How To Install A Drainage System Under Your Deck

Walkout deck with attached pergola

Have you ever been on your deck and noticed rainwater dripping down underneath it? Are you ready to take care of this issue with a drainage system under your deck? Along with a professional irrigation installation, a drainage system under your deck is an easy way to ensure that water doesn’t collect and cause damage. In this blog post, we will show you how to install a drainage system under your deck.

We will also go over the different types of deck drainage systems available, as well as the benefits of installing one. By the end of this post, you will know all the different options for drainage systems under your deck so you can make the best choice for your situation.

Time: Several days

Complexity: Intermediate

Cost: Few hundred dollars (depending on the size of the deck)


  • Measuring tape
  • Power drill
  • Drill bit set
  • Circular saw
  • Safety glasses
  • Work gloves


  • Corrugated fiberglass or plastic roofing panels
  • Gutter and downspout (optional)
  • Deck screws
  • Roofing screws
  • Pressure-treated 2×4 wooden boards

Before we get into how to install a deck drainage system, let’s quickly go over what a drainage system under your deck is. 

What Is A Drainage System Under Your Deck?

A drainage system under your deck is simply a drainage system that prevents water from flowing through the deck and pooling below. Instead, the water is redirected with a deck drainage system that keeps the area beneath the deck dry and useable.

How To Install A Drainage System Under Your Deck

If you’re looking for an easy, cost-effective way to add more usable space to your outdoor living area, you should consider installing a deck drainage system. Not only will this create a dry and comfortable place to relax, but it can also help keep the area free from water damage. 

Here’s a step-by-step guide to building and installing a drainage system under your deck.

Step 1: Measure Your Deck And Cut The Boards

Person cutting a board with a Circular Saw

The first step is to measure the underside of your deck, so you know what length to cut your pressure-treated boards. Cut the boards the same length as the deck boards on the floor of your deck.

The boards should be oriented perpendicular to the direction you want the water captured by your drainage system to flow. For most homeowners who have a second-story deck on the back of their home, this means that your boards will be installed and oriented parallel to the back side of your house.

The pressure-treated boards will be what you attach the roofing panels to.

Be sure to save any pieces that you trimmed off your boards. You will use those pieces to create spacers (more on that in a little bit).

Step 2: Measure And Cut Spacers

You want the roofing panels under your deck to direct water away from your home’s foundation. So you will need to ensure that the panels are sloping away from your house.

We recommend a slope of approximately 1/4 inch per foot. This means that if your deck extends 10 feet out from your house, the roofing panel furthest from your house should be about 2 1/2 inches lower than the panel closest to your house (10 feet x 1/4 inch= 2 1/2 inches).

Measure and cut the spacers so they will create an appropriate slope for your deck size. Remember to cut enough spacers for all the joists under your deck. Attach the spacers to the bottom of your deck joists with the deck screws, making sure to attach the shortest spacers closest to your home and the biggest spacers furthest from your home.

Step 3: Attach The 2×4 Boards To The Spacers

Now use deck screws to attach the 2×4 boards to the spacers. If done correctly, the boards should slope approximately 1/4 inch per foot away from your home.

Step 4: Measure And Cut The Roofing Panels

Plastic piece of corrugated roofing panel

Unless the size of your deck matches the standard size of a roofing panel, you will need to cut your panels to the correct size. Measure the roofing panels to ensure they match the length of your 2×4 boards. Use the circular saw to trim any excess length off.

Step 5: Attach The Roofing Panels

Use the roofing screws to attach the roofing panels to the 2×4 boards under the deck. It’s crucial to use roofing screws (not deck screws) to attach the roofing panels because roofing screws have rubber washers on them that will prevent water from leaking through the holes created by the screws. Tighten the screws until they are snug but not so tight that they compromise the integrity of the rubber washers.

Remember to orient the corrugated roofing panels, so the channels direct the water toward the outer edge of your deck (away from your home). Overlap one or two channels of each piece of the corrugated roofing panel.

Pro tip: In order to avoid having to trim any excess roofing panel off the side of your deck, take a moment to plan out the minimum number of roof panels you need. Then determine how much you need to overlap each piece of the corrugated roof in order to fit perfectly under your deck.

If you prefer that the water that is captured by your under-deck drainage system be directed into a gutter system rather than falling on the ground, you should proceed to Step 6 of this guide. Otherwise–Congratulations! You have successfully installed a drainage system under your deck. 

Step 6: Attach A Gutter System And Downspout Under The Outer Edge Of Your Roofing Panels (Optional)

While not absolutely necessary, you can also attach a gutter system under your deck to minimize any splashing caused by water draining off the end of the corrugated roofing panels.

  • Cut a gutter to the correct size that matches the size of your drainage system under your deck.
  • Attach the gutter hangers to the underside of your deck, so it captures any water that runs off the roof panels. The method for attaching a gutter system under your deck will vary depending on how your deck was constructed. You may need to attach additional boards to the edge of your deck so you have a continuous level surface to attach the gutter hangers. The most essential thing to remember is that you have the gutter sloped correctly, so the water flows toward the downspout.
  • Attach an endcap and a downspout on each end of the gutter.
  • Direct the downspout outflow to an appropriate area of your yard, or attach it to an in-ground drainage system.

What Are The Types Of Drainage Systems For Under Your Deck?

Above-Joists Deck Drainage Systems

Above-joist deck drainage systems are installed between the top of the deck joists and the floor of the deck. These types of systems typically use a waterproof membrane that covers the joists and prevents water from pooling on top of them. The membrane drapes over the joists, creating channels that direct water away from home and keep the area under the deck dry.

These systems are best installed before the floor of the deck is nailed in place since you need to be able to completely cover the joists. The Trex RainEscape and the DEK Drain system are both good examples of this type of drainage system for under your deck. The main advantage of this type of system is that it prevents your deck joists from getting wet. This can extend the life of your deck since the joists are a major component of your deck that is prone to moisture damage.

Under-Joists Deck Drainage Systems

An under-joist deck drainage system is any drainage system for your deck that is installed under the joists. This type of system includes the roofing panel system that we outlined above. The main advantage of this type of system is that it can easily be installed on an existing deck that is already completely built.

Besides the DIY system outlined above, there are many manufacturers who make readymade under-joist deck drainage kits. Some examples of this type of system include Zip-UP UnderDeck and Under Deck Oasis. While the readymade under-joist systems are typically more expensive than the DIY versions, they do have a sleeker, more professional appearance because they include an under-deck ceiling.

What Are The Benefits Of Installing A Drainage System Under Your Deck?

Young man sitting on swing under wooden deck

There are many benefits to installing a drainage system under your deck. Here are some of the biggest ones.

Reduces erosion and rot: A drainage system under your deck ensures that water does not collect and pool under your deck, resulting in erosion of soil and wood rot.

Aesthetics: A drainage system under your deck can add to its aesthetic appeal, as drainage systems are available in various styles to complement the appearance of most decks.

Creates additional outdoor living space: Installing a drainage system under your deck increases the usable area of your outdoor living space. With a drainage system under your deck, you can set up seating furniture on the lower level of your deck without concerns about getting wet. 

In short, when it comes to preserving the longevity of your deck and enjoying long-term value out of one of your home’s central gathering spaces, installing a drainage system under your deck is a smart decision!

Simmons Landscape & Irrigation Are Your Drainage Experts

Do you have proper drainage set up around your home? If you’re not sure or would like help deciding the best type of drainage system for your needs, fill out our contact form today. One of our expert representatives will be in touch to discuss your options and help you find a solution that works best for you. Don’t wait until it’s too late and water damage has already been done–act now and protect your property with a reliable drainage system.