The Best Underlayment In The Bathroom
Is Cement Fiber Panels
Whether you’re planning on remodeling an existing bathroom or adding a completely new one, you will probably be doing some kind of ceramic tile work. While laying tile in a bathroom (or any room for that matter) looks intimidating at first glance, it’s really not that difficult for even the beginning do-it-yourselfer. But before you tackle the tile, you need to have something to lay it on and that something should be cement fiber panel underlayment.
The 2006 International Residential Code (IRC) will not recognize moisture-resistant, paper-faced drywall (a.k.a. "green board") as an appropriate tile backing material in wet areas like tub and shower surrounds. A cement based tile backerboard will now be specified to meet IRC requirements.
What are Cement Fiber Panels? 
Exactly what they sound like. ¼” or ½” thick 3’ x 5’ panels made from 90% cement that weigh around 40 pounds each. Cement fiber panels are readily available from big box home remodeling centers and most lumberyards. Economically priced, you’ll normally pay about $10 per sheet. They’re installed using roofing nails or special square-drive screws that are available wherever the panels are sold.
When you bring the panels home, don’t make the mistake of leaning them up against a wall to get them out of the way. Despite being made of cement, they will bow. The preferred storage method is to simply stack them flat on the floor.
So what do you do if you’re faced with a bowed panel? Lay it flat somewhere and wait a while. It’ll straighten out in short order.
Why use Cement Fiber Panels?
Cement fiber panels are the easiest way to attain a smooth, rigid base for ceramic tile floors. At first glance it would seem that particle board, thick masonite, exterior grade plywood or even an existing hardwood floor would form a perfectly acceptable underlayment (also known as substrate). Despite appearances to the contrary, that’s wrong. Tile absolutely cannot be installed over hardwood floors, masonite, particle board or vinyl tile. It’s possible to use exterior grade plywood as your substrate, but do so at your risk. The problem is that moisture can cause the plywood to delaminate.
When it comes to walls, tile can be installed over drywall (but not wallpaper that may be on the drywall). Still, cement fiber panels can be installed over the drywall and will make your tile job more durable. Cement fiber panels in tub/shower surrounds, as you will see a bit further down in this article, are mandatory.
What’s the best thickness?
That depends. If you’re planning a tile a floor and are putting cement panels down over a solid base that has no flex to it, ¼” will work just fine. For a sub-surface that gives even slightly, ½” would be the best choice. The object is to wind up with an underlayment that is completely rigid in order to eliminate eventual cracking of the tile grout.
Walls can be either thickness, depending on a number of factors. The regular walls of an existing bath can have ¼” panels applied directly over the drywall (sheetrock). New construction would normally use ½” drywall over open studs with ¼” cement fiber over that. But there’s certainly no reason why you couldn’t use ½” cement fiber over the open studs in place of the drywall. Just be aware that if you do so, a good bit of additional framing will be necessary in order to properly mount the cement panels. Do keep in mind that if you live in an area with soil that moves a lot, attaining a rigid base (especially for floor tile) can border on the impossible. Still, the ½” thickness can minimize problems to the point that you’ll never have to worry about cracking grout.
Tub/shower surrounds
Tub/shower surrounds are a special case due to water, steam, and heat exposure. Virtually every builder or remodeler uses greenboard as a base surface over the studs. For those of you who don’t know, greenboard is nothing more than a water resistant gypsum board. Gypsum board is, of course, drywall. It doesn’t take kindly to moisture, especially the high level that exists in a shower. Tiling over greenboard leaves absolutely no margin for error. A poor grouting job on the tile can let enough moisture thru to the greenboard that will set the stage for eventual deterioration. But there is a solution.
Since cement fiber panels are impervious to water, some of the more enlightened contractors are beginning to use cement panels in place of greenboard and the practice is gradually spreading. It’s also an ideal choice for the do-it-yourselfer. If your contractor is not planning to use cement fiber panels in you shower you should request it. If he is bullheaded about it, you should demand it. It is your house and you are the one who will be dealing with problems 5 years down the road.
Installation
Regardless of the thickness used or the surface you’ll be tiling, it’s recommended that each panel be nailed or screwed on 8” centers. Using that pattern on a 3’ x 5’ panel means that you will use 54 fasteners per panel. Can you use fewer? You can (and in some cases you have to), but however many (or few) you use, make sure that the end result is solid with no give to it. Assuming your bath is large enough, you will want to overlap the joints of your cement panels. Sometime that isn’t possible. Regardless, once the panels are down and before you begin to lay tile, you will want to cover the joints with 2” wide backer tape (available from your cement fiber panel supplier) pressed into a thin layer of tile adhesive. The end result looks very much like taped and bedded drywall.
When working with cement fiber panels, you want to be sure you use the appropriate safety equipment. These panels are normally cut by simply scoring with a special blade (available where you buy the panels) and snapping, much like cutting glass. However, if you need to cut narrow strips or other shapes with a portable power saw, do not do it indoors and wear a double-element respirator. These products contain silica. Silica dust is easily inhaled and can cause silicosis. So wear that respirator! You’ll also need a carbide saw blade designed especially for cement panels. They’re available wherever the panels are for around $8 or $10 for a two-pack.
When installing cement fiber panels over the open studs of a tub/shower surround, you probably will not be able to maintain the recommended 8” on center dimension for the fasteners. This really isn’t a problem as long as you add enough supports (cripples) between the wall studs that will result in a rigid installation. The above photo of a partially complete tub/shower surround using cement panels over open studs clearly illustrates the correct approach.
Conclusion
Tackling a tile bathroom, whether floor, walls or tub/shower surround, is not something to be afraid of. Certainly it deserves respect and careful planning as does any kind of construction or remodeling. If you take your time, research the proper methods, and use the correct equipment then you too can have that one of a kind bathroom that will be trouble free for years to come.
Steven Pollack is the owner of Bath Plus, an importer of exotic hardwood bathroom vanities. This line can be found at www.BathGems.com You may reprint this article unedited so long as the attribution and links remain in the reprint. | 
