Joist tape and the idea of protecting joists from the elements is a relatively new concept in deck building.
The main two reasons we recommend and see joist tape used on decks is to protect the new-growth wood framing, and to protect existing joists on decks where the decking is replaced but the framing is not.
“Old-growth wood” is lumber that has been harvested from an area where trees grew naturally, and have a tighter grain. This tighter grain is caused by trees growing close together with less light, and each growth ring is not as thick, so trees are smaller for their size but also denser. Imagine the rings as layers that are getting packed around each other more tightly, and so creating a stronger and heavier wood.
On older decks where this wood was used, the framing has lasted longer because less water has been absorbed, causing less rot and breakdown. Newer wood that is lighter and has allowed more water into the grain is more likely to break down sooner.
Why Use Joist Tape On Old-Growth Wood?
Whatever outdoor project you have with old-growth wood, it’s likely that there will be screw and nail holes on the tops of the joists from where deck screws have been removed. Even though this wood is tougher than the new wood on the market, water is going to want to soak into the holes and wood more easily than if it was fresh, virgin lumber. Adding joist tape protects the top surface and keeps water and rot out of the top of the joist, where water likes to sit and soak in.
Most, if not all lumber available today is going to come from areas that have been forested for years, where the trees grow faster and the grains/rings that are formed each year are further apart and the wood between is not as dense.
When building deck frames with this less dense wood, builders often use joist tape on new wood to protect just the top of the joist with tape, because it’s the most susceptible to standing water that will try and get in and rot the joist. The less dense wood allows water to soak into the wood more easily than if it was old-growth wood.
Why Use Joist Tape On New-Growth Wood?
Because decking nowadays can last anywhere from 15-50+ years, keeping the joists from rotting is especially important, because it’s likely that the decking will outlast the framing every time. Getting your money’s worth out of your decking only works if your framing is built well and protected as much as possible from the elements. Replacing framing is not easy to do when you have to remove all the decking and railing, and putting the decking and railing back on in good condition is nearly inpossible.
How Does It Work?
Joist tape is laid down across the entire length of the joists, beam, and any blocking, and decking is laid and fastened over the top of the tape and framing. When water falls between the decking gaps, it runs off and down the sides of the joists, rather than sitting on the top of the joist surface and soaking in and potentially rotting the wood. When a thicker tape is used, any water that tries to run between the decking and the tape is kept out of the screw holes by the thick tape that has sealed around the screw, and squished into all the gaps and air spaces. Instead of getting into that screw hole and running deeper into the body of the joist, it runs off the top of the joist and either evaporates or drips off the bottom of the deck frame.
Being that joist tape only costs 15-30 cents per square foot (a small percentage of your overall deck cost), it may be a worthy investment on your deck. The point of low-maintenance decking is to give your deck the longest life possible, but it’s only as tough as the framing it’s resting on. Make your framing last as long as your decking, and you’ll have a deck that will last a lifetime.
Joist Tape brands available at Pro Deck Supply: