Tom  McNall
Floor Restoration Contributor

Restoration on Crack

When I first heard the term “Crack Suppression Mem-brane,” I envisioned some sort of Wile E. Coyote-inspired trap set up by the ATF to haul away inner city drug users and dealers involving a net attached to a helicopter.

Notwithstanding, I still think that is a good idea, but what Crack Suppression Membrane actually is turned out to be a handy tool in the stone trade’s arsenal.

Essentially in my experience in restoration, CSM (I’m not going to type out the full phrase/name throughout the article) is used when we see one or more semi-straight cracks in an installation. It is not something we can apply over a stone to fix, it must be applied under the stone. That means we have to remove the stone to get to the crack before we can use it. But let’s not get ahead of ourselves just yet.

Just what is CSM? The layman’s explanation is that it involves covering a crack in the substrate with rubber cement, then applying a specially reinforced fabric over that, and then coating over the fabric with another layer of the rubber cement. There are, however, dozens–if not more–systems available that employ slightly different methods and/or materials.

Why is CSM needed? Well, when I was taking a course on substrate and subfloor inspection to be a certified inspector, I learned a valuable lesson about concrete slabs. That lesson was, “there are two types of concrete slabs: cracked and about to crack.” The reasons for this being true are endless: slab/earth under slab settling; slab curing; extreme temperature changes; poor mix; concrete doesn’t like us…  

Either way, you end up with a crack in the subfloor and it will always transfer up to the stone and grout above. Why? Because unlike rubber cement and fabric, stone tile and grout will not bend, fold, expand and contract or shift when the slab beneath it does (technically the slab isn’t either, but it is separating or pulling away on both sides of the crack).

Therefore, when the stone above the crack is securely adhered to the subfloor on both sides of the crack, it needs to “give” when the slab does and it does this by telegraphing the crack from the slab up through the setting bed and stone above. CSM has a way of getting between the two rigid surfaces like a marriage counsellor and taking the sting out of the innocent stone on top, thus “suppressing” the crack.  Hence the name.

Now, in restoration, I have seen competitors try to fill the cracks and move on. Well, guess what? The cracks magically re-appear like unwelcomed in-laws.  This is because the fill is usually just as rigid as the stone. You need to pull up the damaged stone and count them as waste because the odds of them being pulled up, glued together and re-laid are just slightly better then the odds on winning the lottery when you don’t buy a ticket. That means you need to find a similar looking stone tile before you attack the repair.

Once the cracked stone is up, you need to clean the surface and smooth it as best you can around the crack. For cracks that run horizontally through a row of tile, we will remove the whole row and ensure we have at least 4-5 inches on each side (we prefer 6-8˝) of the crack.

If the crack is closer to one edge of the tile and you are dealing with 12˝ x 12˝ stone, you may need to remove a second row. Cracks that travel diagonally through the tile require that you remove two or more rows of tiles on both sides so that you can properly cover the crack with the flexible membrane.

Once the membrane is down and the required curing time is up, you can re-tile the area and grout when appropriate. If this is a restoration job, you may now have a row of tiles higher than the rest of the floor (but not always). If this is the case, you will need to grind the row of tiles level with the rest, and refinish accordingly to the desired sheen.

Construction in this new millennium is moving fast. We are building bigger and faster. Unfortunately, concrete is not curing or settling much faster than it did 20, 30 even 50 years ago. For that reason, construction companies cannot wait to find out when and where the next crack in a slab substrate is going to show up, and instead opt to use CSM in an all-out approach, covering the entire surface.

No matter what your situation, if you are going beyond a quick repair (quick in the stone sense, anyway) of one row of stone tile, I suggest that you be up to date on your regional building codes for use of CSM as to what type (waterproof, soundproof, vapor transmission) and how much coverage you are required to use, if any, for your situation. As well, use a true CSM system and try to avoid the cheap and quick fix methods like tar and Kraft paper.

I hope you have found this primer on CSM helpful and until next month, keep your stick on the ice.

Tom McNall is founder and owner of Great Northern Stone, an Ontario-based stone cleaning and restoration company servicing Ontario and Chicago, IL. Tom also offers corporate and private consultations as well as speaking at conventions. He can be reached at .