You asked for clarification on the matter of span. The "L" rating of the floor is basically how bouncy it is. The higher the #, the stiffer the floor. A floor system flexes both between the joists and along the entire span. If the span is too long the flexing of the floor can lead to cracks in the stone. stone is rigid, where porcelain is slightly flexible, hence the difference in deflection requirements between the two. If your joists are fir (especially old fir) then it should be good enough for stone. aged fir has a reddish brown appearance with distinct darker grain, where spruce or pine will be dark yellow with indistinct grain and nearly black knots.

From your description I feel confident in saying that it is spruce. That being said, should I put up a beam with jackposts at the midway point of the 12' length?

Edited for spelling

that would be the best possible solution. If it was my house, I'd probably just go for it and hope for the best. If it were for a customer (with warranty) I'd put in the beam or twin the joists with fir 2x6 glued and screwed.

Unfortunately, I've never seen fir in our home improvement stores, so I would be left to using spruce. I'm assuming by stating that you'd "twin" the joists, you mean sistering it? Is the beam a viable solution? Honestly, it would be easier than sistering each beam; that would mean spending less time folded like a jacknife in my crawlspace. Lord knows I been there enough. Believe me, I have thought about it just putting the tile down, but my gut tells me to re-inforce the floor. I will by no means call myself accomplished in tiling, but have done a number of jobs that experienced workers have commended, and I really just want to do this once. Again, thanks for the advice.

After re-examining the crawlspace area, I think you're idea of sitering the joists is far more sensible than the beam. Now, when I sister the new to the old, I will glue them and screw them, however should I use ordinary wood screws, lag bolts, carriage bolts? Thanks.

I use 3" deck screws. It's good to have the screws that have no threads for about an inch below the head so it'll draw the two pieces together.

I've also found it best to start the screw into the existing joist then into the sister which also helps draw them up together tight.

Another thing to keep in mind is that you only have to sister 2/3 of the span, specifically in the middle. So if you have a 12' span, the sister can be 8', centered on the 12' span.

I don't know what formula would be used to calculate the deflection on your joists once they are sistered with 2x6's, but are you certain that's going to be enough? I ask because there's not a lot of difference in price between a 2x6 and a 2x8, and although there's a little more effort involved to use the 2x8, it would also provide quite a bit more strength than the 2x6. In this area I think there's less than $2 difference between the two.

I will be using 2x8"s and since I have access at one one end to the footing, and at the other end to a main support beam in the crawlspace, I would think it would make the sistering that much more secure. It may be some overkill, but I'll never have to worry about it. Thanks for all the advice, gentlemen, and if anyone is free Monday night I have beer and pizza!!

The only downside to using full-span sisters is that they can be difficult to wedge into the space in an upright position. When your original joists were set, there wasn't a subfloor or walls in place. Now you have to work the joists in that tight space on both ends with only 14 1/2" of space between the existing joists.

Another potential problem is that occasionally your new joists might be slightly taller than the existing ones, making them impossible to fit properly. If that is the case, you'll end up trimming the ends anyway.

The weak point of the joist is in the middle, which is the reason for centering the sisters. You have little or no deflection in the first few inches or so nearest the supports, so running the sisters onto the supports is of little or no value.

It may work for you, but I wouldn't count on it. Good luck.