When you tile a shower stall or tub surround, you need a moisture-resistant backing. The one recommended by almost every building authority is cement board. The rest of the bathroom walls, on the other hand, are usually covered in drywall. This means you have a transition between the two materials at the edge of the tiled area. Your mission, should you choose to accept it, is to make this transition unnoticeable on the finished wall, and it isn't that difficult to accomplish.
Drywall and Cement Board Transitions
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Anyone who has installed drywall knows that you finish the joints with drywall tape, which is usually paper, and joint compound, a gypsum-based plaster commonly referred to as mud. The procedure for finishing cement board seams is almost the same, except you use fiberglass cement board mesh tape instead of paper and thinset mortar instead of joint compound. Which materials do you use at the transition between cement board and drywall?
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You want the joint to be able to stand up to moisture, so you should use fiberglass tape instead of paper, and you should apply at least one coat of mortar. However, mortar is too coarse to finish the drywall side of the transition. After it has dried, you can smooth the transition, especially on the drywall side, by applying extra coats of joint compound, scraping them flat with a drywall knife and sanding with 120-grit sandpaper. When done with care, this technique produces a smooth wall at the transition, and the seam won't show up after you're finished tiling and painting.
How to Reinforce the Seam
If you're constructing the bathroom from scratch, it's easy to put a stud at the edge of the tiled area where the seam occurs. That allows you to anchor the edges of the cement board and the drywall. If you're remodeling, though, and you're working with existing framing, you may have to add a 2x4 at the seam. It's a good idea to set this new wall stud so its wide side faces out rather than setting it on edge. This gives you a generous 3 1/2 inches of wood for driving screws rather than the more limiting 1 1/2 inches provided by the edge of the stud.
If you can't fit a whole stud into the wall because of obstructions, an alternative is to install horizontal blocks between existing studs at the lowest and highest points you can access as well as any accessible points in between. If you're tiling a bathtub, the lowest and highest points would typically be at the top of the tub surround and the top of the wall. As long as you can anchor both the cement board and the drywall for a significant part of the wall, the joint should remain stable and crack-free.
Dealing With Different Thicknesses
Cement board used on walls typically is 1/2 inch thick, and so is the drywall you use in most rooms in the house. In the bathroom, however, water-resistant drywall is recommended, and this is often 5/8 inch thick. If you're installing the cement board yourself as part of a tiling project, the best way to bring the edges flush is to add shims to the wall studs before you hang it. However, if the cement board is already in place and you're installing new drywall, you need a different strategy.
If enough of the cement board is exposed, you can simply add extra mortar to the cement board side to level the seam. It may happen, however, that the tiles are set too close to the edge to allow you to do this. In this case, you might want to consider bordering the edge of the tile with decorative wood trim. This may be the only way to avoid leaving a messy joint that you can't hide with paint or caulk.