We have 15 normal stairs with 40'' each. The installer told us that we need 100 sq ft additional wood plank for the stairs plus 15 stair nose. It's the first time we install hard wood floor. Our question is: do we really need 100 sq ft just for the stair riser? Any suggestions are appreciated. Thank you!

Yes, If not more, including the waste, from ripped boards on both tread and risers.I measured for the last one I did this past Thurs. & Fri. @ 125 sq.ft. They were 41½" wide. I ordered the next full cartons worth(29.22sq.ft. per carton=146sq.ft) and I'm glad I did. I only had 5 boards left after all the rip cuts and waste.

There is a lot of waste in stair work.

40" wide steps is approx. 6.5sq.ft. per step, no waste.
6.5 x 15 = 97.5 sq.ft. and no waste.

Then say you have 40" x 3" of waste on both the riser and the tread of each step?
15 steps and that is 25sq.ft. of waste.

Thank you very much!