When we cleaned up the stairs, we took off each tread and sanded and routed the nose. Then put them back in place with some screws for a temporary hold. Later as we finished the project they would be easy to remove when necessary but we could still walk up and down safely -just a little tip in case you are trying this at home, until you are DONE don't re-glue and nail yet!
Anyway, to fill the gap we started off by adding a diagonal molding that runs just above the nose of each stair tread from the bottom stair to the top. This created a triangular space above the stair support. Which meant all we had left to do was fill that space. We (meaning Justin- good thing someone is really patient in this family!) carefully measured each triangle that was left below the molding and cut out a piece of MDF to fill it up.
Nailed in place it looked like this(below). We also traded out the molding we had in the pictures above to a simple basically rectangular molding to simplify the profile.
We had to trim or straighten the cuts on some of the treads, but for the most part, this little triangular molding, just filled the existing huge gaps.
When they were all nailed in place, I caulked any gaps, and we installed the risers (even though they will be covered) and painted a few coats of white paint.
In case you want to follow along, check out these posts too:
post 1: Starting the project!
post 2 : Demo & Rebuilding
post 3: More Rebuilding
post 4 : Filling the gaps
post 5 : Walnut Stain… at last!
post 6 : Adding the Stair Risers, Finishing the Runners
Post 7 : Custom Built Newel Posts
New Enclosed Loft Space At The Top Of The Stairs
Social Retargetingby Chango