I'm remodeling a 1965 Northern California home and currently working on the bathroom. I want to put 12" tile in the bathroom. After tearing out the old linoleum and particle board backing I found a TnG 2x4 subfloor with joists 24" apart. With all those grooves is it OK to lay down thinset and 1/2" durock? With the heat range of the area being 30 degrees in winter to 100+ in summer I'm worried the expansion and contraction of all those joints will create cracks in the tile grout or maybe even the tiles. Thanks.

Is that 2x4 joist 24" apart? and if it is, what do the joist sit on and how long is the clear span and width of the floor? Is there insulation in the joist bay or is there a basemsnt under the bathroom? Or is it Joist ( thick)? and 2x4 lating flat over the joist as the actual subfloor and how warm does the bathroom stay or vary in hot and cold during the winter? The warm summer part is fine. how big are the grooves? I would prefer to use 1/4" luan first to cover up the groves and then you will be more accurate on seeing the dips to fill in with the thinset and if its really dipped you can thinly screet the thinset to level (flatten) out the dips and bumps. Also the Luan will be a plus for you or someone else if someone wants to change the floor back to another finish product later. The luan will greatly help take away the temp movement between the finish floor and the subfloor. The tile will act like it's a big solid sheet and you laid it down like a big stone and its one with itself. Check your thinset to see if there is latacrete in it. This gives it good properties to expand and shrink and not fracture. Use this latacrete when setting the tile. The 1/2" duro rock is perfect. Don't forget to seal the grout joints after it drys. This is far as I can take it until I know more details on the joist itslf. Hope this helps and Happy Holidays.

The subfloor is 2x4 TnG nailed to 2x6 joists 24" apart that sit on the foundation on the edges and are supported by concrete piers under the house. The gaps between the 2x4s vary from almost none to 1/4". The house is a single level home and the crawl space under the house is about 2 feet high and the floor is not insulated. When the central heating is not on it can get quite cold in the house in the winter.

What kind of a house is this? (age, joist span/ house width, #floors etc). It seems a bit underbuilt. Are you sure of all dimensions? I have never seen 2x4 T&G... or 2x6 used as joists on a main floor... or 24" center joist spacing under a main floor.

There are two things to do before considering tiles:
1) Put in more floor joists. IMHO, 2x6 on 24" center is not enough to support tile properly. 2) insulate the floor

Both of these actions will reduce the chance of cracking the grout/ tiles.

It's a 1965 single store ranch style home in Sonoma, CA. 2x6 joists run the width of the house and from what I understand from the local hardware store, 2x4 TnG was used in the area during that time. I haven't been under the entire house (crawl space is really tight) but from the access hatch I have measured the distance from joist to joist and confirmed the 24" distance. The nails attaching the subfloor to the floor joists in the bathroom also measure 24" apart.

I don't want to raise the floor up too much but really don't want to risk cracking tiles/grout so would putting down 1/4" or 3/8" plywood then putting down 1/4" durock over the 2x4 TnG be enough to prevent too much flex?

Thanks again.

wow! The size of floor joist is way below standard building practices here in southwest Pa and national building codes. Here standard is 2x10 16" O.C. with 3/4 T.G. ply. or 1x4 T.G.(old style) Our crawl spaces is insulatied on the inside foundation walls ( Old way) New standard is Min R19 in joist bay and cross ventlaon in foundation. Your house reminds me of a cabin floor. The way your floor structrial load and live load set up is the 2x4 is heaver and is spreads put your live load and catches more floor joist in pressure and spreads the load in that fashion The ols houses here is 2x8 16" O.C. with 3/4 1x4 angled on the joist. I personally feel the 2x6 should be min 16" O.C. or less top 12" O.C. The size and no insulation in joist ( to small ) is not good for your temp changes for the floor I don't know how much heat it geat gets into that area. I would get some 2 to 3" ureathene ( Blue Boards) they come in 2" wide and 4"wide 8 or 10 feet long. I would liquid nail that { and short Galv drywall screws) to the inside joist bay (crawl space) against the 2x4 floor. The blue board is good and it doest collect moisture like fiberglass insul. To the bottom of the 2x4. I would double up on the floor joist where the bathroom part is and the reat for tha matte and nail some sister joist on. 2x8 sisters is even batter. And Insulate. A vapor barrier stapled underneath the sub floor is more of a perfect prep before blue board. Save your heat and floor and add this before the tile. Have a happy new Year

Thanks for all the info. Looks like I have a bit of work ahead of me.

If the spans of the 2x6 joist are a short distance from bearing point to bearing point and the 2x4 tng is running perpendicular to the floor joist. Then I wouldn't have a problem with placing the durock over the 2x4 tng, if when you put your body weight on the 2x4 sub floor it's flex is next to none. Make sure you place the durock edges over the floor joists and end the other edges in the middle of the 2x4 tng. If its small space and the spans are short you will probably be OK. I would insulate under the floor though.