How to Waterproof the Underside of a Wood Deck

Many people have an upper level deck with a ground level space underneath they would like to transform into a usable space, but the problem being that water drips through from the deck above.  So what is the simplest way to make the underside of the deck waterproof ?

There are of course many under deck ceiling systems which generally work by installing some type of panelling or channelling system underneath the deck , which then drains all water away to one of the sides. Generally these systems are installed by licensed installers so are not normally a DIY option, and the cost can be relatively high.  On the other hand one of the benefits of many of these systems is that they can create an attractive lining to the underside of your deck.

But if you are looking for a do-it-yourself option to waterproof a deck, that is both quick to install and relatively economical , then possibly the best alternative is to install interlocking deck tiles on your deck in conjunction with a membrane under the tiles . The way this is achieved is as follows.

You purchase a continuous sheet of EPDM which is typically used as a liner for outdoor ponds or as a waterproof membrane on rooftops and will provide a durable , long-lasting waterproof membrane. EPDM is available in widths up to about 50′ so you can lay down a continuous sheet without any seams over most decks. At the wall side , you need to ensure that water cannot seep down the side of the wall into the area below. The best way of achieving this is to run the EPDM sheet a short distance up the side of the wall as flashing and to use an adhesive specially designed for use with EPDM to provide a good seal between the wall and the EPDM sheet.

After you have laid the sheet over the existing deck , then you simply lay down modular interlocking deck tiles over the entire area . These tiles can be purchased in a number of different materials including not only wood but also composite wood, slate, granite, sandstone and ceramic . So if for example you decide you don’t want the regular maintenance issue associated with a traditional wood deck, then you could use the stone or composite wood tiles instead . And with products from some suppliers you can mix and match the tiles as well .  So for example you could have a composite wood deck with an infill panel of slate.

And perhaps the main benefit of interlocking deck pavers is that they simply snap together, automatically ensuring accurate spacing , with constant gaps between the tiles. So even if you don’t have any particular DIY skills, laying these deck tiles is essentially foolproof.

For the perimeter of your deck you can normally purchase edging pieces or transition strips to create a sloping transition to the deck level and ensure a neat professional finish.

A couple of points you need to take note of if you are planning to waterproof the underside of your deck with this particular method.  Firstly, since the deck tiles will raise the level of the existing deck about1 1/2″, you need to ensure this will not interfere Edit this text with any outward opening doors.  The second consideration is that your deck must have a slight slope away from the wall, because any water must drain away from your walls.

But apart from these two points , you’ll probably find that using interlocking deck tiles installed over an EPDM membrane is one of the quickest, easiest and most economical methods of waterproofing an upper level deck.