Shower curb waterproofing before tile

The curb detail must match the shower receptor method. Traditional liner curbs, bonded membrane curbs, foam curbs, masonry curbs, and hot-mop curbs are not interchangeable. IRC, IPC, UPC, TCNA methods, local AHJ amendments, and manufacturer instructions control the accepted assembly.

Identify the curb type and the waterproofing method before it is covered. Confirm whether the pan uses a loose-laid liner with clamping drain, a bonded waterproof membrane with bonding flange drain, or another approved local system. Match all visible layers to the written instructions.

- Traditional liner and surface membrane details mixed without an approved transition

- Foam curb used with a membrane or mortar system not listed by the manufacturer

- Installer cannot identify whether the curb is waterproofed by liner, liquid, sheet membrane, or hot mop

- AHJ-required inspection skipped before covering the curb

The curb must be stable enough for the waterproofing, mortar, tile, and future door or glass loads. Manufacturer instructions determine whether wood, masonry, foam, mortar, or board products are allowed under the selected waterproofing. The curb substrate must not move, twist, wick moisture, or break the waterproofing layer.

Push gently on the curb and look for movement, loose blocking, bowed lumber, crumbling mortar, gaps at the floor, or unsupported foam. Confirm the substrate is clean, dry or conditioned as required, and ready for the liner, sheet membrane, liquid membrane, lath, or mortar specified.

- Loose stacked lumber under the waterproofing

- Pressure-treated or wet lumber shrinking after tile is installed

- Foam curb cut or patched without manufacturer detail

- Debris, dust, or old thin-set preventing membrane bond

In a traditional liner shower, the liner must turn up the walls and wrap over the curb as part of the shower receptor. The exact height, fastening limits, and turn-up requirements depend on the adopted code, AHJ amendments, and liner manufacturer. The curb top and inside face are part of the wet side of the receptor.

Confirm the liner runs from the shower floor up the inside face, over the curb top, and down the outside face without cuts on the top or inside face. Look for stretching, sharp folds, holes, open seams, or areas where the liner stops at the curb instead of wrapping over it.

- Liner only run to the inside of the curb

- Top of curb cut to make the liner lie flat, leaving an unsealed slit

- Liner folded so tightly that it lifts or bridges at the corners

- Outside face left without enough liner for the next layer to hold safely

The curb-to-jamb and curb-to-wall transitions are common failure points. Traditional PVC or CPE liners usually require preformed dam corners or manufacturer-approved patches bonded with the correct adhesive or sealant. Bonded membrane systems require their own inside and outside corner treatment.

Inspect both curb ends, rough jambs, and any return walls. Verify preformed dam corners, sheet membrane corners, liquid reinforcement fabric, or approved patches are present where the curb meets the vertical surfaces. Confirm they are bonded flat, continuous, and not merely tucked behind backer board.

- No dam corners at the curb-to-jamb intersection

- Wrong adhesive used for PVC or CPE liner corners

- Patch installed after the lath or board already hides the leak point

- Bonded membrane corner has fishmouths, pinholes, or uncoated fabric

Fasteners through the liner or membrane in the curb's wet-side risk zone can defeat the waterproofing. Codes and manufacturers restrict nails, screws, staples, and penetrations near the receptor, curb top, and inside face. Exact limits vary by adopted code, AHJ, and system.

Look before backer board, lath, mud, or curb cap hides the surface. There should be no screws, nails, staples, backer-board fasteners, glass-track anchors, or temporary brads through the curb top or inside face unless the manufacturer has a specific approved detail for that system.

- Cement board screwed to the top or inside of a liner-wrapped curb

- Metal lath fastened on the shower side instead of only where allowed

- Temporary staples left through the liner to hold it in place

- Future glass door hardware planned with curb-top screws and no waterproofing strategy

Traditional liner curbs are commonly covered with metal lath and mortar because tile cannot be bonded directly to most loose-laid pan liners. The lath and mortar method must protect the liner and follow TCNA-style details and manufacturer instructions. Backer board screwed through a liner-wrapped curb is not an equivalent method.

If metal lath is planned, verify it can wrap over the curb without sharp edges cutting the liner and without fasteners through the top or inside face. Confirm the lath is formed before mortar covers it and that the mortar will fully encase the lath without crushing or tearing the liner.

- Cement board installed on a traditional curb with screws through the liner

- Sharp lath edges slicing the liner at the curb corners

- Lath fastened through the curb top

- Mortar packed before the curb liner and dam corners are inspected

For ANSI A118.10 bonded waterproofing systems, the curb must be waterproofed as part of one continuous surface-applied assembly. Required overlaps, fabric, corner pieces, wet-film thickness, cure time, thin-set type, and substrate preparation are system-specific and come from the manufacturer.

On sheet systems, check for full bond, proper laps, inside and outside corners, and no fishmouths at curb edges. On liquid systems, verify coverage on the inside face, top, outside face, ends, and transitions, with reinforcement where required. Confirm the membrane ties into the shower floor and wall waterproofing.

- Liquid membrane too thin on curb edges and corners

- Sheet membrane stops at the curb top instead of wrapping the full curb

- Uncoated screw heads or seams in a board curb

- Curb waterproofing not tied into the drain-side membrane system

The finished curb should shed water back into the shower, and the waterproofing below should support that intent. Code language, accessibility rules, curbless exceptions, curb dimensions, and flood-test timing vary by AHJ and system. The curb should be documented before tile, stone cap, or glass hardware hides it.

Use a small level or straightedge to confirm the curb top is not pitched toward the bathroom side. Photograph the curb liner or membrane, dam corners, fastener-free areas, lath placement, and any planned glass attachment detail. Record open corrections before flood test or tile work proceeds.

- Curb top pitched outward toward the bathroom floor

- Waterproofing hidden before owner or AHJ inspection

- Glass installer later drills through the waterproofed curb

- No photos of the dam corners or top-of-curb membrane before tile