Under-Joist or Over-Joist Deck Drainage System? Make the Right Choice!

You’ve made the smart decision to install a drainage system under your new two-story deck, so you’ll have dry space beneath for entertaining, storage or relaxation.

Next, you have an important choice to make. Should you install an over-the-joist or under-the-joist deck drainage system? An over-the-joist system is just that: the drainage system is placed over the joists so water never comes in contact with the joists and beams. An under-the-joist deck drainage system allows water to trickle through the slats in your decking, run over the joists and collect in the drainage system beneath. When the joists repeatedly get wet, they will eventually rot.

Choosing the right deck drainage system will make a drastic difference in the life of your deck.

The Problem with a Below-the-Joist Deck Drainage System

A below-the-joist deck drainage system causes damage and wood rot in three ways:

• First, it allows the wood to get wet over and over as rain drips through the deck boards, over the joists and into the drainage system below. The water hits the pan and runs out, but not before drenching the joists.
• Second, this type of system can trap in moisture. If a ceiling is beneath the deck, there’s no air coming in to dry the wood. The only ventilation is through the quarter-inch or less space between the top deck boards.
• Third, in the hot months of summer, the temperature further cooks this trapped, wet wood and accelerates the rotting. Over time, this is the perfect formula for wood rot: moisture + heat – ventilation = destruction.

Watch this video to see what happens to the wood with an under-the-joist deck drainage system.

Choose an Over-the-Joist Deck Drainage System

The best deck drainage system is one that is installed above the joists. This will provide 100% protection of all the joists and beams from moisture penetration. An above-the-joist system also helps to hold the screws in place longer and acts as a shield between galvanized metal and wood.

If you’re building the deck yourself, choose an over-the-joist system that’s easy to install. Use this handy chart for comparison. As you’ll see, Trex RainEscape offers a high quality, easy to install a system that is affordably priced.

The bottom line: Install an above-the-joist deck drainage system and your deck substructure will last as long as the decking it supports.