The shower in the Findlay Creek bungalow looked rough. The grout had gone from off-white to mottled grey, with black streaks at the bottom of the wall and a few hairline cracks along the corner. The owner asked the obvious question: do I need to rip this out and start over, or can I clean it?
The honest answer depends on three things. Most homeowners assume the worst when the answer is usually an inexpensive bottle of cleaner. Here is how to read your grout before you spend money you don't need to spend.
What grout actually does, and why it fails
Grout fills the joints between tiles. It does two jobs: it stops water from running down behind the tile, and it allows the tiles to flex slightly without cracking each other. It is not structural. The thinset under the tile is what holds the tile to the wall.
Grout fails in three ways. Surface staining (black mildew, dirt buildup, soap scum embedded into the porous surface). Structural cracking (the grout splits along a line, usually at a corner or along a long wall). And separation (the grout pulls away from the tile edge, leaving a hairline gap on one or both sides).
Each one has a different fix. The trick is reading which kind of failure you have.
The three-step diagnostic
Before you decide cleaning vs replacing, run these three tests on the worst-looking section.
Step 1: the fingernail test
Press your thumbnail into the grout joint with firm pressure. Healthy cement-based grout resists. It feels dense, like setting concrete. Failed grout crumbles, flakes, or leaves chalky residue under your nail.
If the grout is solid: cleaning will work. If it crumbles: regrouting is required. The grout has lost its binder and no amount of cleaner will restore it.
Step 2: the moisture test
Spray a fine mist of water on the grout from a spray bottle. Watch for 60 seconds. Healthy grout beads water on the surface or absorbs it slowly and evenly. Failed grout absorbs water immediately, often turning visibly darker, and the moisture spreads into the tile body.
The water that disappears in 5 seconds is going somewhere. Usually behind the tile. That's the failure mode you actually need to worry about, because the next step after grout failure is the substrate going soft.
Step 3: the corner inspection
Look at every change-of-plane joint: where the wall meets the floor, where two walls meet at a corner, where the tile meets the tub or shower base. These joints should NEVER be grouted. They should be caulked with a colour-matched silicone or a polyurethane caulk that flexes.
If those joints are grouted, the grout will eventually crack, not because the grout failed, but because the building itself moves slightly with the seasons and the rigid grout cannot flex. The fix is to remove the grout from those joints and recaulk them. The wall-field grout may still be fine.
Safety warning: black grout staining is not always mildew
Persistent black staining at the bottom of a shower wall, especially below the lowest 12 inches, is sometimes mildew on the surface. But it can also be water that has gotten behind the tile and is staining the substrate from the inside out. If your cleaning effort doesn't budge the staining and the tile sounds hollow when you tap it, the issue is behind the tile. At that point, DIY stops. Water inside a wall cavity rots the substrate, the framing, and eventually the floor below. Pull a tile or two and see what's behind. If you find black mold or soft drywall, call a renovator. This is not a clean-and-recaulk fix.
When cleaning is the right answer
Grout that passes the fingernail test (dense, no crumbling) and the moisture test (slow, even absorption) is structurally fine. It just looks bad. Three cleaning approaches in order of effort:
- Hot water and a stiff brush. Surprising how much surface dirt comes off with no chemicals at all. A grout brush (small, stiff bristles, narrow head) plus near-boiling water removes most light staining in 20 minutes per shower.
- Oxygenated cleaner (OxiClean or similar) plus the brush. Mix per package directions, apply to grout, let sit 10 minutes, scrub. Removes mildew and embedded soap scum that hot water alone won't touch.
- Steam cleaner. Rentable by the day at most home centres. The steam pressure breaks up grime that brushing won't reach. Best for floor grout in kitchens and entryways. Be careful with steam on caulked joints; sustained heat will degrade silicone.
What to avoid: bleach (whitens cement grout temporarily but degrades the binder over time), muriatic acid (etches the grout and the tile glaze), wire brushes (scratches porcelain). Stick to oxygenated cleaners and elbow grease.
After cleaning, seal cement-based grout with a penetrating sealer (Aqua Mix Sealer's Choice Gold or Miracle 511 Impregnator). Two coats, 24 hours apart, on a fully dry surface. Sealer pushes back the next round of staining by 18 to 24 months.
When regrouting is the right answer
If the fingernail test reveals crumbling, or you see structural cracks running along grout lines, regrouting is the fix. The job: dig out the old grout to a depth of at least 1/8 inch, vacuum the joints, mix new grout, work it into the joints, wipe the surface clean, let it cure.
The DIY truth: regrouting a shower is doable, but tedious. The grout removal is the slow part. A carbide grout blade on an oscillating multi-tool (an inexpensive accessory) cuts the time in half compared to a manual grout saw. Budget 4 to 6 hours for a typical 3-wall shower, plus another 2 hours for the regrouting itself.
Three things that catch first-timers:
- Grout type matters. Sanded grout for joints over 1/8 inch wide, unsanded for joints under 1/8 inch. Mix them up and the wider joints crack in six months.
- Match the colour or accept the difference. New grout in a few joints will look new against old grout in adjacent joints. Either regrout the entire wall or accept the patchwork look. There's no in-between.
- Don't grout the change-of-plane joints. Wall-to-wall corners, wall-to-floor, wall-to-tub: these get caulk, not grout. Grouting them again will crack again, same as before.
When neither cleaning nor regrouting will fix it
Three failure modes mean the tile itself has to come out:
- Hollow tile. Tap each tile with a wooden handle. A solid tile sounds dense. A hollow tile sounds drum-like. Hollow tiles have separated from the substrate. Even if you regrout around them, water will keep getting behind them. The tile and the thinset have to come out and be replaced.
- Soft substrate. If the wall flexes when you press on it, or the floor gives slightly underfoot, the substrate behind or below the tile is rotted. Water has been getting in for a while. The tile, the thinset, and the substrate all have to come out, the framing has to be inspected, and the wall has to be rebuilt with a proper waterproof membrane (Schluter Kerdi, Wedi, or equivalent) before new tile goes back.
- Tile that's discoloured from inside. Some natural stones (marble, travertine) absorb moisture and stain permanently. No surface cleaning will fix it. The tiles have to be replaced or the whole wall refinished.
The grout is rarely the real problem. The grout is the messenger. What you actually need to know is whether the wall behind it is still dry.
The honest call for an Ottawa shower
Most Ottawa bathrooms we see at the 8 to 12 year mark need cleaning, sealing, and recaulking the change-of-plane joints. Total DIY cost: a handful of inexpensive supplies and half a Saturday. The grout is fine. The owner just thought it was time to replace it.
At the 15 to 20 year mark, regrouting becomes a real conversation. By then the original grout is often crumbling, the caulk has long since failed, and there's enough surface staining that cleaning won't catch up. DIY regrouting works if you have the patience.
Past 20 years, we typically recommend a tile-out. The substrate behind older tile (often greenboard or unsealed cement board) is unlikely to be still in good shape. Rebuilding the wall with a modern waterproof membrane gives you another 25-year run with no leaks.
For a complete tile and waterproofing rebuild as part of a larger bathroom project, see our bathroom renovation Ottawa page. We use Schluter Kerdi or Wedi systems on every wet wall and warrant the work for 1 year, plus the manufacturer's 25-year warranty on the membrane itself.