Intro: An Alternative to Using Backer Board Over Subfloors Prior to Tiling
When I built the house I wanted all my bathrooms, hallways and the kitchen floor to be tiled which was close to about 1500 square feet. The main floor is actually the second floor of the house so it's built from 3/4 inch tongue and groove plywood over 2 by 12 yellow pine resting on some steel beams because I have a 20 foot span over the Crawlspace where the Top Secret Tornado Bunker is to be located then a 27 foot span for the bottom level which is a workshop, 3 car garage, storage room and bathroom. I was worried about tiling the kitchen mostly because it was such a high traffic area and a big span over nothing. The room is 14 by 27 with a large pantry and its sitting above the 3 car garage. You can't tile directly to the wood because the wood expands and contracts differently than mortar will so generally people use backer board which is like a 3 by 5 foot sheet of 1/2 thick masonry product and works very well but it's close to $10 a piece and I was gonna most likely need 170 sheets of the stuff.
I also worried what to do about the seams. I am sure they must want them taped and mudded before you tile them but I'm no expert and haven't researched it but I'd cover them with screen and mortar them just because.
I was worried about the seams expanding between sheets and maybe cracking the tiles plus its pretty dusty if you use a skill saw to cut it. You can use a razor blade which is fine if you just have to do a bunch of rectangular areas but it's a pain on a big job and another reason was as far as building funds were at the time, we were running on fumes so to save $1000 would be a big deal.
I googled a while and read about mudpans and then asked a few older builders I know, if they'd ever done any and was told that's how they used to do it so I thought a bit more about the fiberglass they put in concrete so you don't have to put down screen and my mortar mixer and the bounty of sand you can find in Florida and I came up with this method which hasn't shown any cracks or problems in the last 18 months or so that the floors have been down.
Step 1: Basic Method
Basically this is like pouring a concrete slab just on a smaller scale. I poured these mudpans about 5/8 of an inch thick and I wouldn't think anything less than 1/2 would be wise.
I wanted a bit of room between the wall and the floor for expansion and contraction so I decided to use treated lumber for what is essentially the form for the slab. I wanted to leave these in place after the mudpans were done mostly because I figured if I went back and tried to pry them loose I'd crack the mudpan plus I stapled roofing felt down on the floor again for expansion / contraction and the wood worked very well to secure the borders. I left about a 3/8 inch gap between the wood strips and wall and then filled it with expanding foam and cut it flush before pouring the cement. I overlapped the felt the same as you do when doing a roof. I used the heavier felt just out of habit but looking back this would have been a good time to use the lighter felt because you want it to lay down fairly flat and want to be sure the weight of 5/8 of an inch can force down any bumps which would cause a thin spot.
Step 2: Getting Started
When I cut my 5/8 strips of treated wood I cut a few extra to use as cheater boards. I'd lay a strip about three feet away from the wall I started on (Don't cement yourself into a corner!) and use it with a four foot 2 by 4 and rake it back and forth the same way you do a slab. Just slowly drag it flat and do the entire length of the wall you're starting on. It helps to have planned head with your float board and gotten one straight with no rough spots and since you're only working in a small spot at a time its not very difficult to lay it out fairly smooth.
I'd leave that one in place and lay another about 2 feet away instead of three and repeat the process. working left to right or right to left which seems to work best. The reason to skip to a 2 foot gap is after you have done the second row, which is 2 foot wide you need to go back and pull up the first row of cheater boards, then go back and fill in the crack while the cement around it is still fresh and hand trowel it as best as you can. It's better to have a few high spots I found than to have them too low because with this mix unless it is an very hot day you can come back the next day with a brick and use it to sand down any high spots. It cuts very easily if you do it the next day but you'll have sore arms if you let it sit for a couple days so I never poured out more than I knew I could go back and fix the next day if there was a flaw.
Life happens you wouldn't want to do this in a sloppy manner then get called away for two weeks, it sets up fairly hard in fact had I done this in the summer most likely I would have been able to brick it the same day.
To know when its ready to brick if necessary, the surface of the mix should just cut quickly but not come flying off in big chunks. Generally if you are trying to sand something flat, the less time it takes the flatter it generally is because you don't tend to stay in one spot too long compared to having to spend an hour just to do a square yard.
Smoothing the Kitchen took me the better part of a day which is 14 by 27 and I spent more time on it because I wanted a good flat surface to put my tile down on.
Big areas show off flaws easier...
Smaller areas are much easier to float out and if your talented with a trowel you could probably do a small bathroom just by eye so this is a matter of personal skill.
If you doubt you can put down a smooth finish leave longer to brick it flat but if you can lay it out slick there is no need to sand it at all.
Step 3: The Formula
The mix I used to make this was half a bag of fiberglass from the concrete plant which I think was about 7 dollars a bag, a sack of Portland cement and about 40 good sized shovels of sand. I used a higher amount of sand since I wanted it to be less brittle and a higher amount of fiberglass since I wanted it to stay together.
This was a wild guess on my part I just kind of judged the mix by how grey it was and counted shovels to keep it pretty consistent. Someone with more experience might have better knowledge of what ratios to use but as I said it's been 18 months over very large spans and I can't see any obvious failures in it.
When I tiled the room the strips of wood came in very handy. I marked all my rows in them all around the room before I lay down the first piece then just used a Laser straight line to connect the dots so to speak and did the whole room in no time. When I did the hallways and bathrooms I did the same thing and it made it very easy to lay a few tile down, get distracted by something else, then come back a day or two later and just pick up where you left off. I tiled over the treated wood around the perimeter and again I can report no failures. I do have one squeak in the house in one of the bathrooms and I have been watching that floor closely for cracks and nothing.
I ended up I think using about 6 sacks of Portland cement, 3 packs of fiberglass and some masonry sand I had left over from doing the brick I'm not sure how many rolls of felt but I most likely spent less than $150 max to do all my floors except the master bath where I used backer board because I used Radiant heat in there.
Since I got the bulk of the tile on special none of my floors including the tongue and groove Oak cost me more than $3 a square foot finished