Leaking Deck Repair and Waterproofing in Seattle

$10–$25 per square foot installed, according to a Puget Sound waterproofing contractor, which puts a typical 200 square foot deck between $2,500 and $6,000. The spread covers the system itself, the condition of the plywood underneath and the fiddly parts: drains, posts and the threshold under the door.

A Seattle remodeler prices the same work in tiers, which is a useful way to see where your deck sits:

• $600–$1,200 for surface prep plus installing the waterproof membrane
• $1,300–$2,200 when structural repairs and upgrades go in at the same time
• From $150 for a site evaluation and deck condition assessment, and from $180 for a moisture inspection and report

Timing moves the bill more than the brand does. Recoating a membrane that is still sound costs considerably less than a full tear-off, and once the deck has leaked into the space below you are paying for substrate repair too. The deck cost guide covers new builds, including the ledger and flashing work that sits behind most of these leaks.

Look at the ceiling under it before you look at the deck. The signs a Puget Sound waterproofing firm lists are water stains or dripping on the ceiling or walls below a deck or balcony, soft spots underfoot, and coating that is cracked, bubbling or peeling, which means the membrane has lost its seal.

Flashing is the usual culprit and it does its damage quietly. When the flashing above the ledger fails, water runs behind it on every wet day and the rim joist inside the wall rots over three to five years, according to a Seattle repair contractor. By then the lag screws holding the deck to the house are gripping soft wood. That is the failure worth paying to rule out early, and what it costs to put right is in deck rot and structural repair.

Not because of the rain. A Puget Sound contractor is blunt about it: what kills deck systems early is installation shortcuts, meaning missing or wrong flashing, no slope to drain, and coatings applied over damp or damaged plywood. Done properly, the same firm puts a membrane system at ten to twenty years in Western Washington, with a maintenance recoat roughly every five to seven years extending it.

So the questions that matter are about sequence and detail rather than product. Was the plywood dry and sound when the coating went on? Is there slope to the drain? How is the membrane turned up at the door threshold and around the posts? A bid worth comparing answers all three in writing, and the hiring checklist covers the rest of what belongs on it.

Yes. Seattle's exemption for a low platform only holds when the structure is less than 18 inches above grade and not over a basement or other building story, so a deck over a room is outside it from the start. Two more of the city's rules catch waterproof decks in particular:

• A roof deck, meaning one built on top of part of the building, needs a permit
• Deck flooring that is a solid surface with no gaps between boards pushes the job from the quick subject-to-field-inspection permit up to an addition or alteration permit, and a membrane deck is exactly that surface

The suburbs write their exemptions the same way. Redmond exempts detached decks no more than 30 inches above adjacent grade and not over any basement or story below, and Edmonds and Bellevue both draw their line at 30 inches with the same proviso. In every case a deck over something habitable fails the test. Deck permits across the Seattle-area towns sets the seven cities out together.

What happens when the plywood comes up. That one line decides whether your price holds:

• Recoat or full tear-off, and which one this deck is getting
• What the price does if wet or damaged substrate is found underneath, with a rate agreed up front
• Flashing at the door threshold, at the wall, and around posts and railing penetrations
• Slope to drain, and where the water is meant to go
• The membrane system by name, and who warrants it
• Who applies for the permit, and whether the fee is in the price

Then look the company's registration up on the state's L&I Verify tool before you sign. How we vet our deck pros explains the check we run on our side. When you are ready, describe the leak and our vetted local deck pro takes it from there.