It doesn't make much difference if you are overlaying the OSB with backer board as you indicated. By "backer board" I assume you mean something akin to 9/16" glass mat cement backer board. With this overlay the mortar shouldn't reach or affect the OSB. The backer board will also stiffen the subfloor system which, in turn, will reduce deflection and subsequent cracking. This stiffening improvement is perpendicular to the joists, and doesn't help deflection along the length of the joists.

The Tile Council of America has an installation specification (F155-03) for tile floors over OSB decking but it includes covering the OSB with 5/8" exterior plywood. OSB has come a long way in the past 20 years but still isn't generally accepted for applications in direct contact with wet materials and this includes mortar (even though it cures to a dry state). Some of the earlier OSB was easily damaged by moisture.

A separate issue is deflection. This is the amount of "flex" in a floor system. When the weight of people and equipment are applied to any floor some amount of measurable deflection occurs. Wood and vinyl materials can tolerate an enormous degree of deflection without defect or failure. On the other hand, tile isn't nearly as tolerant. Deflection limits are measure by the formula l/360 where l = the span of the floor (distance from end of joist to end of joist) in inches divided by a three digit number. The lower the 3 digit number the greater the deflection. Wood and vinyl can easily cope with deflections of l/180. Ceramic tile requires deflections of l/360 or better to prevent cracking. For this reason plywood is more often used than OSB for decking because plywood is slightly stiffer than OSB when applied in the correct orientation (grain perpendicular to the joists). Whether using plywood or OSB, the floor must be stiff. Thus the thicker the decking the better. If your floor joists span 10 feet they should not deflect more than l/360 or 10' x 12" / 360 = 3/8 inch when a 300 lb. load is applied at the center of the span. The 300 lb. load is an arbitrary amount but is the load used by the Tile Council of America.

If your floor deflects more than the limits described above, you need to stiffen the floor by 1) adding extra floor joists or 2) shortening the span of the floor joists by adding a wall below at midspan or 3) installing a heavier deck or a combination of the three. Although a heavy decking is usually a desirable quality, it won't help weak joists very much. The best way to stiffen a floor in all directions is to add new joists between the existing joists. The added joists can be place midway between each existing joist (best method) or can be scabbed (sistered) to the side of each existing joist (easiest method). In the latter application the added joists are not required to sit on the support walls at each end as long as they are within a few inches and are well nailed to the existing joists for the full length.

Considering the time, energy, and expense of remodelling and installing ceramic tile, don't cheat on the foundation. In this case the foundation being the joists and decking. It is better to be too stiff than too flexible.

Regards

P.S Visit the Tile Council of America site for a commentary on OSB.

http://www.tileusa.com/OSB_faq.htm

Regards

Thank you for your very detailed reply however I am a little confused by it. Are you saying that as I am covering up the OSB with backer board (1/4' thick cement Wonder Board) that it is OK? I had origially not intended to put anything between the subfloor and the backer board because of all the warnings about OSB coming apart when it became damp but when I checked the surface there were some elevation differences (1/8-1/4")at the joints of two pieces of the OSB. I had wanted some epoxy or some filler to pour into these low areas so that the backer board would have support and not be spanning a hollow about 5ftx1ft, 1/4" deep.

The OSB is in good shape and this hollow is from poor workmanship not damage.

First of all, never put self leveling cement under the cement board. When the cement board gets nailed or screwed down the SLC will turn to crunchy peanut brittle and become an obstacle to a successful tile installation. The SLC should be poured on top of the cement board.

As pointed out OSB is a questionable substrate and it presence can be tolerated if a 5/8" sheet of BC grade ply is put over it. Technically you could tile directly to the BC ply but it is not advisable if you are a novice to installing tile, that is where the 1/4' cement board comes in, it offers a much better and a safer bond of the thinset.

The ridges on the OSB seams should be ground down with a belt sander to minimize the variation between boards. The cement board must have a layer of thinset under it, use a 1/4" square notch trowel, and then anchor the boards with galvanized roofing nails or backerboard screws. The thinset is under the board to fill in the voids much like you talked about with the OSB, it is not for bonding, it is strictly a filler.

After the SLC is applied and dries you can set the tile using a good grade modified thinset. The thinset is best applied using the method outlined by the National Tile Contractors Association in a video they sell called "Trowel and Error". Go to www.tile-assn.com and look for it there. Its 10 minutes for $10 and well worth it.

Well guys, it looks like I've backed myself into a nasty corner. There is no way I can add 5/8 plywood, thinset, backer board and then tile on top of that. It will be much too thick and will make a difficult step up from the ajoining room.

Is my next option cutting out the OSB sub floor and replacing it with that 5/8" plywood?

Gmoore:

On all new construction I specify only 1" decking with tongue and groove edges. If you really need the space and considering the low quality of plywood being produced, the least I would advise is 3/4 inch. My experience has been that 5/8 flexes too much for ceramic tile. With any plywood, be sure to run the grain perpendicular to the joists. Apply a bead of "liquid nail" to the top of each joist just before laying the decking and fasten with screws or ring shank nails.

As a note of interest, many of the recomended plywood thicknesses were established using full dimension products. This means that 5/8 inch plywood recommended was actually 5/8 inch thick. Although full dimension plywood is still available by special order, most lumber yards now stock only "nominal dimension" or "span rated" products which are never the full thickness alluded to. A sheet of plywood sold as 5/8 inch is usually labelled as 19/32 and actually will measure at 9/16 or less. This is a 10% reduction in thickness and about a 20% increase in deflection (flexing under load). I have bought 3/4 inch plywood that actally measured to be 5/8 inch.

Once the decking has been replaced you can install the cement backer board as Tileguybob described.

Regards

Thank you all for your help. This should keep me busy for awhile and I will check with this web site BEFORE I start any other projects.

Thank you again.

George