How Deep Do You Have to Rake Out Cracked Grout Before Adding New Grout?

New grout doesn't chemically weld to old grout — it locks in mechanically against the sides of the joint. That's why the standard trade rule, echoed in Custom Building Products' grout repair instructions, is to remove at least two-thirds (66%) of the total joint depth before packing in new material. Anything shallower doesn't have enough mass or side-wall contact to resist the tile's normal micro-movement, and it flakes or cracks along the same line within weeks.

Even on a shallow joint, like sanded grout between small mosaic tile where the whole joint might only run 3/16 inch deep, don't go below the absolute floor set out in TCNA's grout repair notes: 1/8 inch (0.125 in) of removal minimum, no matter how thin the joint is to begin with. A skim coat thinner than that has no body to flex with foot traffic.

On standard floor tile, joint depth roughly tracks tile thickness, and ANSI A108.10 guidance treats 3/8 inch (0.375 in) as a typical full joint depth for standard porcelain and ceramic floor installations. Two-thirds of that works out to roughly 1/4 inch (0.25 in) of actual removal on a typical floor joint — the real number depends on the tile you're matching, so check thickness before you start cutting. If you're planning a full regrout alongside new tile and want joint width and depth worked out together instead of guessed at, the /tile-spacing-calculator lays that out by tile size before you touch a grout saw.

Showers, tub surrounds, and other wet areas get a stricter rule than dry floors: remove grout to 100% depth, all the way down to the substrate or backer board, per TCNA Handbook wet-area guidance. Partial removal in a shower leaves old, potentially saturated grout sitting behind a thin new skim, and moisture trapped there works its way back out through the same crack line as mold and staining. If you're scoping a wet-area regrout alongside a tile refresh, the /bathroom-tile-calculator helps size materials for the whole job rather than just the joint repair.

Crack width changes the answer too. Hairline cracks under about 1/16 inch (0.0625 in) that sit only in the surface layer of an otherwise well-bonded cementitious joint sometimes can be addressed without full-depth raking, per Custom Building Products' crack evaluation guidance. But anything wider, or any crack that runs the full visible width of the joint, means the crack has already reached the substrate side and needs the standard 66% minimum removal at least.

Switching grout type changes the rule as well. Moving from sanded cementitious grout to epoxy, or the reverse, means the new material needs to anchor into clean, exposed joint walls rather than layer over chemically dissimilar residue — epoxy won't bond to old cementitious grout the way a fresh cementitious mix keys into itself. Any material change gets treated as a full-depth job regardless of how narrow the visible crack looks, and the same goes for movement joints and inside corners, which should never get a partial fill.

A grout skim raked to less than the 66% mark has almost no mass to resist normal thermal expansion and foot traffic, so the new grout cracks along the exact same line within weeks or months. Each repeat patch job leaves the joint slightly wider and shallower than before, until eventually there's not enough depth left to hold grout at all — the only fix at that point is cutting the joint out and starting from the substrate.

Shallow patches also trap a hairline void behind the new surface layer where water collects with nowhere to evaporate. On a kitchen or entry floor that shows up as discoloration and a musty smell along the grout line; in a shower it shows up as mold well before the joint's first year is out, because the trapped moisture never fully dries between uses.

On floors specifically, a debonded topcoat eventually pops loose entirely underfoot, and by that stage the repair isn't a quick regrout — it's removing tile at the joint edges to get a saw or oscillating blade far enough in to finally hit full depth, which runs more in labor and materials than doing the removal correctly the first time.

A carbide-tipped grout saw or an oscillating multi-tool fitted with a grout removal blade gives more control over depth than a manual grout rake once the joint runs deeper than 1/8 inch, because you can gauge progress against the exposed length of the blade instead of eyeballing a channel you can't fully see into. Wrapping a strip of tape around the blade at the target depth turns a guess into a measurable stop point.

Once the old material is out, vacuum the channel and check the exposed tile edges and backer board for chips or damage before mixing new grout. Confirming total joint volume — through the /tile-spacing-calculator or by working out grout quantity for the reopened joints — keeps you from mixing a batch that's short once you're committed to filling at full depth instead of skimming the surface.