Field guide

Shower Pan Flood Test: The 24-Hour Check Before Tile Goes Down

Plug the drain, fill the shower pan to within an inch of the curb top, mark the water line with tape, and leave it 24 hours. If the line has not moved and the ceiling below is dry, the pan holds. Do it before any tile goes on, because afterward there is nothing left to look at.

Tile is not waterproof. The membrane is

The most expensive sentence in bathroom remodeling is the assumption that tile keeps water out. It does not. Water gets through grout lines into the mortar and the backing behind it, and the membrane under the tile is what keeps the shower dry. Ontario's building code lists tile among the materials it calls a waterproof finish, which is where the idea comes from, but the makers of shower waterproofing say it plainly: contrary to popular belief, the tile covering itself is not waterproof.

That is why the flood test exists. It is the one moment anyone can see the waterproofing work, and it happens before the tile goes on, because afterward a leak is found the expensive way: a stained ceiling, a swollen subfloor, a gut renovation of a two-year-old bathroom. A membrane maker calls the test strongly recommended before setting tile, and some plumbing codes require it.

How the test actually runs

The procedure comes from Laticrete's Technical Data Sheet 169, which draws on ASTM D5957, the standard for flood testing horizontal waterproofing. Flood the pan with potable water to a minimum depth of one inch and a maximum of four inches at the low point, with a mean depth not over two and a half inches. Then wait. The test runs 24 to 72 hours, counted from the moment the water reaches its maximum depth. A builder's rule of thumb is simpler: plug the drain with a test plug, fill to within an inch of the curb top or to a marked line on a curbless floor, mark the water line with tape, and come back in 24 hours.

Two details decide whether the test means anything. First, the membrane has to be cured before it is tested. A sheet system typically needs at least 24 hours before a flood test, and a liquid membrane at least 12, longer in cool or damp conditions. Second, the water load itself stresses the structure, so the Tile Council of North America deflection requirements have to hold through the whole test. A pan that passes on a flexing floor has not really passed.

What a failed test looks like

A failed test is usually quiet. The water line drops a quarter inch overnight, or the ceiling below shows a damp spot that was not there yesterday. That is a leak, and the fix is to drain the pan, find it, repair the membrane, and run the test again until it holds. The temptation is to call a small drop evaporation and move on. Do not. A quarter inch over 24 hours in a small pan is a real volume of water going somewhere it should not, and the somewhere is usually the framing.

The two systems fail differently. A traditional pan liner lets water through the mortar bed and drains it out through weep holes at the drain, so a blocked weep hole is a failure even when the liner holds. A bonded membrane stops water right under the tile, so the shower dries between uses, but a missed seam or an unsealed penetration is a straight leak. The flood test catches both, which is why it applies to either system.

Ask for it by name

Your municipality may or may not inspect the pan, so assume no one will and photograph the test yourself. The photo of the taped water line at the start and the same line 24 hours later is the record that the shower was built right, and it is worth more than any warranty paragraph if a leak shows up later. If your remodeler says the test is unnecessary, that is information about the remodeler. The test costs a day. A failed shower costs a bathroom.

Related reading

Readers working through “Shower Pan Flood Test: The 24-Hour Check Before Tile Goes Down” often find these earlier guides useful as well: Basement Egress Windows: The Code Rule That Decides the Bedroom, Bathroom Exhaust Fan: The 50 CFM Code Rule and Why Fans Fail Inspection, and Remodeling Change Orders: Who Pays When the Plan Changes.

Frequently asked questions

How long should a shower flood test run?

24 hours at minimum, and up to 72 hours per Laticrete TDS 169 and ASTM D5957. The clock starts once the water reaches its maximum test depth. Some manufacturers accept less, but none objects to more.

How deep should the water be for a flood test?

A minimum of one inch and a maximum of four inches at the low point, with a mean depth not over two and a half inches. In practice, fill to within an inch of the curb top and mark the line with tape.

Does the flood test happen before or after tile?

Before any tile goes on. The test proves the membrane holds, and once tile covers it there is nothing left to look at. It also has to wait until the membrane has fully cured, usually at least 24 hours for sheet systems.

What do I do if the water level drops during the test?

Drain the pan, find the leak, repair the membrane, and run the test again until the level holds for a full 24 hours. A small drop is still a leak. Do not write it off as evaporation.