Decks Over Living Spaces: Solving Bubbling, Moisture, and Venting Challenges

Out in the countryside, land is relatively inexpensive compared to the Bay Area. As land prices rise closer to the cities, suburban lots get smaller. With smaller lots, homeowners often look for ways to maximize space, like building decks over living areas. A deck over a kitchen or bedroom not only adds usable space but can also provide stunning hillside views.

Much of our work involves deck coatings. Often, we’re re-coating existing decks that are failing, whether with urethane or epoxy. Deck coating failures usually happen at transition points, where the deck meets the wall, at door pans, or where metal flashing meets posts. Daily temperature changes cause expansion and contraction in these areas, stressing the membrane. Once we locate the source of a leak, we repair the area and apply a new coating.

Sometimes, coatings simply degrade over time due to UV exposure. In these cases, we partially grind off the old top coat, apply an epoxy layer, and then add a sand coat. The sand coat is a base urethane with kiln-dried sand pressed into it until the base layer “refuses” to absorb more. This creates a tough, rubbery “wear coat,” which we then top with a colored urethane layer. The top coat is slightly harder, allowing you to place furniture, BBQs, and potted plants on the deck without damaging the surface.

Some decks are designed with a slope over a flat layer to improve water drainage. The slope directs water either to a drain or off the edge of the deck. One of the trickiest problems we encounter is bubbling in the deck coating. Bubbling typically happens for one of three reasons:

Poor adhesion: Sometimes a new coating doesn’t bond properly to the original layer.

Trapped moisture: If a sloped deck sits over a flat deck and a tiny leak occurs, water can get trapped between layers. Heat turns it into steam, causing the coating to bubble. Once the leak is repaired, we grind out the bubbles and re-coat the deck.

Venting issues: This is common when there’s living space below, such as a kitchen. Cooking, running a dishwasher, or even normal breathing introduces moisture into the ceiling space. On hot days, the moisture turns to steam, pushing the membrane off the plywood and creating bubbles. Fixing this requires looking at the building as a whole to determine the most efficient way to remove the trapped moisture. Once addressed, the repair follows our standard process: grind out bubbles, repair, then apply epoxy, sand, and top coats.

Even experienced contractors can overlook venting problems in decks over living spaces, so careful attention to detail is essential. When done right, a well-coated deck not only protects your home but can also create an elevated, enjoyable outdoor space with amazing views.