Deck Joist Waterproofing: 7 Popular Options and Solutions

You’re building a deck and you want it to last a long, long time. You’ve heard about using stains and sealers to extend the life of the decking, but what about all that framing that’s holding it up. It won’t make much of a difference if, 10 years down the road, the decking is in pristine condition, but the joists holding it up are rotting away.

Just as you’ll need to treat and protect the deck boards you’ll be walking on; you’ll also need to protect the framing that’s holding it up. This guide will provide you with deck joist waterproofing options and solutions, so you can ensure that your deck remains standing for many years to come.

Deck Joist Waterproofing

Why Waterproof Deck Joists

I know what you’re saying to yourself. Deck joists are already treated. Why do I need to apply more treatment? There are a few reasons. First of all, because the way decking is generally attached to joists, there is a multitude of cracks and crevices in the joints between these pieces of lumber. Because these nooks and crannies are poorly ventilated and slow to drain, they trap moisture, causing rot.

And though the pressure treatment that the wood has undergone will protect it to some degree, water will eventually triumph if the wood isn’t able to dry quickly. Trapped moisture, even in treated lumber, will eventually cause rot.

Waterproofing deck joists

It’s also important to understand that treated lumber isn’t treated to the core. The pressure treatment only penetrates the wood a few inches. With large joists and beams, that means the center is just plain old wood. Why does it matter if the core of each joist isn’t treated?

The long nails and screws that are driven into each joist provide dozens of entrance holes to that untreated core. This can cause the joist to rot from the inside out. It can even cause damaging expansion and contraction if water penetrates these holes and freezes.

How do you protect your deck framing from these assaults? There are a variety of methods to choose from.

How to Waterproof Deck Joists

The options for waterproofing your deck joists are many and varied. From good old fashioned tar paper to the more modern flashing tape to the home remedy of used motor oil, there are plenty of methods to cover. So, let’s get started.

1. Tar Paper

Deck joist protector

Tar paper was the material of choice before companies began manufacturing and selling all those fancy joist tapes that are available now.

This is the tried-and-true method that builders have been using to protect deck joists and beams for many many years.

This also happens to be one of the cheaper methods as a roll of 15lb felt tar paper is generally inexpensive. The installation process is also relatively easy. Cut the tar paper into strips, then attach the strips to the deck-facing edge of each joist using a staple gun.

Just as it does in its intended application as a substrate for roofing, the tar paper serves as a waterproof barrier for the top of the deck’s joists.

2. Deck Joist Flashing Tape

Deck flashing tape

Don’t want to fool around with cutting tar paper strips and using staple guns? Want a solution that’s designed specifically for the job and super easy to install? Then consider deck joist tape. Deck joist tape comes in rolls and can be applied to the deck’s joists by peeling and sticking.

Rolls come in a variety of widths to fit single width and double-width joists. The tape covers the top surface as well as the top inch or two of each side. As with felt strips, the tape works as a moisture barrier, preventing water that collects on the tops of the beams or joists from infiltrating the wood.

Deck joist tape generally comes in two materials, asphalt, which is the same material as tar paper, and butyl, which is a synthetic rubber. Butyl is the superior product as it holds up better against extreme heat, is stickier, and can be applied in a broader range of temperatures.

It also does a better job of creating a seal around screw heads, preventing water from infiltrating your joists.

Flashing tape also comes in a wide variety of thicknesses, ranging from 20 mil up to 40 mil with asphalt tapes being on the thicker end. The thicker tape can be a little more challenging to install but will generally last longer than thinner tape.

3. Seal Deck Joists

sealing deck joistsNot keen on tape or felt? Maybe you have a taller deck, and you’re concerned about the tape becoming unsightly. Consider treating your deck joists with sealant in the same manner you would treat your deck boards.

Purchase a water-resistant sealant designed for decks. Before installing the joists, coat each one, making sure to apply an extra coat to the ends.

While this method will provide more protection than tapes that only cover the top of the joist, keep in mind that it will take significantly longer to treat each joist. Also, be aware that the sealant won’t protect the screw or nail holes created when you install the decking.

4. Liquid Rubber

deck joist sealerWhile deck sealant is a good option for resisting water, liquid rubber can provide your deck joists with an even better water barrier.

You can find liquid rubber foundation sealant at your local home improvement store. This thick liquid can be applied with a brush to all sides of the joists or beams. Make sure to apply two heavy coats to ensure maximum protection.

Liquid rubber offers perhaps the most comprehensive protection by creating a thick waterproof coating around the entire joist. There are, however, some downsides to this method that you should be aware of.

Applying liquid rubber is time-consuming and messy. The coating has the consistency of tar and will take you some time to coat all of the joists and beams you plan to use, especially if you are building a large deck.

This also isn’t for every deck as the treatment can be unsightly. In addition to having the consistency of tar, liquid rubber foundation sealant also has the appearance of tar. As such, the finished result, while thoroughly waterproofed, is not aesthetically pleasing. If the underside of your deck is going to be visible, consider going with another option.


5. Coil Stock

If you like the strength of metal, then you might consider using coil stock to protect your deck joists. Coil stock is the aluminum siding that has been used for decades to protect trimmed areas around homes. It usually comes in rolls in a variety of different colors.

deck joist flashingInstallation can be difficult as the sheeting must be bent to fit over the tops of the joists. This is a process that can’t be completed by hand. You’ll need to use a sheet metal brake to bend the sheets.

If you don’t have access to one, you’ll need to get your metal sheets bent for you by a local metal shop, which can be a chore.

Alternatively, you can use two L-shaped pieces of trim together. Simply overlap the pieces over the trim and use silicon adhesive to hold them together.

Since coil stock is metal, this option offers a very strong and durable water barrier for your joists. That said, it does have some downsides. Water will still be able to get through the holes that are created by the screws or nails you’ll use to attach the deck boards.

Sheet metal is also hard to work with. Aside from the fact that bending it requires special tools most people don’t have, cutting it can also be a pain.

6. Joist Caps

If you want metal protecting your joists, but don’t like the idea of bending and cutting metal, then check out joist caps. These metal caps, modeled after the edge caps used on roofs, are shaped to fit around the top of the joist.

There are caps designed to cover single joists and double-width joists. As with the coil stock option, joist caps provide a strong barrier against water. These caps, which clamp over the top of the joists, can be quickly installed.

Use a chop saw to cut them to length and pop them on. Joints between joists and beams are protected with tape.

Joist caps offer excellent protection but expect to pay more than other options on this list. Also, they do have their downsides. Since they don’t fit as tightly as an adhesive tape, water can get trapped between the cap and the joist, creating a bigger problem than if there was no cap at all.

With this in mind, the caps must be correctly installed. This means they must be properly taped at the joints, as these areas represent the most likely points of entry for moisture.

It’s also a bad idea to use joist caps for standard decking as you’ll have to punch dozens of holes in the caps, creating numerous places for water to enter. As such, this option works best with hidden fastening systems such as ShadoeTrack.

7. Old Motor Oil

I know what you’re thinking. Can I treat my joists with old motor oil? Yes, you can. Let’s start with a history lesson. Pressure-treated wood has only been around since the 1930s. Before the invention of treated lumber, farmers had to come up with some way of treating fence posts to prevent them from quickly rotting.

One solution they came up with was painting motor oil onto posts, with the idea being that the motor oil would soak into the fibers of the wood, thereby protecting the wood from moisture since, as well know, oil resists water.

Installation is very similar to staining. Use a paintbrush to paint the stain onto each joist. Then wait about a day for the stain to soak into the wood and dry.

If you do choose to use this method, you’ll need to plan. As with wood stain, for this to work, the wood has to be able to soak up the oil. And for that, it needs to be dry. Using wet treated lumber isn’t going to work.

You’ll need to wait for the lumber to dry out. This makes this method somewhat inconvenient and maybe impossible, depending on what your construction schedule is.

On the upside, used motor oil is free. Simply save the oil from your car’s next oil change and you’ll have plenty of free water sealant for your deck joists.



Building a deck is no small job. It’s a major project that requires a hefty investment in both time and money. Don’t cut corners by failing to properly protect the framing that will hold your deck up.

By using one of the methods on this list, you can protect the joists and beams on your deck, ensuring that it will be sturdy and rot free for many years to come.

Leave a Comment