Why is my grout crumbling and powdery?

The three reasons grout turns to powder — mixed wrong, cured wrong, or a wet substrate behind it — and why the third one matters most.

Difficulty: beginner
Time: 1 hr 30 min
Read: 3 min

Short answer

Powdery grout that rubs away with a fingernail was almost always mixed too wet, washed off too soon, or has been permanently damp. Rake it out and regrout properly. But if it is crumbling in one area only, and especially low down in a shower, suspect water getting behind the tiles before you regrout anything.

Cement grout gains its strength by hydrating slowly. Too much water in the mix, or washing the surface down before it has set, dilutes the cement at the face and leaves a chalky layer that never gains strength. Constant saturation from behind does the same thing over a longer period.

What you'll need

A grout rake or oscillating tool
New grout suited to the joint width
A grout float and sponge
Eye protection

Step by step

Test how deep the problem goes.Scrape with a screwdriver. A thin chalky skin over sound grout is a finishing defect. Grout that crumbles all the way through has either failed or has been permanently wet.
Map where it is happening.Everywhere in the room points at mixing or curing. Only in the shower, only low down, or only on one wall points at water behind the tiles.
Tap the tiles around it.A hollow ring where the rest sound solid means the adhesive bond has failed, usually because the substrate is wet. Regrouting over that is wasted work.
Check the sealant joints first.A failed silicone joint at the tray or bath lets water straight into the wall. That is the commonest reason grout in one area disintegrates.
Rake out all the failed grout.Take it back to sound material, at least 2mm deep, wearing eye protection. A grout rake for small areas, an oscillating tool with a grout blade for larger.
Vacuum and let it dry properly.If the wall was wet, give it days rather than hours with the extractor running before regrouting.
Choose the right grout for the joint.Narrow joints take an unsanded grout; joints over about 3mm need a sanded one. In a shower, use a grout rated for wet areas.
Mix to the stated ratio and let it stand.Mix to a stiff toothpaste, leave it to slake for the stated few minutes, and remix. Adding extra water to make it easier to spread is what causes powdery grout.
Wait before washing off.Let the grout firm up until a thumbnail leaves only a light mark before sponging. Washing too early pulls cement out of the surface.
Use flexible sealant in the corners, not grout.Every internal corner and the tray or bath joint. Grout there will crack out again regardless of how well the field is done.

Tips

Buy a little more grout than you need. Mixing a second, thinner batch mid-job gives a visible colour difference.
Damp-sponge in circles, then finish with straight passes in one direction. Circular finishing pulls grout out of the joints.
If the wall behind is wet, everything else is cosmetic. Find the water path first.

Common mistakes

Adding extra water to make grout easier to spread — The water-to-cement ratio determines the final strength. A wetter mix is easier to work and dramatically weaker, and it dries chalky and porous.
Regrouting over a wet substrate — New grout in a saturated wall fails the same way as the old. The water source has to be found and stopped first, and the wall allowed to dry.

If it doesn't work

Thin chalky layer, sound grout underneath

Cause: Washed off too early, or too wet a mix — Fix: Rake the surface back a couple of millimetres and regrout.

Crumbles right through, one area only

Cause: Water getting behind the tiles — Fix: Find and fix the water path — usually a failed sealant joint — then dry out and regrout.

Tiles sound hollow nearby

Cause: Adhesive bond failed on a wet substrate — Fix: Those tiles need lifting; regrouting will not hold them.

New grout crumbles again

Cause: Still-wet wall, or the mix was too wet again — Fix: Check the substrate is genuinely dry and mix strictly to the stated ratio.

Questions people ask

Can I just paint over crumbling grout with a grout pen?

Only if the grout underneath is sound and the problem is colour. A pen over powdery grout hides it for a few weeks and does nothing about the water path behind it.

How long should grout cure before the shower is used?

Follow the bag, but typically 24 hours before light use and several days before sealing it. Cement grout continues gaining strength for weeks.