Why Shower Grout Cracks, and What the Crack Is Telling You

Grout cracks, someone regrouts it, and six weeks later it cracks again in exactly the same place. That second crack is the useful one. It tells you the grout was never the problem.

Grout is cement. It is hard, it is brittle, and it has close to no ability to stretch. Put it somewhere that moves and it will break, every time, no matter who mixes it or what it cost. So the question is never why the grout cracked. The question is what moved.

Where the crack runs answers that.

A crack in the corner is not a grout failure

Look at where the shower wall meets the other wall, and where the wall meets the floor or the top of the tub. If the crack is in one of those lines, the grout is doing exactly what cement does when two surfaces move independently of each other, which they always do.

Those lines are called changes of plane, and the tile industry's own standard has said for decades that they do not get grout. They get a flexible sealant, usually silicone, colour matched to the grout so you cannot tell by looking. Silicone stretches. Grout does not.

This is the single most common thing we find in a shower that keeps cracking in the same spot. Somebody grouted the corners. Every seasonal movement in the house, every time a full tub loads the floor and unloads it again, that rigid line takes the strain and splits. Regrouting it just starts the clock again.

The fix is a genuine fix: rake the grout out of those joints, clean them properly, and run silicone. That is an afternoon, not a renovation.

A crack in the field of the tile is a different story

If the crack runs across the middle of a wall or a floor, through the grout lines between whole tiles rather than in a corner, something structural is moving underneath.

Tap the tiles either side of it with a knuckle or the back of a screwdriver. A tile bonded properly to the wall sounds dull and solid. A tile with a void behind it rings hollow, like tapping a cardboard box. Walk the sound along the wall and you can usually map the void.

Hollow tile normally means the mortar never made full contact. Tile is supposed to be set into a combed bed of thinset and pressed until the ridges collapse, and on a wet wall you want the back of the tile almost completely covered. What we find instead, in work that fails early, is spot bonding: five dabs of mortar, one in each corner and one in the middle, tile pushed on. It looks fine on day one. It leaves four fifths of the tile with air behind it, no support, and a direct path for water.

That is not something you repair with grout. Those tiles come off.

Movement joints exist for a reason

A tiled surface expands and contracts. Not much, but tile is unforgiving, and across a long run it adds up. The standard calls for soft joints at intervals across large areas, at every change of plane, and wherever the tile meets a different material or a restraining surface such as a door frame or a window.

Almost nobody puts them in a residential shower, and in a small shower you can often get away with it. In a long floor run, or a steam shower, or a wall that catches direct sun through a window, you cannot. When there is nowhere for the movement to go, it goes into the weakest line available, which is the grout.

The wall behind the tile

In older New Jersey housing, and there is a lot of it in Kearny, Harrison, Newark and the towns around them, we open showers and find tile set straight onto drywall or onto green board. Green board is moisture resistant, which is not the same as waterproof, and it was never meant to sit behind a shower.

Once water gets past the grout, and it will, because grout is porous and was never a waterproofing layer, that board softens. A soft board cannot hold tile still. The tile flexes slightly every time it is touched, and the grout lines open up.

You can often tell from outside. Press the tile firmly. If there is any give, or if the grout is dark and damp long after the shower has dried, the problem is behind the tile and no amount of surface work will reach it.

The same goes for a bench or a curb that cracks along the top. Those are the two places a shower takes the most weight and the most water, and they are the two places that get built badly most often.

Joints that are too tight

Two more things we see, both avoidable.

Grout joints that are too narrow for the tile. Pressed porcelain is not perfectly flat or perfectly square, particularly in larger formats, and a hairline joint leaves the grout no body at all. A thin ribbon of cement in a joint that moves will crack.

The wrong grout for the width. Traditional sanded and unsanded grouts are formulated for different joint widths, and unsanded grout used in a wide joint shrinks as it cures and crazes. Modern high performance grouts are more forgiving, but they are not a licence to ignore the joint.

What to do about yours

Work through it in order.

Find where the crack is. If it is only in the corners and where the tile meets the tub or the floor, that is a silicone job and the rest of the shower is probably sound.

Tap around the crack. If it all sounds solid, you are likely looking at a movement problem at the surface. If it rings hollow, the tile is not bonded and it will keep failing.

Press the tile and look at the grout after the shower has been dry for a day. Movement or lasting damp means the substrate is wet, and the only honest answer is to open the wall and look.

Check the ceiling below if there is one. A stain there means water has been getting through for a while, and at that point the shower is not a grout question at all.

When regrouting is the wrong answer

There is a real temptation, when a shower looks tired and the grout is cracked, to grind it all out and put fresh grout in. On a sound shower that is a genuine improvement and it buys years.

On a shower with hollow tile, a soft substrate or no waterproofing behind it, it is the most expensive way to lose six months. The new grout cracks along the same lines, the water keeps going where it was already going, and the damage underneath carries on quietly.

We would rather tell you that on the first visit than after we have taken your money for a regrout that was never going to hold. That is why we look before we quote, and why we ask to run the shower and watch what the tile does.

If your grout keeps cracking in the same place, call us and we will come and look at it. Nobody can tell you from a photograph whether a shower needs a silicone joint or a new wall, and we are not going to pretend otherwise. We look first, in person, then we quote.