Deck Waterproofing: The Only Guide You’ll Ever Need

Protecting a deck from moisture damage has undergone numerous improvements over the past decade or two. It’s not just a matter of using pressure treated, plastic, or composite decking, it’s about protecting the whole structure. If you’re interested in deck waterproofing, we’re here to help!

Waterproofing a deck begins by protecting the posts, beams, ledger, joists, and blocking from moisture damage and rot. Stringers, steps, and risers, along with railings also need to be waterproofed. Installing an above or below joist drainage system can offer protection from moisture damage to many components of the deck support system too.

In this guide, we’ll discuss different materials and methods to protect various deck components. We’ll explain how to waterproof your whole deck, prevent water from pooling, maintenance, and the overall cost of waterproofing a deck. Our aim is to provide you with a definitive guide to protecting your deck and those who use it.

Deck Waterproofing

Deck Waterproofing Materials and Products

Decks are a great way to take advantage of the outdoors. Unfortunately, most deck support structures and surfaces are made of materials that can be damaged by precipitation. Rain and meltwater typically percolate through the spaces in the deck boards and become trapped between decking, joists, and beams.

This trapped moisture can quickly lead to mold, fungal growth, and rot, causing maintenance issues and destroying your outdoor enjoyment. There are different ways to protect the deck structure from moisture damage which can add years to its lifespan.

Some options need to be considered during the planning or prebuild stages, while others can be implemented in an existing deck. Here are some options that will improve deck waterproofing.


Deck Waterproofing Membranes

There are two styles of membrane that can be used to protect a deck. One is a waterproof, slip-resistant vinyl material applied and glued to wood deck surfaces. Available in rolls, it’s usually laid parallel to the house from the outer deck edge to the house wall, with seams overlapping and taped to prevent seepage. The built-in slope of the deck helps shed water to the outer edge and off the deck.

The other method is a large rubber or vinyl membrane or sheet, such as a pond liner, that is draped over the joists prior to the deck boards being laid. The membrane is allowed to sag between joists to create troughs that slope to the outer edge of the deck or to the house.

Specially formed hard plastic downspouts are attached at the down-slope end to the inner face of the rim board, beam, or ledger board and redirect runoff to a gutter that diverts and carries it away.



Staining a deck

Coatings are thick, waterproof liquid materials designed to protect wooden decking by preventing water or moisture intrusion. Commonly made of a resin or rubberized base, they form a durable, slip-resistant waterproof coating. They can be applied by brush, roller, or sprayer to plywood or other wood decking to reduce maintenance issues. Depending on product, exposure, and climate, reapplication is between 3 and 7 years.

Another type of coating for solid deck surfaces is fiberglass. Rolls of fiberglass and liquid resin can be used to form a low-maintenance, long-lasting, waterproof coating in a wide range of color choices.

It is ideal for high-precipitation regions and will outlast many other options. Unfortunately, while the coatings seal cracks and protect the wood, they also fill and hide screw or nail heads, making repairs more difficult.


Grace Vycor Deck Protector Self Adhered Flashing - 4' x 75' RollNumerous brands of self-adhering, slip-resistant joist tape or joist-flashing are available in different widths and lengths. Butyl-based tapes won’t ‘bleed’ or streak like asphalt-based tapes, nor will they curl or peel at the edges as they age.

The tape is applied to the top face or edge of joists, ledgers, rim boards, blocking, and band boards, and even to the top of beams. The tape seals the surface and upper edges of the structural members improving protection from moisture damage.

Many installers apply the tape wherever wood members connect with or to each other and where metal fasteners attach.

Deck Flashing

Deck Flashing

Deck flashing commonly refers to a non-corrosive strip used to prevent moisture from entering the wall where the ledger board attaches. It may be aluminum, galvanized, stainless steel, copper, plastic, or vinyl, and may come in a roll or formed L or Z-shaped strip.

The flashing tucks under the existing exterior wall siding and over the ledger board and caulking is used to fill any seams, gaps, or fastener holes. It helps to direct water away from the house and ledger board, helping to prevent moisture issues.

It is becoming increasingly common to also install eave or rake flashing around the perimeter rim and band boards of the deck to move moisture away from the deck edge. It is so common that some manufacturers are identifying it as ‘deck flashing’. The flashing tucks between the decking and framework, and sheds water away from the edges of wooden framework members.

It’s important to remember that galvanized steel and aluminum deteriorate in contact with ACQ pressure treated lumber, so take precautions. Also, if using copper, only use copper or approved fasteners. Additionally, vinyl or plastic don’t handle extreme temperature fluctuations well, so while less expensive, they are limited in use.


Ready Seal 512 5-Gallon Pail Natural Cedar Exterior Stain and Sealer for WoodDeck sealers are oil or water-based liquids that are applied to dry deck boards and penetrate into the wood’s pores. They protect the lumber from moisture, insects, mold, mildew, and UV damage. They also help to prevent fading, graying, splinters, checking, cracking, and rot. Available in clear to bring out the natural color and grain pattern, they also come in semi-transparent or tinted to help even out or brighten the natural color.

Sealers can be applied by brush, roller, or sprayer and often require two applications, however, it’s best to read the manufacturer’s specs for proper use. Most sealers last 2 to 3 years before requiring reapplication, but much depends on exposure to the elements and climate.

Reapplication typically requires the deck to be cleaned, washed, and allowed to dry before reapplying a sealer of the same base type. If you want to switch base types, it can be done but usually involves more work.

Waterproofing Paints

How to Repaint Deck

Waterproofing deck paints come in oil and latex-based liquids that can be applied with a roller, brush, or sprayer. Oil-based paints tend to be more weather-resistant while latex withstands heat and sun exposure better.

Both are available in a wide range of colors and help protect decking from the elements and foot traffic. Paint is often used for older decks that have grayed or become worn, and when sealers or stains won’t improve their appearance.

The solid opaque nature of paint hides imperfections and provides an even color, and can complement or accent any home’s color palette. Deck paints usually require two applications and must be reapplied every 3 to 10 years depending on climate, exposure, and paint quality. In high use or harsh climates, touch-ups or a single recoat may be required every 18 to 24 months to freshen the look.

Interlocking Decking

Interlocking decking comes in several formats – plank and tile. Interlocking deck planks are manufactured from composite wood, plastic, fiberglass, and even metal. They usually have a formed U-channel along one edge and an upside-down U on the other edge, allowing the planks to hook, lap, or interlock together. The overlap creates a channel to divert moisture to the outer edge of the deck. This helps to protect the substructure from moisture damage and rot.

Interlocking deck tiles are square or rectangular tiles with a rubber or plastic web backing. The exposed part is short lengths of wood, composite, or plastic planks evenly spaced and attached to the backing.

The tiles can be applied in a checkerboard or parallel pattern over a joist framework or over an existing wood or concrete surface. The web provides airspace between the boards and support structure, allowing the wood to dry and reducing the possibility of rot.

How to Waterproof Deck Frame

Protecting the deck framework from moisture damage helps protect the whole deck and extends its lifespan. Moisture trapped between decking and joists, beams, or the ledger can accelerate rot and fungal growth, and corrode fasteners and connectors.

Decks should have a built-in 1/8” to 1/4″ per foot slope away from the house to move moisture away from the house. Below we identify different ways to protect your deck frame from water damage.

Ledger Board Flashing

Ledger cap flashing

Moving water away from the ledger board is important as the ledger stabilizes and usually carries half of the deck. Plus, it’s attached to the house you don’t want to damage it.

Once the siding is removed from the ledger location, install flashing that goes up under the siding and covers the exposed wall where the ledger will attach – ensure you transfer marks where bolt holes go first. If there is siding below the deck, the flashing needs a formed drip cap to direct moisture outward.

When attaching the ledger to the wall, add two washers to the through bolts or lag screws to create a gap between the wall and the back of the ledger. The gap will allow any moisture to escape and help keep the wood dry.

With the ledger attached, apply ledger tape – a self-adhering waterproof butyl membrane – to the wall above the ledger and wrap the top and exposed face of the ledger. The tape will create a waterproof seal.

Slide L or Z flashing under the siding and over the top of the ledger, and fasten it in place. The ledger tape also protects the ledger from moisture between the joists’ ends and hangers.

Deck Posts

Waterproofing Deck Posts

Deck posts are often pressure treated timber or rot-resistant wood like cedar, but all cuts, through holes, and contact points should be protected from moisture. Posts that rest in post brackets set in concrete piles are usually 4” to 6” above grade which helps keep the base dry. Posts set directly into the ground should be coated with a liquid rubber foundation sealer or wrapped in a waterproof post membrane.

To protect the posts above ground, clear or tinted waterproof sealers are commonly used to seal cuts and fastener holes. Many pros are also using joist or ledger tape to wrap post ends and where connectors fasten or beams rest and moisture could cause problems. Remember to use appropriate fasteners and connectors when using pressure treated lumber as the chemicals can corrode the metal, causing it to fail.

Deck Joists

Deck joist protector

Deck boards fastened to joists can trap moisture and cause the joists and fasteners to rot more quickly. There are different methods to waterproof and protect deck joists and extend their lifespan, and thus that of the whole deck.

Cutting tarpaper into 2” or 3” strips and stapling it to the top surface of all joists was and still is a common practice. It will shed moisture and decrease rot and is very inexpensive.

Painting the upper edge of joists with used engine oil was common before pressure treatments came about. The drawback, though, is that the wood must be dry, which can take planning and time.

Self-adhering butyl joist tape in single or double widths is an improvement on tar paper and has become very popular. Just peel and stick it to the top and upper sides of the joists and it shields the wood from moisture. If you’re wondering what’s the best joist tape, check out this article on the top 7 products.

An alternative is to apply water-resistant sealers or liquid foundation sealants to the top and upper sides of the joists. Using metal coil stock or rolled flashing works similar to tarpaper strips, or purchase more expensive pre-bent metal joist caps in single or double widths.

Under Deck Waterproofing

DEK Drain

Under-deck waterproofing protects the framework that supports the deck boards, as well as the open space or roof under the deck. Below are several manufactured systems and some other methods to waterproof the area under the deck surface.

Under-Deck Drainage System

Under-deck drainage systems protect the joists, beams, and posts from moisture exposure and damage. Plus, depending on the deck’s elevation, they provide a relatively dry storage area under the deck. The systems catch rain and run-off that makes it between the deck boards and into the space below them.

The drainage systems usually include pre-formed components of vinyl, plastic, rubber, metal, or fiberglass panels that fasten to the top or bottom of the joists. They create a sloped trough between the joists to collect and divert moisture to a gutter, where it is carried away to prevent erosion, damage, or rot.

Those that cover the top of the joists protect the joists as well as everything underneath, while those mounted under the joists offer significantly less protection. For information on 12 popular options, check out this article.

Under-Deck Ceiling Systems

Under-deck ceiling systems help turn usable space under a deck into additional outdoor living space. There are different systems available made of metal, vinyl, plastic, or rubber, as well as DIY alternatives. The key is to ensure the joists are fully protected from moisture and that the system can be cleared of debris as needed.

They create a finished look, help keep bugs and precipitation out, and can often be enhanced with lighting. Those that mount to the bottom of the joists usually channel moisture to one end where a gutter system carries it away. However, if the joists are still exposed to moisture, they may rot unless protected. Some systems combine a drainage system that protects the joists and a finished ceiling panel system.

There are other rigid systems that mount to the joist or beams and create drainage channels with the deck boards fastening directly to built-in flanges or supports. Thus, creating a ceiling and drainage system all in one. One DIY method is a sloped membrane between the joists coupled with soffits or siding attached to the underside of the joists to form the ceiling.

Sloped Membranes

Decks typically have a 1/4” per foot slope away from the building so moisture is moved away from the structure. It is installed prior to the deck boards and protects the whole deck frame from water damage.

Draping a manufactured deck membrane of rubber, plastic, or vinyl, or a pond liner or other DIY membrane over the joists and space between them will prevent moisture damage and rot. Depending on joist orientation, the membranes can even slope from end to middle to reduce over costs.

Use a long board to create a valley with the desired slope in the loose membrane between the joists and fasten the membrane to the joist to maintain that slope. The board helps form a sloped membrane that will transport moisture in the desired direction.

Move the board to create each valley and tape all seams. Where the membrane encounters the rim board or beam, a molded downspout-like diverter redirects the flow to a trough that takes it away from the deck area. Membranes also prevent joist rot where the deck boards fasten directly to the joists.

Decking Flanges

Rain and runoff tend to drip or seep through the gap between deck boards and collect on the joists, blocking, beams, and other horizontal surfaces. It also falls to the ground underneath making everything wet. Deck flanges are designed to work with plastic or composite decking that has a groove in both edges for invisible fasteners.

Deck flanges are commonly ‘+’ shaped rubbery material that slides into the parallel grooves in manufactured decking. The vertical part of the ‘+’ fills the exposed gap between the boards.

The deck boards are attached to the joists using screws through the top and hidden with plugs. Deck flange styles close the space between the planks so moisture drains off the end of the deck, keeping everything underneath dry.

Wall Flashing

Installing wall flashing is easiest with new deck construction. Flashing is inserted between the house and the siding and down behind the ledger. A second layer of Z or L-flashing and/or self-adhesive ledger tape slides under the siding and over the ledger.

The double layer protects the wall and ledger from damage by moisture. An additional safeguard is to apply a strip of self-adhesive flashing tape to the edge of the deck board where it parallels the wall and fasten the board tight to the wall flashing. This helps to prevent water from seeping its way behind and under the decking and rotting the board nearest the house.

If the decking is perpendicular to the structure, apply the flashing tape to the ends of the deck boards where they will butt against the house.

How to Waterproof Deck Boards

Deck boards, whether pressure treated or composite require maintenance to prevent graying or discoloration, rot, and mildew. The boards should be clean and dry and any damage repaired or replaced before applying a protective sealer or stain. Also, clean between the boards and where they rest on joists, especially if the joists aren’t taped or protected.

Applying waterproofing to the exposed joists will help improve lifespan too. If you’re wondering what the differences are between sealers and stains, and which is best for your deck, check out this article.

Always check the manufacturer’s site for recommended products and procedures for waterproofing composite boards. Pressure treated decking can take up to 6 months to dry from the treatment, which is why many wait a year or two before applying sealers or stains.

A splash test in random locations will identify if the wood is absorbing moisture or beading. Once the decking is clean and dry, use a roller, sprayer, and/or brush to apply the stain and/or sealer. Allow the sealer or stain to dry before applying a second coat or moving furniture back and using it.

Deck Railing

Deck Railing

Wooden deck railings, like other wooden deck components, also need regular maintenance and protection from rot-causing elements. Clean dirt and debris from the railing and lightly sand or replace rough or damaged pieces. Apply a waterproof stain or sealer that contains UV protection using a brush and/or roller, and ensure that drips and puddles are wiped up.

If the railing has composite, plastic, vinyl, or metal components, wash and clean seasonal dirt and debris off and inspect for damage. Also, inspect on-deck mounted posts to ensure fasteners are tight and seals or caulking undamaged. The seals or caulking prevent moisture seepage that can rot decking and important structural connections.

Deck Stairs

Deck stairs

Deck stairs may have wood or metal stringers but often have wooden or composite stairs and risers, or they may be all metal or all concrete. Regardless of composition, they should receive regular cleaning and maintenance.

Metal components should be cleaned, primed, and painted as necessary with appropriate weatherproof paint. Concrete should also be cleaned and sealed with appropriate sealers.

Wooden and composite stair components should be seasonally cleaned and regularly waterproofed to prevent rot and damage. The upper surfaces of stringers can be protected with self-adhesive joist tape or other waterproof materials.

Waterproof paints, stains, or sealer will protect stringers, stairs, and risers from moisture damage too. However, they can also make the steps slippery, so use anti-slip strips, add sand to the coating on the steps, or use slip-resistant stair paint.

Deck Fascia

Composite Fascia Boards

Deck facia is vertically oriented, so it sheds moisture well, but it is also fully exposed to the brutality of the elements. Wooden and composite fasciae can be cleaned and maintained on the same schedule as the deck boards, and even protected with the same coating.

Clean the fascia of dirt and debris, repair any damage, and lightly sand any rough sections. Apply a UV-resistant sealer, stain, or paint depending on your aesthetic scheme. To minimize maintenance, consider installing PVC fascia or wrapping the existing fascia with metal flashing.

How to Stop Water Pooling on Decking

Water pooling on the deck is different than waterproof sealers causing water to beading. Beads tend to evaporate or run off. Water that is unable to drain off, however, will pool and can damage wood decking or cause slip hazards. Decks should have a slope between 1/8” and 1/4″ per foot to help prevent water pooling.

Unfortunately, thermal expansion, contraction, and cupping can trap water, as can the settling of footings or supports. Check the slope and insert wedges or shims to elevate low spots between decking and joists, or to improve the overall slope.

Ensure the fastener spacing is as recommended to prevent cupping. If the spacing between planks prevents drainage, then adjust them. Water shouldn’t pool if properly sloped.

Maintaining Your Waterproofed Deck

A well-maintained waterproof deck should provide decades of outdoor enjoyment. Set up a maintenance schedule based on the recommendations from manufacturers and installers.

  • Seasonally clean and wash dirt and debris off your deck to keep it looking good.
  • Clean up food slaters and drink spills as they happen.
  • Use dish soap, TSP, or a biodegradable detergent with a soft bristle scrub brush or cloth to wash decking.
  • Be cautious if using a pressure washer on decking as wood is easily damaged.
  • Check under planters and other deck furnishings for trapped moisture or damage.
  • Remove large accumulations of snow as they can damage deck supports.
  • Check for damaged or loose fasteners or components and address issues as they arise.
  • Reset nails or screws if necessary, and check nuts and bolts yearly, or prior to major storms.
  • Inspect caulking or sealants and flashing in the spring to ensure they aren’t cracked, lifted, or separated.
  • If sealer, stain, or paint is worn, refurbish it as it protects the decking from rot and deterioration.

Additionally, check all seams and connections of under-deck drainage, if applicable, and ensure it is clear of dirt and debris. Blockages in drainage troughs between joists, downspouts, and water removal systems can cause significant damage if not addressed quickly.

How Much Does It Cost to Waterproof a Deck?

The cost to waterproof a deck depends on the size of the deck, who is doing the work, the quality of the materials, and what is meant by waterproofing. It also depends on if the deck is already built, or if it’s a new build. For purposes of simplicity, we’ll go with an existing deck.

Cleaning the deck boards and applying two coats of quality sealer or stain will range from $0.20 to $0.60 if doing it yourself, and $0.75 to $3.50/sqft or more if hiring it out. Depending on railing and stair styles and complexity, the cost ranges from $1/linear foot for a DIYer and up to $7 or more for a hired job. These tasks usually need to be done every 2 to 5 years depending on exposure climate.

An under-deck drainage system is a one-time cost that will vary based on the kind installed, type of material, color, deck size, and who does the work. The retail cost of materials for a 10’x20’ above or below joist deck drainage system ranges from $1,200 to $1,600 or $6 to $8 per square foot.

Hiring the work done will add about 25% to 50% to the material costs, although some installers may charge much more.

Leave a Comment