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How do I build a waterproof ceiling under a deck?

Updated July 20, 2017

A deck is a great addition to any home; it adds usable square footage and provides an outdoor entertaining area. Many homeowners who have a second-story deck increase the versatility of their decks by making use of the area underneath it. Lower-level patios and sitting areas are popular and keeping rain from coming through the deck above is important. A drainage and gutter system can be installed to keep your under-deck space clean and dry. Many systems are available.

Install the rain diverters between joists. Tuck the diverters underneath the existing ledger board flashing and nail it to the ledger board. Caulk the seams with silicone caulk.

Measure a starting point according to the manufacturer's instructions and snap a chalk line down the side of each joist. The line should have a downward slope with the high side closest to the house.

Install the joist rails to the sides of the joists following the chalk line you marked in Step 2. Mount the joist rails with galvanised nails, then caulk along the top seams. The manufacturer's instructions specify how to cut the joist rails to allow for expansion. Apply tar tape to seal along the top surface of the joist rails to seal them.

Install the joist collector panels. The directions on how to cut and install them are outlined in your installation manual. Seal all seams of the collector panels with silicone caulk.

Install the joist gutters lengthwise along the bottom of each joist.

Install the rim joist gutter. This gutter mounts perpendicular to the joist gutters and collects water from them. The gutter should have a slight pitch to properly channel water away.

Install the downspout to the low side of the rim joist gutter. Caulk all the seams for a water tight seal. Install a downspout kick-out elbow if necessary to channel water away from the house.

Tip

Ensure your deck is level to begin with and double check the slope of all gutters with a level. Do not rely on measurements alone.

Warning

Always use safety gloves with vinyl or aluminium gutters and flashing. The cut edges are razor sharp.

Things You'll Need

  • Under-deck drainage system/kit
  • Drill
  • Screwgun
  • Tin snips
  • Ladder
  • Tape measure
  • Chalk line
  • Galvanised roofing nails
  • Flashing
  • Tar tape
  • Silicone caulk
  • Caulking gun
  • Hammer
  • Pencil or marking pen
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About the Author

Emrah Oruc is a general contractor, freelance writer and former race-car mechanic who has written professionally since 2000. He has been published in "The Family Handyman" magazine and has experience as a consultant developing and delivering end-user training. Oruc holds a Bachelor of Arts in political science and a minor in economics from the University of Delaware.