As we last left off, the bathroom sub-floor had been entirely pulled up and was shoved into a bin. The next step was obviously putting the sub-floor back in place – but how do we do that, exactly? What do we really need to do?
We had a bit of a decision ahead of us as to what was really needed for strength, versus what was recommended by the manufacturer, versus final floor height. The floor height was a visual/tripping concern considering the adjoining floors in both the master bedroom and the hallway consisted of just the finished 3/4″ plank boards directly on the joists. That means that any finished floor will likely have to have some kind of transition from the finished floor to a raised tile area, but we clearly don’t want a 1″ step or anything insane like that, if we can avoid it.
Our final plan for the sub-floor bathroom layers essentially came from Schluter, the makers of Schluter-DITRA, a uncoupling and waterproofing layer that we intend to put underneath the tile layer. The Schluter-DITRA Installation Handbook is probably the best document I’ve seen regarding the way you can lay out various types of sub-floor, and we certainly used its recommendations to the fullest – page ’23’ is highly recommended reading. Just like the diagrams in the handbook, we start out by putting a layer of 7/16″ OSB across the joists. The internet recommendation I found was to never to use anything underneath 3/8″ for a layer and you should be around 1″ for the total height of the sub-floor when you’re done adding up your layers. On top of the OSB, we are planning to add 3/8″ plywood (because bonding to OSB is apparently difficult in general and not recommended by the self-levelling cement manufacturer). We are also planning to add an in-floor heating system, which means there is an included layer of self-levelling cement/mortar on top of the plywood layer, but before the DITRA layer, so that’s how we figure we can get away with a slightly thinner wood layer – we’re just over 3/4″ with our layers just with wood, but will be just over 1″ once we add the cement. (Besides, shouldn’t cement be stronger than wood and offer less deflection? I sure hope so!)
All of this restructuring is to get to what the industry terms as an ‘L/720’ deflection rating on the floor – normally, ceramic tile installations require a minimum of L/360, but natural stone tile installations require L/720. You’ll note the one of the recommendations for getting to L/720 is to “re-enforce the floor joists”. Well, after placing our first sheet of OSB down, we immediately noticed we had a bit of a problem with our floor joists:
They’re not level. At all. They’re also severely warped towards the middle of the room. They’re also weakened in several places where a previous plumber cut through them.
So that was a bit of a problem. We decided the best way to fix this problem was to simply butt new (straight) 2×8″ joists right up beside them, level them, and glue and screw them together. So that’s exactly what we did (…and skip to the finished product):
Hopefully with several tubes of this stuff tying everything together, there shouldn’t be any floor shifting or floorboard creaking now:
Jenn also worked really hard at cutting more of the old square nails off of the mouldings, which makes our job a lot easier for moving them around now. Perfect!