||06-24-2008 07:55 AM
To install tile of any kind on stairs, you need the stairs to be stiff. If they have any deflection in them, the grout joints won't survive long. The weight of the stone will contribute, but won't be as big of a factor because it is a static load on the stairs at all times and doesn't cause movement of the whole system. A dynamic load...Someone walking up and down the stairs, will be the real issue.
Do you have access to the underside of the stairs? There isn't a given stair jack requirement because there are just way too many variables (stair seat cut dimensions, jack material, length, etc.). If you have access to the underside of the stairs perhaps a wall could be built underneath the jacks (perpindicular to them) to keep them from deflecting.
If you're re-building the stairs altogether, I'd suggest 11-7/8" LVLs for jack material. TJI Timberstrand rim board can also be used for stair jacks, and it is nice and stiff. Both products have excellent dimensional stability. If your stairs are three feet wide, I'd use three jacks.