Q: Our 35-year-old house has a 7-foot cantilevered deck on the top floor — about 25 feet above ground level. The untreated fir joists feel solid but have large black areas on the sides — especially on the outboard end (the area closest to the house is protected by an overhang). I intend to remove the decking to repair any rot on the tops of the joists; I was told by a pest control guy to pressure-wash the black areas on the sides. I guess that is a good idea.

My concern is how to protect the sides of the joists in the future. (I will use aluminum tar tape on the tops.) Should I consider coats of copper-green, water sealer, paint, stain or something else? I want long-term preservation because of lack of access. Any advice would be appreciated.

A: Untreated Douglas fir joists with black areas 25 feet off the ground could be a recipe for disaster. Fir is susceptible to fungus infection, also known as dry rot. In addition, bugs such as powder post beetles can wreck havoc with wood framing.

Since you mention that you are planning to repair any rot on the top of the joists as part of replacing the deck, we assume you have some rot — hopefully not too much.

Water is the friend of rot — and mildew and bugs. The black areas you see are most likely mildew.

A cantilevered deck is built with floor joists that seem to grow out of the side of the building. To some degree they do. Only one third of the joists are visible from the exterior. The remaining two thirds of the joists are buried inside the structure, attached to the interior in a fairly complex framing scheme.

The worst-case scenario is that the deck joists are so compromised by rot or bug infestation that they will have to be replaced. That is a huge job. That being said, we doubt that your structure is that far gone. The lesson here is that a little preventive maintenance will pay exponential dividends by eliminating the need for that very expensive fix. So stay on top of it.

Sounds like you are doing a total replacement of the decking boards. This is the time to do all the preventive maintenance to extend the useful life of the substructure. After the deck boards are removed, inspect the joist thoroughly for rot damage and repair it. Use an ice pick to probe the joists for soft spots, as rot tends to develop inside a board and isn’t always visible from the outside.

Depending on the extent of the damage, you may find that the easiest fix is to sister another joist onto the existing ones, especially if there’s just a little rot on the tops and ends of the joists. If you go this route, use carriage bolts, staggered on 18-inch centers to marry the new board to the old. Make sure the "sister" joist is pressure-treated lumber.

Check the area where each joist exits the house. Make sure the penetrations are well caulked to prevent water infiltration. Also, when reinstalling the deck boards, it is a good idea to install flashing to direct any water away from the house wall out on the deck so that it can be channeled away from the house.

The pest control guy is right, pressure-wash the joists to remove the surface mildew. We’d go one step further and give all the exposed parts of the deck a good scrubbing with a strong chlorine bleach solution to kill any residual spores.

To finish the job, apply wood preservative to the joists whether you paint, stain or leave it au naturel. We don’t recommend "Copper Green" due to its toxicity and because it stains and doesn’t paint well. Check the home centers or hardware stores for other alternatives. Make sure to be generous with the wood preservative and pay particular attention to the tops of the joists, as that is the area most susceptible to rot because of the nails or screws used to fasten the deck boards. The tar tape you plan to use will be a big help sealing the penetration, but the wood preservative gives added protection against decay.

A final piece of advice: If the new decking is wood, give each board a good dose of clear preservative on all four sides before installing them. It will extend the life of the decking and make future maintenance much easier.


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