The spaces between decking boards on outdoor decks serve an important function—namely, drainage. They help to prevent puddles from forming and enable the surface to dry more quickly after rain and snowstorms.
Along with routine waterproofing application, spacing between boards helps to prolong the useful life of the decking material. But if you want to use the space beneath a deck for additional outdoor living or storage space, it’s better to install a barrier that prevents water from draining to the surface directly below.
The dominant solution for waterproofing areas below decks is to drape plastic membranes within joist spaces below decking to form troughs. The Trex RainEscape deck waterproofing system, shown below, is an example of a membrane-and-downspout system.
The Trex RainEscape troughs divert runoff to downspouts positioned near the joist header. A length of gutter is attached to the structure just below the downspouts to collect water and lead it away from the protected area.
A number of recently introduced under-deck ceiling systems use corrugated aluminum or plastic panels designed to catch water that drains through decking and channel it away from the area below. Among the newest is the RainTight system, introduced at Deck Expo 2014.
RainTight panels are made of heavy-gauge aluminum. The seamless panels are custom-fabricated, based on the size of the deck. They lock together along their long edges to form leak-proof joints.
A perimeter frame and graduated spacer-nailers are fastened beneath the deck joists so that the plane of the ceiling slopes slightly (at least 1/4 inch per foot) away from the house toward the joist header. A system-compatible aluminum trough is fastened below the joist header to carry runoff to one or more downspouts that drain to the ground outside the protected area.
Under-deck ceiling systems provide an all-in-one drainage and ceiling finish that is particularly suitable for areas beneath a deck that are high enough off the ground to provide a sheltered outdoor living space below. Special framing and moisture-protection measures can provide mounting points for overhead light fixtures and ceiling fans.
Simpler than membrane and ceiling systems, waterproof decking flanges offer an effective means of diverting water that would otherwise drain through conventionally spaced deck boards. Dexedry flanges are designed to act as spacers between boards and also as mini-gutters to channel water toward deck edges. The rubber flanges are pounded into decking edges with a rubber mallet.
Dexedry flanges are available in a number of sizes that match the edge grooves found on major synthetic decking brands. The edges of wood decking boards can be milled with a dado blade, shaper or router to accommodate the flanges.
Whether you use gutters, flanges or an under-deck ceiling, creating additional patio or storage space under your deck can add to your enjoyment and to the functionality of your home.
Michael Chotiner is a home-improvement expert who has owned his own construction business and who is also a master carpenter. Michael writes on homeowner DIY projects and how-tos for Home Depot. For a selection of tools you might use if you are planning to install waterproofing on an outdoor structure, you can visit the Home Depot website.