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Why Peninsula Showers Leak: What Should Be Behind Your Tile

August 5, 20267 min read By Job & Benito, Job's Clean Jobs Inc

Here is the sentence that explains most bathroom remodels gone wrong: grout is not a waterproofing material. It’s cement. It absorbs water, holds it, and passes it through to whatever is behind it. Sealing it helps with staining and slows absorption; it does not make it a barrier.

So what keeps the water out of your framing? A dedicated layer behind the tile. If that layer is continuous and every penetration through it is sealed, the shower lasts decades. If it has one unsealed seam, water finds it, because water is patient and gravity never takes a day off.

The four things that actually fail

1. Seams

Waterproof board comes in sheets, and sheets have edges. Every edge, inside corner and outside corner has to be sealed with the manufacturer’s banding and sealant. A corner that was merely butted together is a crack with tile over it.

2. Screw holes

The board is fastened to the studs, which means dozens of holes punched straight through the waterproof layer. Each one needs sealing. This is tedious, invisible, and the first thing skipped when someone is behind schedule.

3. The pan slope

A shower floor has to fall toward the drain — roughly a quarter inch per foot. Water that doesn’t reach the drain sits. Water that sits finds the weakest point. Flat pans and, worse, pans with a low spot away from the drain are common in older conversions, and no amount of good tile work fixes one.

4. The niche and the curb

The two places with the most cuts and corners are the shampoo niche and the curb, and they’re also the two that collect standing water. A niche with an unsealed bottom corner is a small funnel pointed into your wall.

How to tell if yours was done wrong

You can’t see the waterproofing, but you can read the symptoms:

• Grout that stays dark in one patch for a day after the shower is used. That area is holding water it shouldn’t be.
• Loose or hollow-sounding tiles. Tap around. A hollow note means the mortar bed has deteriorated, usually because it’s been wet a long time.
• Efflorescence — chalky white mineral deposits at grout lines or the base of the curb. That’s water moving through cement and leaving minerals behind. It is direct evidence of water where it shouldn’t be.
• A stain on the ceiling below, or on the wall on the other side of the shower. This one is unambiguous.
• Soft or springy floor at the shower threshold. Check the framing before anything else.
• A persistent musty smell that cleaning doesn’t touch.

Five questions to ask before anyone tiles your bathroom

Ask these of every contractor you interview, including us. The answers tell you a lot in about ninety seconds.

1. “What waterproofing system are you using, by name?” You want a specific product system, not “we use the green board.” Moisture-resistant drywall is not a waterproofing system.
2. “Are you sealing every seam and every fastener?” The only correct answer is yes.
3. “Is the pan pre-sloped, and where’s the drain relative to the low point?” They should answer instantly.
4. “Will you flood-test before tiling?” Plug the drain, fill the pan, leave it overnight, check the ceiling below. It costs one day and catches the failure while it’s still free to fix.
5. “Can I see photos of the waterproofing before the tile goes on?” Anyone doing it properly is pleased to be asked. It’s the best work they do and nobody ever sees it.

That last one is why our work page shows jobs in stages rather than finished photos only. The bathroom sequence starts with sealed board, a pre-sloped pan and floor tile going down — before any of the pretty part exists.

What this has to do with the Peninsula specifically

Two local patterns come up again and again:

Second bathrooms in older homes. Plenty of older Palo Alto, Menlo Park and Redwood City homes have had a bathroom added or a tub converted to a shower somewhere along the way. Conversions done before modern waterproofing systems were standard — or done fast — are where we find most flat pans.

Long dry spells. Our summers mean a slow leak can run for months without anyone noticing, because nothing outside is adding moisture to make it obvious. The damage shows up as a surprise rather than a gradual warning.

If it has already failed

Be wary of anyone who offers to fix an active shower leak by re-grouting. Sometimes new grout and fresh caulk at the joints genuinely is the fix — if the failure is only surface-level and the barrier behind is intact. But if water has been getting past the barrier, new grout is a cosmetic delay on a bill that grows the whole time.

The honest answer sometimes is: this shower needs to come out. That’s an expensive sentence to hear, which is why plenty of people won’t say it. We’d rather tell you now than re-grout it, take your money, and have you call us again in a year.

Common questions