Expert: Dan Griffin - 4/11/2010

Question
QUESTION: Hi, I have a project for my unheated summer home in upstste NY, and I was wondering if it is feasible. I would like to install a concrete floor in my bathroom with decorative accent tiles and electric radiant heat underneath. The bath is 14'x9' and the 2x6 (1.5x5.5) floor joists are on 12" center with a span of 9'6". There will be a 6' claw foot tub (cast iron) in one corner perpendicular to the floor joists. The opposite end of those same joists will have a glass block shower divider wall which will get an extra double joist under it.  I would like to know how thin I can apply the concrete,  and if the joists will be able to handle the load. Will the concrete add strength if I use reinforcing mesh? Is there a better product, or type of concrete for this job?  Will the home being unheated in the winter be a problem, and should I leave room for expansion around the edge? Thanks

ANSWER: Ron, that's a lot of load.  126SF.  Concrete needs to be 2" thick minimum, so 21 cubic feet of lightfweight concrete (about 120#/CF) would be about 2520 pounds which is about 20 pounds per SF,  Tub 300+.  Water 350. Human 200.  Glass block - 160@6, so about 1000.  Looks like we're headed for about 35 pounds/SF of live load.

Now, let's look at your framing.  
http://www.mcvicker.com/resguide/page013b.htm
and
http://www.awc.org/calculators/span/calc/timbercalcstyle.asp?species=Mixed+South
looks like you'll be all right.  Make sure to use high grade lumber for these joists.  I used l/480 deflection because of the ceramic and No.1 yellow pine on the awc calculator.

Here is one manufacturer of light weight in your area:
http:http://packagepavement.com/custom_mixes.html
make sure to read their spec sheet.

Wire mesh does not make the concrete stronger, it only helps hold it together.  You are not pouring it thick enough to use rebar which would add to the structural strength.  If it is poured in the summer, it can only get smaller, so I wouldn't worry about expansion allowance.  Being unheated is not a problem for cured concrete.  It will be hard on caulk joints, etc.  I know you realize you must winterize all water usage.

You might explore cement board if the depth of the concrete is going to create a problem.

---------- FOLLOW-UP ----------

QUESTION: Thnks for responding so quickly! Of course now I have a couple of follow up questions. Would reinforcing mesh be required for this application? Would stucco lathe be a better choice? Would using a fiber reinforced concrete lead to exposed fibers with the finished product (I would like to leave the concrete exposed for most of the area)? How would using concrete board allow me to save on weight?

Answer
ron, a lath product would probably be better, they make a plastic lath.

I'm going to suggest another web site that specializes in tile.  They will have information on full mud beds, light weight self leveling subfloor, and concrete board installations:
http://www.johnbridge.com/vbulletin/showthread.php?t=36020&highlight=radiant+hea

I've not ever used light weight concrete as a finished floor, but the packagepavement site I gave you discussed coloring, so I would guess it can be appropriate.

1/2" thick concrete board made with light weight aggregrate would be quite a bit lighter than 2" poured in place.