How To Install An Under-Deck Drainage System?

Having an elevated deck or second-story terrace allows you to enjoy the pleasures of outdoor living. However, you may encounter the frustration of water seeping through the deck and pooling beneath it. This situation renders the area under the deck damp and unusable.

More seriously, persistent moisture can erode the deck’s overall structure. Even composite deck tiles with superior moisture resistance can be affected. It may cause joists and fasteners to rot, and metal components to rust. Ultimately, this compromises the safety and lifespan of the entire deck. The solution lies in installing an under-deck drainage system.

This article will guide you through planning and installing an under-deck drainage system. It can transform the space beneath your deck into a dry, comfortable living area.

What is an Under-Deck Drainage System?

An under-deck drainage system is a design that removes standing water from beneath your deck, protecting the deck structure. It effectively prevents rot, mold, and other issues caused by water accumulation. The under-deck drainage system is primarily divided into two categories. One type is installed beneath the balcony joists. The other type is installed above the balcony joists. These two systems operate differently and yield significantly varying results.

The optimal time to install an under-deck drainage system is during home construction. However, existing decks can also be retrofitted with this system. Understanding the different types of drainage systems is crucial before beginning any work.

Above-Joist Drainage System

The above-joist drainage system is typically installed before laying the deck boards. It is mounted directly on top of the joists. The system consists of a series of gutter boards and downspouts. Together, they form a waterproof layer that covers the entire deck frame like a “roof.” This design directs water to collect and drain quickly. It also reduces the likelihood of clogging from leaves and debris.

The above-joist drainage system prevents water from contacting the joists and fasteners below. This eliminates the root causes of rot and corrosion. Consequently, this drainage system significantly extends the structural lifespan of the deck. Additionally, with the substructure completely dry, you can confidently remodel and utilize the space beneath the deck. Dry joists also facilitate the installation of features like recessed lighting and ceiling fans.

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Below-Joist Drainage System

The under-joist drainage system is a relatively simple retrofit solution. This system is installed beneath the joists of existing decks. It catches water dripping from the deck boards via sloped panels, then channels it to the edge for drainage.

A drawback of this system is its inability to protect the deck structure fully. Water flowing down can still wet the joists and metal connectors. Improper design may even trap moisture within the joist cavities, accelerating wood decay. Additionally, decaying wood debris may fall onto the panels. This can further obstruct drainage, creating a vicious cycle.

In summary, for new decks or comprehensive renovations, installing a deck-mounted drainage system is the optimal choice. It provides a permanent solution.

Steps for Installing an Under-Deck Drainage System

Installation of the drainage system beneath the terrace must be carried out according to the type and structure of the deck. The following are reference steps:

Installing Downspouts

Funnel-shaped downspouts are typically installed on joists spaced 16 inches apart. If joists are spaced 12 inches apart, you can rotate the downspout 90 degrees for installation. For 2×10-inch joists, the downspout height is ideal. For smaller joists, you may need to trim the funnel outlet. For larger joists, attach a section of PVC downspout to the funnel outlet with screws.

Place one downspout in each joist slot along the deck edge. After placement, temporarily secure with staples.

Laying and Securing Drip Edge

Next, install the drip edge. Before proceeding, ensure the wall is properly sealed to prevent water from flowing back into the wall. Follow these steps for drip edge installation:

• Take one gutter board, unfold it, and let it rest for a moment. This allows it to adjust to the ambient temperature.
• Fold it lengthwise to create a slight crease. This helps form the drainage groove later.
• Place the gutter board with the smooth side facing up. Tuck one end as far as possible under the wall waterproofing board. Secure it to the top of the support beam with staples.
• Pull the gutter board taut. Then secure the other end to the edge of the support beam opposite the wall. Keep the sheet flat and taut.
• When reaching the downspout location, fold the sheet sharply backward to eliminate any curling. Trim it to align with the center of the downspout opening.

Forming the Slope and Checking Drainage

Following the guidelines on the sheet, staple both sides of the sheet to the joists one after another. This process automatically creates a slight V-shaped slope directing water toward the downspout. Repeat until all support beam channels are fitted with gutter panels.

Finally, pour one cup of water into each gutter channel. Observe whether water flows smoothly and rapidly into the downspout. Ensure no standing water remains. Adjust the slope promptly if issues arise.

Handling Special Areas

Certain areas may not accommodate downspouts, such as angled sections or cantilevered sections. For these areas, create short gutter segments and slope them backward. This allows water to flow into adjacent main gutters and ultimately drain into the downspout.

Seal All Seams

After installing all gutter panels, trim any excess overlapping material with a utility knife. Ensure no wood remains exposed. Then, proceed with sealing as follows:

• Beginning at one end of the house, unroll butyl tape toward the downspout. Peel off the backing paper and adhere the tape tightly over all gutter panel seams and overlaps. Also, seal the junction where the gutter panels meet the wall waterproofing panels.
• Press firmly to ensure the tape adheres completely to the surface without wrinkles or air bubbles.
• Similarly, carefully seal all seams around downspout edges with butyl tape.
• For complex or uneven areas where tape may not achieve a complete seal, use butyl caulk to fill gaps. Ensure all potential leak points are thoroughly sealed.

Installing Fixtures

Your deck now has a dry space underneath. Easily install recessed lighting, ceiling fans, or other electrical fixtures onto the joists following the manufacturer’s instructions. These additions can transform the space beneath your deck into a comfortable outdoor room.

Installing Gutters

Install PVC, aluminum, or steel gutters beneath the downspout outlets. Secure them firmly using gutter hangers. Ensure each downspout outlet extends approximately 1 inch into the gutter. Additionally, the gutter must be properly sloped. Verify water flows toward the downspout and drains away from the house foundation.

Installing Deck Boards

Once the drainage system is complete, deck boards can be installed. Avoid using nails during installation, as they can compromise seals and loosen over time, leading to leaks. Opt for screws or hidden fastening systems instead, which provide superior waterproof sealing.

Conclusion

Installing an under-deck drainage system is a forward-thinking home investment. Efficient drainage creates a completely dry, protected space. Your deck structure will become more durable and safer. At the same time, you’ll gain a functional under-deck area.

With careful planning and installation, you can complete this project yourself. Alternatively, you can hire an experienced contractor. Start planning now to transform your damp under-deck space. For more deck-related knowledge, feel free to contact us. COOWIN provides a premium composite lumber deck for you.

FAQ

Can an Under-deck Drainage System be Installed on an Existing Deck?

Yes. However, installing a drainage system above the support beams may prove challenging. This is because it requires first removing all deck boards, railings, and other accessories. For existing decks, most people consider the simpler under-joist drainage system. Of course, this approach has some drawbacks. Therefore, if your old deck is nearing the end of its lifespan, a complete renovation might be preferable.

Which Deck Material Offers The Best Waterproofing?

Composite decking materials inherently possess strong moisture-resistant and rot-resistant properties. They also require virtually no maintenance. You can pair composite decking materials with an under-deck drainage system. Together, they provide lifelong waterproofing performance. In contrast, wooden decking requires regular painting and sealing to achieve adequate waterproofing.

Can Drainage Systems Get Clogged with Leaves and Debris?

Over-beam drainage systems are designed to minimize clogging risks. Their smooth surfaces and effective slopes make it difficult for water and debris to accumulate. Most leaves or debris will be washed away by rain. Cleaning is also straightforward.

You can rinse the gaps between deck boards with a garden hose. The water flow will naturally carry away any loose debris in the channels. For stubborn debris, use a soft-bristle brush or a leaf blower to clear it from the board gaps. Additionally, install a filter screen at the end of the drain channel. This prevents clogging of the downspout.

Is a Deck Drainage System Really Worth The Investment?

Absolutely. The benefits of a deck drainage system are multifaceted. It protects the wooden and metal structures of the terrace from water damage. This significantly extends the overall lifespan of the terrace. Simultaneously, a well-designed drainage system enhances your home’s functionality and market value. It increases usable outdoor living or storage space. A dry, comfortable under-deck area greatly improves your living experience.