# Subfloor question for tiling a bathroom floor with heat mat under the tiles

Discussion in 'Remodel Forum & Blog' started by Drewski123, Sep 2, 2011.

1. ### jadnashuaRetired Defense Industry Engineer xxx

Joined:
Sep 2, 2004
Occupation:
Retired Systems engineer for defense industry.
Location:
New England
Look at blocking this way: take a rope, hand a 50# weight on it, it will stretch or deflect a certain amount. Now, take a couple of ropes hung say 16" away and tie them also to the weight...it won't stretch as much. But, now add another 50# weight for each rope you add...they'll all deflect or stretch as much as the single rope with the weight on it. Tile puts a significant weight on the entire floor, just like hanging weights on each individual rope, regardless of whether they are tied together...bottom line, if you need a stronger floor, blocking won't do it. Thicker, deeper, shorter, or closer together will give you decreased deflection.

2. ### johnfrwhippleBATHROOM DESIGN & BUILD for both Canada & the US

Joined:
Jul 20, 2009
Occupation:
Design Work World Wide: Bathrooms Vancouver Area
Location:
North Vancouver, BC
Post(s) deleted by John Whipple

Last edited: Mar 18, 2014
3. ### jadnashuaRetired Defense Industry Engineer xxx

Joined:
Sep 2, 2004
Occupation:
Retired Systems engineer for defense industry.
Location:
New England
Blocking DOES change the resonant frequency of a floor...it does not change the deflection rating of the overall joists. It DOES help point loads, but does not change the effect of a total load on the floor and the amount it deflects. Place a piano on a floor with and without blocking, and it will deflect the same. Jump up and down on the floor and it will be sturdier (vibrate less) with blocking. The two are related, but not the same. Blocking is sort of like fingering a string instrument...shorten the length, the pitch changes, but it does NOT make the string stronger - it will break at the same point regardless. Not exactly the same, but a joist has sections in compression and tension...that effect occurs along the ENTIRE UNSUPPORTED LENGTH, blocking doesn't change that, nor does it change the ultimate amount it deflects from the applied load. Blocking does prevent the joists from twisting under load, so in that manner (as I've said before), it MAINTAINS the strength the joist has. The subflooring does that to a great degree, and if sheathed on the other side, that helps too. It really helps make things stable before the subflooring is installed.

Last edited: Sep 7, 2011
4. ### johnfrwhippleBATHROOM DESIGN & BUILD for both Canada & the US

Joined:
Jul 20, 2009
Occupation:
Design Work World Wide: Bathrooms Vancouver Area
Location:
North Vancouver, BC
Post(s) deleted by John Whipple

Last edited: Mar 18, 2014
5. ### jadnashuaRetired Defense Industry Engineer xxx

Joined:
Sep 2, 2004
Occupation:
Retired Systems engineer for defense industry.
Location:
New England
The joists themselves must be strong enough to meet at least the L/360 rating, blocking won't increase it. It will change the resonant frequency. Studies have shown that a load on an inadequate floor will sag, and continue to bow to a greater extent as the structure ages. It can take a decade, but eventually, that floor or beam or whatever will take a set from the load. Take a look at the ridge of houses, it is rare to see a new one that sags. Look at some that are 10, 20,...50 years old, some are nice and straight, but some sag radically from one end to the other - the result of using too small of a beam. Crushing the fibers to jam in a long piece of blocking doesn't help in the long run, either. Crushed fibers aren't as strong as uncrushed fibers, so you've effectively created a weak point in the joist. Now, a gluelam is a different thing...crushed, but with glue to reinforce and hold the things together.

6. ### Key2013New Member

Joined:
Jan 6, 2013
Location:
Bc
Hi guys thanks for the info, new to this board and had a question. Doing a main floor in 30" by 20" tiles in an older home from the 70's I believe. Joists then ship lap subfloor and then 1" ext plywood glued and screwed on top of that. Is this acceptable to tile on top of? Or should I be adding another underlayment such as ditra? What about thinner membranes such as easy at or mapegard?

Thanks

7. ### jadnashuaRetired Defense Industry Engineer xxx

Joined:
Sep 2, 2004
Occupation:
Retired Systems engineer for defense industry.
Location:
New England
Tile wants no plywood with a 'D' face, and exterior sheathing may have that. Do you know? a 'D' face means voids, and those are not acceptable underneath tile. Gluing ply to dimensional lumber is also iffy - the goal is to provide a stable surface, and the dimensional lumber moves much more than plywood - gluing it will improve between joist deflection (not really an issue with that thick of material), but you really want some decoupling. With tile that big, you want a VERY flat floor or you'll be having problems with lippage. Also, lay some down on a flat surface and see if they have any bow or twist in them. Hopefully, they are quite flat. Your subfloor is stiff enough, if you don't have a 'D' face, but the joist structure is equally as important, if not more so. If the joists are okay, I'd consider using something like Ditra. You may want to look into something like TLS or QEP leveling system - these are clips you install during the setting of the tile that hold the edges in vertical alignment. After the thinset cures (well, not completely, but at the earliest the next day), you break off the vertical portion and can then grout it. They are designed to break off near the bottom of the tile, so do not show once grouted. Larger tile are much harder to get to lay perfectly level with each other since any small difference gets magnified over the long edge of the tile. Thus, the leveling system will speed things up and almost guarantee better results.

8. ### Key2013New Member

Joined:
Jan 6, 2013
Location:
Bc
Thanks for the info, I am just doing the setting the contractor has done the 1"plywood sub. The tiles are quite flat I have checked them and going with a "stacked pattern" compared to a brick or offset so should be ok. So you do recommend going with another underpayment? Would you recommend setting on top of the 1" ply with a high performance mortar?

9. ### jadnashuaRetired Defense Industry Engineer xxx

Joined:
Sep 2, 2004
Occupation:
Retired Systems engineer for defense industry.
Location:
New England
Tiling directly to plywood requires a second layer. You're better off using something like Ditra on what you have. If the existing stuff is something like C-D grade, I'd put at least 3/8" additional layer of ply on top of it before I tiled, something like B-C or A-C with an exterior or exposure I glue. Do NOT glue it to the existing layer, but do screw it down, oriented across the joists, not along them. You could probably get by with what you have with that large of a tile unless a corner happened to be right over a void, and you happened to put a big point load there. Not likely, but possible, that's why they specify no 'D' faces, as everything is either solid or plugged to fill in any voids on anything above a 'D' face.

10. ### Key2013New Member

Joined:
Jan 6, 2013
Location:
Bc
Ok I will most likely go with ditra, I was hoping I could tile directly over the 1" plywood to keep costs down the contractor also keeps assuring me that it is solid. Any issues down the road and I will be the one to call tough, so for future reference there must be 2 layers of plywood even over ship lap, I may also look into some other membranes that are more cost effective maybe a peel and stick membrane such as easymatt or mapegard

11. ### jadnashuaRetired Defense Industry Engineer xxx

Joined:
Sep 2, 2004
Occupation:
Retired Systems engineer for defense industry.
Location:
New England
That's not what I said! If the layer you have has a 'D' face, then I'd want one on top that didn't! You never want a void in a floor you are going to tile. The fact that it is strong overall has nothing to do with a point load over a void. A small deflection there can crack a tile. Natural stone wants two layers, but normally, one over dimensional lumber is fine for either (if it meets industry standards, that is).