Building A Water Resistant Deck

Techniques to prevent water from undermining your next installation.

Water Resistant Deck
(All photos: Trex RainEscape)
By Dave Kile
From the June 2023 Issue

Top-of-class decks maintain their good looks for years—decades even—without needing much upkeep. Usually, they’re designed to shed water and built using waterproof flashing and/or an under-deck drainage system. Unfortunately, improper drainage is one of the biggest threats to a deck’s longevity. Here’s how to prevent water from undermining your next deck installation.

1. Survey the site.

Assess the slope/ grade of the surrounding landscape. Water must drain away from the building and deck structure. If the deck includes an underdeck drainage system, decide where to discharge the water. When necessary, add a French drain (engineered trench), reservoir, or other means to redirect the water or collect it for use in a garden or planting area.

2. Erect the structure.

The lumber used in deck beams, joists, and stair stringers is pressure-treated to be water-resistant, not waterproof, and only some pressure-treated lumber is suitable for ground contact. Check the label. Also, the biocide in the lumber doesn’t usually penetrate to the core, so when you cut it, seal the cut end by brushing on a preservative.

Set the deck framing at a pitch of 1% to 2% (1/8” to 1/4” per foot) to slope away from the building. Use only fasteners approved for pressure-treated lumber. To reduce the number of footings and support posts, install engineered beams with longer spans. These beams are designed much stronger and more stable than standard dimensional lumber beams. It is possible to achieve spans up to 20 feet. Glulam beams bond layers of wood laminations or “lams” together. Parallam strand lumber (PSL) beams glue together long strands of wood, bonding them using a microwave process. With fewer support posts, there’s less surface that needs waterproofing.

Water Resistant Deck
Flashing tape applied on joists, rim joists, beams, stair stringers, and blocking helps preserve the deck.

3. Apply impermeable flashing.

To protect the deck’s structural elements from water and rot, apply flashing tape over all the joists, rim joists, beams, stair stringers, and blocking. It’s easy to do and gives you an inexpensive way to preserve the structure’s integrity.

Flashing tape comes in asphalt, acrylic, or butyl formulations. Asphalt tapes are inexpensive, but they’re difficult to handle without making a mess, especially in hot weather. Acrylic tapes are more expensive and aren’t fully set for 24 to 72 hours. Butyl tapes strike a balance. They don’t ooze like asphalt, and they set faster than acrylic tapes. Whatever flashing tape you use, be sure it meets or exceeds the requirements of AAMA 711-13. (The requirements for deck flashing are outlined in section R703.4 of the International Residential Code).

Flashing tape is also an excellent alternative to traditional flashing used at the ledger board, such as aluminum, galvanized steel, stainless steel, copper, and vinyl, because it provides a waterproof seal that is fully compatible with pressure-treated lumber. For example, a butyl-based flashing tape for ledger boards by Trex includes an aluminum lining that gives the tape “memory” so it can be shaped to fit inside difficult corners.

Water Resistant Deck
Over-joist drainage systems make installing lights, fans, and other electric simple.

4. Install under-deck drainage.

Underdeck drainage systems steer the water away from the building and deck. They also keep the below-deck area dry, enabling you to use it as storage or living space. Drainage systems installed above the joists offer more protection than systems that use below-joist panels because they prevent the water from contacting the joists and fasteners.

In contrast, below-joist panels can trap moisture in the joist bays, accelerating deterioration. With the benefit of dry joists and joist bays dry, over-joist drainage systems make installing lights, fans, or other electrical accessories below simple. If you enclose the below-deck area, add an access panel to periodically inspect the drainage system and structural elements.

5. Use low-maintenance decking.

Wood decks can be beautiful. But any wood, whether pressure-treated lumber, cedar, or exotic hardwood, must be cleaned regularly and coated every few years to keep it looking good and prevent water damage. That’s why composite and PVC boards, which are practically impervious to water, continue gaining in popularity. They require little maintenance and last decades. Homeowners just need to clean the deck surface with a mild detergent twice a year and composite deck boards will maintain their appearance and waterproof performance for many years.

Design Build, Deck
Low maintenance decking, such as composite and PVC boards, continues to gain in popularity with homeowners.


6. Spot other threats to deck longevity.

It’s not just rain and snow that can threaten decks. Water and other hazards can come from elsewhere, too.

  • Roofs. If an eave overhangs the deck, redirect the runoff with a gutter. Otherwise, over time, the dripline can leave a stain on the deck boards.
  • Sprinklers. It may seem obvious, but water the plants and lawn, not the deck.
  • Pressure washers. Most manufacturers recommend against using pressure washers on their composite deck boards. If you use one, fit it with a 25˚ nozzle and operate it at the lowest effective setting. When washing wood, resist the urge to go full tilt on old, deeply creviced boards. If you do, the surface could become fibrous and pulpy, making the boards more difficult to refinish.
  • Plants. Keep trees and shrubs from contacting or crowding the deck to ensure good airflow in and around the deck and structure.
  • Tree litter. Trim or remove large limbs or branches that extend over the deck. This will reduce the amount of tree litter that accumulates on the deck. With less debris, there will be less trapped moisture. Removal also means no large branches that can break off in a storm and damage the deck.

Selling Customers On A Waterproof Deck

Most customers are happy to pay a little extra for a waterproof deck. If not, point out the long-term cost savings and peace of mind that come with it. Show them photos of decks that have suffered from water damage and explain how waterproofing techniques can prevent these issues.

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Offer customers a warranty and/or a package deal that includes deck cleaning and maintenance services. The added value of the services will help customers justify the marginally higher cost of waterproofing.

Learn how customers plan to use their decks. Showing them photos of different decks will help spark a conversation. Also, be ready to incorporate personal touches, such as hand-painted tiles, flea market finds, and bright colors. Many decks integrate into game or play areas, edible gardens, water features, and outdoor kitchens. In recent years, more decks have become open-air offices. Other trends include small intimate decks, hidden decks, and rooftop decks.

Dave Kile, CEO, IBP, LLCKile is CEO of IBP, LLC, which owns and licenses Trex RainEscape and Trex Protect. For more information visit and

To read about more design-build projects, see:

Estimating Landscape Projects For Profit
Get Equipped: Design-Build Equipment
Fireplaces: The “Hot” Outdoor Trend

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