[Henryhomework]

Is it ok to use regular tape to hold thinset on the bottom old floor. Keeping it from dripping through prior to intalation of the backer board. I currantly have 7/8 by 10 wide boards and some of them have small gaps between them. Or is there some other way to cover the gaps IE tarpaper???Please help..

[joecaption]

Personaly I'd never lay cement board over that type floor.
Not likly to be flat and can flex.
If it was my job I would cut it out and use Advantec subflooring , 1/2" underlamnet then the tile board.
You need to at least go over it and add screws then use 1/2 subfloor rated subflooring before the tile board.
Sounds like a really old house your trying to do, what's the unsupported spans of the joist and how wide are they?

Tile can not have any flexing going on under it.

[JazMan]

Henry,

There is no choice in the matter. You can not install any backer over plank subfloor, whether they have gaps or not. Too unstable. If your 10" planks were in perfect condition 1/2" ply underlayment exposure 1 might do. Often the planks are cupped or bowed in which case a thicker ply is necessary. You still need to re-screw them with at least 2 screws per joists of course.

Once the new ply is fastened only to the planks, properly spaced etc, you can install your favorite concrete backer board using the specified thinset mortar. Then tape and tiles etc.

Jaz