How to Waterproof Under-Deck Storage

Improving the space under your deck for under deck storage can be done easily and effectively. 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 under deck storage. Follow these guidelines to waterproof the area under your deck and make it a functional and beautiful space. Use a deck storage bin under the deck only to protect items from windblown rain and dew.

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.

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 materials 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 6-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 1 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 putty where each flashing meets the joist to which it is attached.

Step 5: Install the Joist Rails

These will support the ceiling panels. Fasten them to the joists at the angle marked with galvanized stainless steel screws, one every 3 feet.

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 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. Caulk them where they are attached to the outer edge with acrylic caulk. Attach the boundary gutters 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 relaxing, reading, and enjoying the outdoors. Run a string of lights around its perimeter to provide light for evening activities and parties.