QUESTION

Installing an Exterior Deck - I'm installing tile on 3/4" plywood subflooring (interior grade and cannot be changed)on an exterior, covered balcony over occupied interior space. Rain may blow in onto the tile. Any recommendations for the layers and waterproofing? I planned to install sloped cement, then EDPM pond liner, then 1/2" cement board painted with Red Guard, and then the tile with drains.

ANSWER

ANSWER - I'm not sure where you are starting on this project if the interior plywood is already installed, but I will give you some key steps and options.

First the structure needs to be structurally sound and not have any deflection more than L/360 for ceramic tile aor L/720 for stone.

Next the two part drains with weep holes need to be installed, so the weep holes will be located at the waterproof membrane level. Or you can drain off the side of the deck into a gutter system.

If possible I would paint the interior plywood with a liquid applied breathable waterproof membrane like the Red Guard from one of the tile installation product manufacturers you can find on our website.

Apply the pre-sloped concrete so there is a 1/4" per foot slope to drain. I would put a hotmop roofing application over the concrete for your primary waterproof membrane. You can use a sheet membrane liner like the pond liner, but there may be less expensive shower pan liners you can use too. No penetrations into membrane and properly weld all seams.

Unless you have perimeter drains with a uniform slope the cement backer board won't accommodate the slope of the deck. It would be better to install a 1-1/4" thick dry-packed wire reinforced mortar bed. Be sure to protect the weep holes with gravel so they don't get plugged with mortar. I would apply the liquid applied waterproof membrane over the mortar bed as a secondary membrane.

Be sure to achieve full thin-set contact and no voids under tile.

Be sure to install movement joints at the perimeter of the tile work, transitions and every 8 to 12 feet using an ASTM C920 sealant such as a structural silicone or a urethane.

Good luck!