How to Waterproof Under-Deck Storage

A backyard wooden deck with an adirondack chair on top.
What You'll Need
Tape measure
Six-foot carpenter's level
Chalk line
Gloves and safety eyeglasses
Vinyl flashing and sheets
Silicone and acrylic caulking
Caulking gun
Power drill
1 1/2-inch galvanized steel screws, at least four for each rail
Joist rails
Ceiling panels
Joint gutters
Boundary gutters

It can be difficult to utilize the space under your deck for storage if you have to deal with water pooling below. However, if you waterproof the deck from above, you will lose the beauty of your wood deck planking. A better solution is to install a self-draining under-deck ceiling for the space. Follow these guidelines so you can make this a functional and beautiful space.

Step 1 - Measure the Space to Install the Waterproof Material

Measure the overall size of the area where you will install the under-deck ceiling. Count the support joists and measure the width between them to determine how much flashing you will need as well.

Step 2 - Purchase the Correct Amount of Materials

Companies that sell under-deck ceiling materials will have estimating tools on their websites to help you calculate the exact amount of what you need, based on various sizes of ceiling sheets and the area you are covering. Detailed installation guides are also available, so print them off before starting the installation.

Step 3 - Check Your Deck Is Level and Mark the Drainage Pitch

Use the six-foot carpenter's level to check your deck is level before marking the drainage pitch. This is the angle at which you install the under-deck ceiling so water drains away from your house. Mark it with a chalk line, allowing one inch of downward slope for every 10 feet of the deck from the house to the outside edge.

Step 4 - Install the Under-Deck Water Diverters (Flashing)

Attach the under-deck water diverter flashing at the angle you marked with the chalk line. Put a line of caulking where each piece meets the joist to which it is attached.

Step 5 - Install the Joist Rails

Fasten your joist rails to the joists at the angle marked with galvanized stainless steel screws, one every three feet. These will support the ceiling panels.

Step 6 - Attach the Ceiling Panels

Attach the ceiling panels between the joists by sliding them along the joist rails. Ensure they retain their central bend for proper drainage.

Step 7 - Install the Joist and Boundary Gutters

These gutters are meant to go along the outside edge of the deck to divert water away from the house. Install the joist gutters onto the end of the edge beam supporting the deck, and caulk them where they are attached to the outer edge with acrylic sealant. The boundary gutters will be placed at the two outermost edges of the deck. These will drain off the water accumulated from the joist gutters.

An under-deck ceiling will create a water-free space under your deck for storage or relaxing and enjoying the outdoors. You can even run a string of lights around the deck perimeter to provide light for evening activities and parties held underneath!