Can You Grout Over Old Grout? Why Skim Coats Fail and What Works

You can grout over old grout only if you first remove enough of it to leave real depth for the new grout: at least about two thirds of the joint, or down to sound grout, and never less than about 1/8 inch. A thin skim coat of new grout spread over intact old grout does not bond well, shrinks differently, and usually cracks or pops out within weeks to months.

The short answer, by situation

The question usually comes from one of three situations: the old grout is the wrong color, it is low or eroded, or it is cracked or stained. Each one has a different right answer, and only one of them involves adding grout on top of grout.

Why skim coats pop off

A skim coat is new grout smeared over the face of old grout, usually 1/32 to 1/16 inch thick. It often looks fine on the day it goes in. The failure comes later, and it comes from several mechanisms at once.

No mechanical key

Grout holds in a joint mostly because it is locked between two tile edges along its full depth, with some adhesion to the tile shoulders and the setting bed. A skim coat has none of that. Its only grip is a thin adhesive bond to a smooth, cured surface, and that bond is weak. Every footstep, temperature swing and wetting cycle works at it.

Contaminated old surface

The top of old grout is the dirtiest part. It holds residues of sealer, soap scum, cleaning products, cooking grease and body oils. Penetrating sealers in particular are designed to make the surface repel water, which is exactly what fresh cement grout needs to bond. Even after cleaning, those residues act as a bond breaker. Cutting into the grout removes that contaminated layer; skimming does not.

Thin cement dries before it cures

Cement grout needs water to hydrate. A very thin layer spread over absorbent old grout loses its water in minutes, both into the old grout below and into the air above. What remains is under-hydrated, weak and powdery. The mechanism is explained in how cement grout cures.

Different shrinkage and stiffness

New cement grout shrinks as it cures. Old grout has already finished shrinking. When a thin new layer shrinks on a stable base, it develops tension and cracks or curls at the edges, much like a thin plaster coat on an old wall. See why grout shrinks.

How much old grout to remove

The working rule in the trade is to remove at least two thirds of the joint depth, or down to sound grout if the damage goes deeper, and to leave at least about 1/8 inch (3 mm) of space for new cement grout. Some setters go to full depth whenever the grout comes out cleanly, because a full-depth joint behaves exactly like new work.

Why two thirds and not one third? Because the bottom of the joint is usually the best grout: it was compacted against the setting bed and protected from wear and contamination. What fails is the top. Leaving the bottom third gives a sound, clean base, and the remaining two thirds gives the new grout enough body and enough contact with the tile shoulders to lock in place. On thin wall tile, two thirds may be only 3/16 inch, which is close to the 1/8 inch floor; on those, take it to full depth if you can.

The removal methods, tool choices and dust control are covered in how to remove grout, and the full regrouting procedure in how to regrout tile. For a few feet of damage, use the method in spot grout repair.

Getting new grout to bond to the old grout that remains

When you leave the bottom third of the old grout in place, the new grout bonds to it as well as to the tile shoulders. A few things improve that bond:

• Fresh, rough surface. The cut surface left by a carbide saw is clean and slightly rough, which is ideal. Do not smooth it.
• Dust removal. Vacuum, then wipe or brush with a damp brush. Fine cement dust is a bond breaker.
• Damp, not wet. Dampen the joint so the old grout does not suck water out of the new. No puddles: standing water raises the water ratio of the new grout at the bottom, where you need it strongest.
• Clean shoulders. Scrape grout film and sealer off the tile edges so the new grout grips the tile itself.
• Polymer-modified grout. A polymer-modified or high-performance cement grout (A118.7) generally bonds better to existing surfaces and shrinks less than a basic A118.6 grout.
• Firm compaction. Pack the grout across the joint from two directions with a rubber float so there are no voids at the interface. See grout compaction and joint filling.

Exceptions: products and cases where grouting over can work

Products designed for shallow refill or renewal

A few products are marketed for refreshing or refilling existing grout joints without full removal. They tend to fall into two groups: single-component premixed grouts and urethane grouts with strong adhesion and low shrinkage, and resin-based "grout renew" or "grout refresher" products that are closer to a pigmented coating than a grout. Some premixed grouts list a minimum depth on their TDS that is shallower than cement grouts allow. If a product claims it can go over existing grout, read the TDS for minimum depth, required surface preparation (often etching or abrading the old grout and removing all sealer) and approved locations. Many exclude shower floors, continuously wet areas and exterior use.

Even with those products, the best results come from cutting the old grout down at least a little to remove the contaminated surface. The products reduce the amount of removal, they do not eliminate the need for prep. Background on these categories is in premixed grout and urethane grout.

Joints originally left very low

If a floor was grouted with joints left 1/8 inch or more below the tile face (common with heavily cushioned edges or tile with a deep pillow edge), and the remaining grout is hard and clean, filling the remaining depth can work because the new layer is thick enough. Scrape and abrade the old surface, clean it, and fill. If the joints are low by less than about 1/8 inch, cut them down further before filling. The causes of low joints are in low or recessed grout joints.

Color change only

If the grout is sound and you only want a different color, the product built for that is a grout colorant (a pigmented sealer-like coating). It is formulated to bond to cured grout and is applied in a film measured in mils, not fractions of an inch. It is not a grout and cannot fix cracks or low joints.

Mixing grout types: epoxy over cement, cement over epoxy

New epoxy grout over a cut-down base of clean, dry, sound cement grout generally bonds well, provided the cement grout is fully dry and free of sealer; epoxy is tolerant of the base as long as it is clean and the new layer has enough depth to lock between the tiles. The reverse is not true. Cement grout does not bond reliably to cured epoxy, and epoxy residue on tile shoulders also repels cement grout. If the original grout was epoxy, plan to remove it fully and regrout with epoxy. The comparison of the two families is in epoxy vs cement grout.