of lumber on the floor from the spot beneath the landing to the location of the first step. Shim it until it is level. Then measure the height of the shimmed gap between the lumber and the first step location. If the high spot is between the two key locations, simply add an equal amount to both - e.g., put a scrap 2x4 under each end, get it level and still add just the amount of the shims on the lower end, since you would just subtract the same extra piece on both ends it doesn't count at all.