Deflection and Mud Work

[tilemanct]

Got into a discussion at the local tile distributor the other day with an installer who does a lot of mud floor work. I told him how I often have to decline mudding floors due to added mud height, weight, and deflection. He said that you don’t have to worry about deflection with mud work, as the mud job takes it out. He also mudds to a thickness of 1” to 1 1/8” even on the 500 sq/ft + jobs. I always thought you needed to be at least 1 ½” minimum on anything over 100 sq/ft. I always made sure any added weight of a mud job was accounted for in the floor structure design. He said you don’t have to worry about that, joists can take it. What am I missing? Am I too cautious? I can’t tell you how many jobs I have run through the Deflecto lately that give me thumbs down to ceramic tile. I couldn’t imagine putting 1 ½” mud on top.

[cx]

Sounds like we're mixing some methods here, Dave. Are you're talking about this use over wood framed subfloors?

[tilelayer]

Dave

The Guy who taught me would mud over anything. I remember mudding over some bouncy floors these were over 1000 sq ft too. I am like you I check everything your better off in the long run. If I can't mud I plywood and ditra.

[tilemanct]

Yes CX, Wood Frame Subfloors.

[cx]

Not sure what your installer friend is relying upon for his information, but as far as I know there is still a requirement for a joist structure and subfloor that meet L/360 deflection requirements to use the method I think you are referring to (F145 in TCNA Handbook). Maximum joist spacing is 16" on center and minimum subflooring is nominal 3/4" T&G plywood.

I'm told that method once had a limitation of no more than 100 square feet, but that's no longer in the published method.

You can get a Light Commercial rating if done correctly.

[Davy]

Yeah, I've mudded many bath floors as thin as 1/2 to 3/4 inch without issues. They were all under 100 sq ft, most were 50 ft or less.

[tilemanct]

Well CX,
He believes that mud takes out deflection no matter what the situation is. That was where I began to question his reasoning.

[e3]

I would think that the deflection may be OK(depending on what it is) but the joist may not take the weight.(dead and live)

[vrusso123]

many a fella here in ohio think paper and lathe fix any structural/ subflooring problems and convince the homeowner of this during their sell. i have 2 copies of the T.C.N.A. i give to these homeowners to give to their tile guy as a courtesy. i always get them back when i do the job.

[Bill Vincent]

I'm thinking along the same lines as Eric. So long as the joisting can handle the weight, within reason, you should be okay, especially in a situation where the drypack isn't bonded to the subfloor, and requires wire reinforcement. It SHOULD add structural strength (and stability, with respect to deflection) to the floor, in my way of thinking. I don't know if a 1" bed would cut it, though.

[coping skills]

All I know is I once tore out a mud bed which was 8" thick going down between chamferred 2x8 joists.

I can't think of any deflection having an effect on a mud bed like that. That mud bed was basically a structure independent of the house. A sovereign mud bed.

[John Bridge]

Deflection actually occurs in two areas: in the subflooring between the joists and along the length of the joists. I agree that an adequate mud bed will stiffen the floor between the joists. Whether the overall floor can take the load is another matter. Short answer: yes, a reinforced mud bed can stiffen a floor.

But an additional sheet of plywood will do the same thing.