Does Pressure Treated Wood Need to Be Sealed?

Using pressure treated lumber to build decks, fences, furniture, and other outdoor items is a good practice. It’s less expensive than cedar or other rot-resistant wood and is long-lasting. But, if you’re wondering, does pressure treated wood need to be sealed, we’re here to help!

Pressure treated wood should be sealed to protect it from UV rays, moisture, mold, mildew, and rot. Allow the wood to dry until the droplet or bead test indicates it will absorb the sealant. Wait for the forecast to indicate 2 to 3 dry days between 65°F and 80°F, and then get busy!

In this article, we’ll identify whether pressure treated (PT) lumber is waterproof and if it can or needs to be sealed. We’ll explain when PT wood should be sealed and what happens if it isn’t. Plus, we’ll discuss protecting cuts and notches, sealing decks and fences, and the use of Thompson Water Seal. We’ll also review what the best PT sealer is and how to maintain a deck. Our aim is to provide you with the information to decide if you should seal pressure treated lumber.

Is Pressure Treated Wood Waterproof?

Pressure treated lumber has been soaked in a chemical bath which is then pressurized to push the water-borne chemicals into the wood fibers below the surface. The lumber is then depressurized and dried so the moisture content is around 19%.

The level of chemical retention after the bath determines how well protected the wood is from insects, fungi, and rot. It also identifies what category it falls in on the American Wood Preservers’ Association’s 12-tier grading system.

Most decks, fences, and outdoor furniture utilize UC3 or UC4-rated lumber. UC3 is rated for above-ground use, while UC4, which retains about twice as much chemical preservative, is rated for ground contact or areas with high moisture content. Although pressure treated wood is better against rot, fungi, and insects than untreated wood of the same species, it is not waterproof or even water-resistant.

The pressure treatment process slows the decay process of wood species like pine, fir, hemlock, and spruce that have little or no natural protection. The treatment extends their life span, so it is similar to species like cedar, cypress, and redwood which are naturally decay-resistant.

Unfortunately, treated wood seldom looks as good as natural wood, however, both types of wood need to be maintained and protected from the elements on a regular basis.

Can You Seal Pressure Treated Wood?

Pressure treated wood can be left to fade and weather naturally for years or it can be stained, sealed, or even painted. However, most treated lumber still contains excess moisture when purchased and installed, and needs to be allowed to dry before applying anything to it.

If the lumber still contains excess moisture, it is unable to properly absorb a sealant. Applying sealer, stain, or paint to wet wood will cause the finishing product to flake, chip, or peel as it is unable to penetrate or bond properly.

Does Pressure-Treated Wood Need to Be Sealed?

Wood is a porous material and requires protection from the elements. Sealing will help prevent moisture from damaging the wood. Even though pressure treated wood is protected from insects and rot, it will still fade or gray, crack, split, or warp with exposure to the elements and UV rays.

Sealing the wood on a regular two-to-three-year schedule will lessen the effects of weathering and help keep the wood durable and looking good. Some even recommend applying sealer on a yearly basis, but environmental conditions and the type of sealer often determine the cycle of reapplication.

What Happens if You Don’t Seal Pressure Treated Wood

The wood used in outdoor locations like decks, docks, fences, and furniture is exposed to UV rays, humidity, dew, rain, snow, leaf debris, dust, and dirt. Moisture left on the wood can cause the wood to swell, crack, cup, warp, or rot. UV rays can break down the wood, cause the color to gray or fade, and damage the bond between cells.

Leaves left on the wood can cause discoloration, staining, and lead to fungi, moss, lichen, mold, and mildew growth. Dust and dirt have similar effects to leaf damage but can also scratch or mar the wood surface.

Pressure treated wood, whether sealed or not, can last from 9 to 40 years depending on where and how it is used, and its cycle of care. Treated lumber that is in dry climates will often outlast the same wood in moist damp climates. Additionally, lumber used vertically will shed moisture easier than that oriented horizontally and also usually last longer.

Pressure treated lumber that is maintained and kept clear of snow, dust, dirt, and leaf debris will still gray with age and crack and warp with weathering. Maintaining the wood and sealing it with a UV protective coating will minimize damage, keep it looking good, and can help it last longer.

How Soon Can You Seal Pressure-Treated Wood?

Pressure treated wood can be sealed to better protect it from UV rays, cracking, warping, dirt, mold, mildew, fungi, and rot. However, depending on local humidity levels and the moisture content of the new wood, it may take 3 weeks to 3 months or longer before it is dry enough to absorb the sealer.

Use the water droplet or bead test method to check the absorbency of the wood. Sprinkle water droplets on different areas of the wood, if it beads, it’s not ready. If it spreads and absorbs in, the wood will absorb the sealer, but now you need to let the water droplets dry out.

Do You Need to Seal Cut Ends of Pressure Treated Lumber?

Pressure treated wood has been bathed in a chemical cocktail and then pressurized to force the chemicals deeper into the outer wood. The depth the chemicals penetrate is usually 1/8” to 1/2″, but depends on wood species and the amount and duration of the pressure.

Cutting, notching, ripping, or drilling the wood will expose untreated wood to the elements. If that untreated wood isn’t sealed, then it opens up the rest of the untreated core wood to moisture and insect damage, and rot. Applying sealer to all cuts and drill holes protects the exposed untreated wood and improves the durability and lifespan of the wood.

How to Seal Pressure Treated Wood Deck or Fence

Sealing a new pressure treated deck or fence is best done when the weather will be dry for several days, the temperature is between 65°F and 80°F, and the relative humidity 40% to 70%. The optimum temperature is 70°F and humidity 50%. Use the droplet or bead test to ensure that the wood is dry enough to absorb the sealer; it may take several weeks or months before new wood cures enough, so be patient.

Select a UV-blocking sealer that best suits your aesthetics or color palette. Since it is pressure treated wood, it may have a greenish or brownish coloring, so you may want a color-tinted semi-transparent sealer. It will show the natural grain but a more uniform color. A solid color sealer will cover the wood in a uniform color but still show some wood character, whereas paint will hide most wood characteristics.

Once you’ve identified that the deck or fence is dry enough to absorb the sealer, remove furniture, etc. from it, and sweep it clean. If there are any BBQ grease stains, wine spills, or bird droppings use a soft bristle brush and some dish soap in water to remove them. Let the washed wood dry before applying the sealant.

Test the stain where it can’t be seen to ensure the color is as desired and that it properly adheres to the wood. If it meets your expectations, begin applying the sealer at one end of the deck or top of the fence and work toward the other end or bottom. Do two to four boards from end to end at a time to prevent overlap lines.

Use a brush, roller, or paint pad to apply the sealer. Remember not to paint yourself into a corner. Typically, only one coat is required, but it should dry for 24 to 48 hours before getting rained on, so wait for an appropriate forecast.

Can You Use Thompson Water Seal on Pressure Treated Wood

Thompson Water Seal can be used on pressure treated wood. Use the droplet or bead test to ensure the new wood is dry enough to absorb the sealant and make sure the wood is clean prior to applying the sealer. Thompson Water Seal is available in clear or tinted, transparent, semi-transparent, and solid color options, and protects against UV damage.

Best Sealer for Pressure Treated Wood

Pressure treated wood needs to be protected from the elements, UV rays, and moisture like any other wood. Unprotected wood will fade or gray, crack, check, cup, and warp, and is susceptible to mold, mildew, and rot.

Sealing wood, including pressure treated lumber, will help protect it and retain its natural color, plus even increase its lifespan. Sealers may be water- or oil-based too, so pick the type best suited for your lifestyle.

Look for a product that is reliable for the climatic conditions the wood is exposed to. It needs to protect the wood from UV rays and moisture from rain and snow. It should be durable enough for the type of use it will experience and able to last the reapplication and maintenance cycle you’re comfortable with.

The sealant should also protect and enhance the appearance of the wood and be available in clear or semi-transparent formulas. It may even offer tinting options. The sealer should also offer good value for the money. Does it do what the manufacturers claim? Checking the reviews on different sites or with family, friends, and neighbors is helpful.

Our pick for the best sealer for pressure treated wood is Thompson Water Seal. It’s available in clear, semi-transparent, and solid formulas. The semi-transparent is subtly tinted in light natural cedar, desert tan, chestnut brown, and Sedona red to provide a consistent coloring to enhance the natural wood coloring and grain pattern.

The sealer provides UV and moisture protection and helps prevent graying, mold, and mildew. It ensures durable year-round protection with one coat and cleans up with soap and water.

Thompson Water Seal also exceeds ASTM-D4446 waterproofing requirements for wood in lab testing. It’s good value for your money, it dries quickly, is a sealant and stain in one, and typically lasts 2 to 5 years depending on wear and exposure.

How to Maintaining Pressure Treated Wood Deck

Maintaining a pressure treated deck isn’t much different from looking after any other wooden deck exposed to the elements. Once the deck has seasoned so that it passes the water droplet or bead test, seal it with good quality, durable, UV protective sealant. The sealer will do the daily work, but you should get in the habit of sweeping dust, dirt, leaves, and other debris off it regularly.

If grease, coffee, wine, or other liquids get spilled, spot-clean them as they occur with soapy water so they don’t stain or mar the wood. Snow and rain can accelerate deterioration, so removing excess moisture or snow can be helpful too – it’s not necessary, but it will help the sealer last longer and protect your investment.

The last step is to monitor the sealant. If the decking begins to fade or water doesn’t bead up on it anymore, it’s time to reseal it. The cycle may be every 2 to 3 years, or it may be longer depending on wear and exposure.

The deck needs to be cleared, swept and washed. You may want to use a manufactured cleaner or brightening product or a DIY solution. If using a power washer use the appropriate tip and maintain the recommended distance – you want to wash it, not etch the wood. Once cleaned and brightened, allow it to dry before reapplying your selected UV protective sealant.

Should You Seal Pressure-Treated Lumber?

Pressure treated wood will gray, crack, check, split, warp, and cup over time if it isn’t sealed against UV rays and weathering. Select a sealer that is durable and protects against UV, mold, and mildew, and will last at least 2 years.

Remember to seal all cuts, notches, and drill holes as you build the deck to protect exposed untreated wood. To protect your investment and keep your deck looking good, the wood should be sealed every two to 5 years – depending on exposure and type of sealant.

Monitor the wood for moisture absorption, and when water stops beading and is soaking in, it’s time to seal it again.