Deck and Balcony Waterproofing for Marin Hillside Homes

Marin's hillside neighborhoods — Mill Valley, Tiburon, Sausalito, San Anselmo, Ross, parts of Belvedere — are dominated by homes with substantial deck space cantilevered or post-supported off the hillside. Those decks have three specific weaknesses we see again and again: 1. Wall-to-deck transition leaks. The deck butts into the wall of the house. The vertical wall flashing has to overlap the horizontal deck membrane the same way it would overlap a roof shingle. When the original deck was painted or sealed with caulk, that detail fails within 3–5 years and water tracks down inside the wall — often unnoticed until it reaches the room below. 2. Drainage edge. A deck that doesn't drain correctly to the outer edge ponds water at the wall. Sloped properly (typical 1/4" per foot toward the drip edge), the surface sheds water in seconds. Sloped wrong, every storm puts water at the worst possible spot. 3. Under-deck moisture and rot in support framing. A deck membrane that has leaked for years leaves the joists damp. Replacement of the deck surface alone, without addressing the framing condition, is a band-aid. ALX is the system we install when a homeowner wants the deck to behave like a roof — because that's what it is. A roof you walk on.

Where deck waterproofing goes wrong: - "Deck stain" or "deck sealer" used on a structural deck over living space. These products are designed for stand- alone wood decks above grade with airflow underneath. Applied to a structural plywood deck above a habitable room, they fail immediately at the transitions. - No wall flashing integration. The deck membrane has to step up the wall behind the siding (similar to step flashing on a roof). Stopping the membrane at the wall surface and caulking the joint is the #1 deck-leak failure mode. - No drip edge at the outer perimeter. Without proper edge metal, water wicks back under the membrane at the deck's outer edge. Eventually the OSB or plywood substrate rots from the outside in. See drip edge. - Missing ice & water shield under the threshold to the door. Wind-driven rain at the door threshold needs the same shield treatment as a roof valley. Skipping it means the doorframe is the first thing to rot. - Patching cracked ALX with elastomeric paint. Cracks in the field surface are usually substrate movement, not the membrane. Painting over them traps moisture against the waterproofing layer. Crack repair on ALX has a specific repair procedure — paint isn't it. - Replacing the deck surface without checking the framing. A new $8,000 deck membrane over rotted joists fails in 18 months. Open the deck, evaluate the framing condition, repair the substrate first.

Northern Pacific is a Westcoat ALX-certified installer. On a typical Marin hillside deck waterproofing scope, we: 1. Strip the existing deck surface to bare plywood or OSB. Inspect and repair any substrate damage or framing rot. 2. Install the wall-to-deck transition flashing — typically a 25-gauge galvanized step pattern overlapping the wall sheathing underneath the siding edge, similar to a roof step flashing detail. 3. Install drip edge metal at the outer perimeter, fastened and lapped per Westcoat's published detail (Westcoat ESR-2290). 4. Install ice & water shield at door thresholds and at the wall-deck transition's inside corner. 5. Fasten the galvanized lath. Apply the base coat. Embed fabric. Apply the second waterproofing coat. Finish coat to the homeowner's color selection. 6. Final inspection — drainage test (water from a hose toward the field, confirming clean drain to the drip edge). A properly installed ALX system on a Marin hillside deck lasts 15–20 years with annual visual inspection. We back the installation with our workmanship warranty and Westcoat's product warranty. CA Lic. #812250.

If your Marin hillside deck has visible cracking, ponding water after rain, soft spots when walked on, or any sign of leaking into the room below — that's not a paint job. It's a roof. Get a free deck waterproofing estimate Call (415) 456-3482 For broader roof-system context, see Why Is My Roof Leaking? or Repair vs Replace Your Roof.