I'm planning on installing hardwood flooring on my stairs and I discovered that they are mortised.

I can't see the underside of the stairs, so I'm not sure how the mortising is done. Basically, what I'm wondering is if I cut off the front of my mortised tread if I'll be ruining the structural integrity of the tread. Is the tread supported along it's entire depth in the mortise, or is the stringer mortised just on the front edge of the tread.

Thanks in advance for your help.

I'm assuming the Stair Tread was Grooved and the wood has a tongue on it?

For those who do not know, this poster is referring to a staircase where the treads are slipped in from behind into what is called a housed stringer. The entire width of the tread is supported on both sides and in the middle by the riser, which is also housed into the stringers.
Here is a good description. Read Chapter 5, page 60.
http://books.google.com/books?id=ycAD5m ... Y#PPA60,M1

But to answer your question, you will need to trim flush to the stringers, the front part of the tread that overhangs, called the nosing. It needs to be trimmed flush to the sides and the riser. At that point, you may need to tighten things up with some screws. Also, the remaining section of nosing that is INSIDE the housed stringer will show somewhat AFTER you install your new nosing or tread. I recommend before installing new material, that you sand smooth this area and use a wood patch/filler to match this spot as smooth as the rest of the stringer. Hopefully, your's is paint grade. If not to be painted, You may want to consider veneering it with 1/4" hardwood plywood BEFORE you install the new material.

Gary,

Great answer. Thanks for the pointer to the book.