In a nutshell, my cabin has 2x6 floor joists on 16" centers over a very tight crawspace, spanning 7'-4". I was planning on installing ceramic, if not slate tile overtop. I have a substancial subfloor, doubled 3/4" plywood for a total of 1-1/2".
My concern was the joists themselves.. I know 2x6's arent exactly massive, but on the other hand, the span is rather short at less than 8 feet.
The floor is basically level except for one spot: The five joists under my hearth are sagging down about 3/8" at the lowest spot. The reason is probably simple: the hearth is built upon 2x10s running on top and perpedicular to the 2x6 joists [to get the stove up to waist level] and in addition to the plywood and hardibacker [for fire resistance] it was boxed in with, there is 2" flagstone [and mortar] across the top.. 3'x5' hearth, so that is alot of weight spanning 15 square feet on top of those particular 5 joists, even with such a short span.
I have been doing some research, and HAD decided to full length sister [screw and glue] with another 2x6 on every joist under the floor [I would definatly jack up the sagging joists under the hearth before sistering.] A buddy of mine, one of those, 'jack of all trades' people, suggested sistering, well I guess a more accurate term would be sandwiching, a 5.5" sheet of 3/4" plywood, again, full length, on each side of every joist instead??
Basically, he is saying to rip some 5.5" sheets of 3/4" plywood and cut them to 7'-4", glue them up with contruction adhesive on each side, and put three [vertical] 2" deck screws every 12".
I cant see how that would add any strength, though he ensured me that it would definatly add strength and stiffness for the tile, as well as help the joists under my hearth to resist the dead load overtop.
Thoughts?
I think you're on the right track. Only thing I would change would be to use SDS screws.
