How to repair rotted deck joist: A beginner’s guide

If your deck and its foundation are showing signs of rotting, or the joist has lost significant strength, you need to repair the rotted deck joist at the earliest. If the situation gets worse, you might need to replace the deck joists altogether, which can be difficult, expensive, and time-consuming. 

Besides, you wouldn’t want your wooden deck to be anything but beautiful and flawless, right? A well-maintained deck, made from the best lumber, can last for about 20 years – depending on the construction, materials, and weather conditions.

That’s why it’s important to keep it protected from excessive moisture and wood rot. In this article, we’ll talk about ways to prevent the rotting of deck joists and steps to repair the rotted deck joists.  

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Why do deck joists rot in the first place?

Any wooden structure that regularly gets wet may rot over time. But if you have the best pressure-treated wood, you may be able to avoid having to repair deck joist rot for a longer time. 

If you’re building a new deck, always buy treated lumber, with the highest amount of copper preservatives – one that’s right for direct ground contact or burial. Also, ensure that your deck’s drainage system is properly installed below the joists.

A faulty under-deck drainage system may trap moisture – creating exceptionally high humidity. It may over time collect debris and lead to decay. 

The high temperatures of the summer months further accelerate the rotting. Another reason for rotted deck joists is the painting of the exposed joists by homeowners. This can cause further damage. As the top layer of wood gets wet, there’s no way to release the moisture from within.

How to prevent deck joist rot?

• Deck joist rot prevention is so much easier if you purchase the best pressure-treated lumber – rated for direct ground contact or burial.
• Also, make sure to install deck joist tape on top of the joists before applying the decking. This is to prevent the fasteners from adding to the dry rot. In the absence of tape, the decking fasteners create micro-cracks in the deck joists which allow water to seep under the decking – causing the lumber to swell. 
• If you can’t put joist tape, try your best to check your deck regularly for any accumulated debris. You can use a hard brush to remove the particles and clean the wood. 
• If you see any signs of mildew, mold, or algae, clean the sides and bottom of the deck joists. Or else, these would lead to wood rot.
• The underside of deck joists shouldn’t be sealed or painted as it can trap moisture which will cause rot.
• Install an under-deck drainage system to protect the joists and beams from moisture penetration. Also, it helps hold the screws in place longer.
• It’s a good idea to add a waterproof material to the underside of the joists.
• Add ventilation to your under-deck area with strip vents to facilitate the release of hot air and help mitigate extreme temperatures. Keeping the lumber at least 18 inches away from the soil is a good way to allow air to circulate beneath a deck.

How to repair a rotted deck joist? 

You can probably replace or repair rotten joists yourself if you’re handy. However, we recommend hiring a professional as it may be a complicated task for a DIYer.

Keep in mind that you’ll have to remove any joist hangers or tie-rod connectors before beginning the repair work. 

Here are some of the steps to repair a rotted deck joist.

• Inspect the deck and determine the type of repair work. You can use an ice pick or long-shaft flat-head screwdriver to test for rotted areas along the entire distance of the joist.
• Remove the deck boards to expose the joists. You’ll have to remove the decking fasteners first. It’s a task that needs professional help or else you may end up damaging the top surface of the wood. If the decking is installed with concealed fasteners, the task may be easier. But, it’s still better to leave it to an experienced decking contractor.
• Remove the rotted, damaged wood. You need to cut away all the rotted wood with a chisel or a saw.
• Keep in mind that wood fillers or epoxy may replace the rotted wood but they offer no structural strength to the deck joist. And, they may not match the look of the original floor joist. Or, bond to the treated lumber. If you must use filler, always brush on copper naphthenate to the chiseled wood. It will help extend the joist’s life span.
• Reinforce the joist with sistered material. Sistering newly treated lumber onto a deck joist that has little to no wood rot is the best technique to fix deck joist rot. For example, if the top 1 inch of the joist has rotted, you can screw on a treated 2×4 inch to the existing floor joist. However, if the rot is beyond repair, you will have to replace the rotted deck joist.
• Replace deck boards. It’s time to replace and fasten the deck boards to the new sister joist. Depending on the decking material, you will need to cut the deck boards to size and fasten them with hidden fasteners, nails, or screws.

Last thoughts

If you notice rotting in the substructure, joists, or boards of your deck, it’s time to repair the rotted deck joists. Or else, the damage may be beyond repair after some time. Keep in mind that your deck is susceptible to weathering and rot due to dampness and warm temperatures. 

Therefore, regular deck maintenance along with timely repair or replacements should be important parts of your home improvement. The goal is to have a deck that stays safe, strong, and secure.

Written by Ramona Sinha. November 16, 2022

Ramona is a content writer for Kukun. This experienced blogger uses simple and succinct words to decipher the complex phenomenon called life. An avid traveler, she’s a digital nomad at heart and an animal lover from the depths of her soul.