Waterproofing Under a Screened in Porch

Measure the space between the joists under the porch and cut water diverter panels to that width using tin snips. The panels move water away from the porch and prevents water from pooling underneath the porch. Bend the water diverters slightly down so they form a slide for water to flow through. Nail it into the deck frame. Place a nail into the joist next to the diverter that the panel rests on to help support it and caulk around the side of the diverter. Continue this process between each joist under the porch. Apply silicone caulking to the top of the diverter's edge after it is installed to help seal it against water infiltration. If you already have flashing installed along the porch border, caulking the top edge shouldn't be necessary.

Measure each joist from the bottom to the top on the edge closest to the home and mark a chalk line at 1 5/8 inches up from the bottom. Go across the joist to the side farther away from the home and make a chalk line even with the one you've just measured. This helps you establish a proper pitch line for your waterproofing so water drains in the proper direction once the panels are all installed.

Attach a joist rail to your first joist 1 1/2 inches away from the ledger board, which is the edge of the porch against the house. Match the bottom of the joist rail with the chalk line you drew earlier. Hammer it into place with three nails. Attach another rail to the joist 1 1/2 inches away from the previous one. The rails should form a straight line if they were connected. The spaces in-between rails allow for drainage. Repeat this for all the main joists on both sides and on the inside of edge joists.

Run tar tape along the top edge of all the rails. Caulk the to edge of the tape to further seal off water.

Measure the total length of your main joists. Cut collector panels 1/8 inch shorter than that length with a utility knife to ensure they fit inside the joist space. Use a straight edge when cutting for best results. Use tin snips to cut a notch in each corner of the panels. Measure the space between joists as well from the center of one joist to the inside edge of the joist next to it. Mark that measurement on the collector panel, cutting it with the utility knife and straight edge so it matches. Take the measurement between the joists at three points along the joist because the distance might vary slightly. This gives your panels slightly wider than the space between joists, allowing it to flex inward. This helps hold the panel in place while also providing a pathway for water to travel.

Push the panels into place between the upper and lower edges of the joist rails. Ensure that the panels bend upward into the bottom of the porch. Trim a quarter-inch off the inside edge of the panel closest to the ledger board and then push the panel back to the ledger board. This allows for expansion between the panel and the end rim board.

Cut the joist gutters with a circular saw so they are a quarter-inch shorter than the length of the joist they attach to. Cut a notch on the end of the gutter in the flange with tin snips and attach a foam weatherstrip to the inside of the gutter housing on the side of the gutter facing the house. Use silicone caulking to seal around the strip.

Make four small cuts into the bottom edge of the joist gutter on the side facing away from the house to act as exit points for water. This helps prevent water from back-flowing.

Push each gutter up against its joist until it pops in over the joist rail and presses firmly against the connector panel. This completes a water circulation system that moves any water that comes into the screened-in porch away from the house and out from under the porch.