How to Repair Dry Rot

Brian Barth
wood repair on a dock

Dry rot is caused by a fungus that eats away at wood, eventually turning it into a crumbly mess. Severe infestations can threaten the structural integrity of a home, cabin, shed, dock, or other wood structure, requiring a professional for treatment. Repairing localized cases of dry rot, however, is a feasible DIY project.

Assessing the Damage

signs of dry rot

The first thing to know about dry rot is that it is a result of wood being exposed to moisture, though the name suggests otherwise. It always starts at a place where the wood is wet, but unlike 'wet rot' it is capable of traveling long distances to infect large areas, transporting the moisture it needs in the process.

As dry rot progresses, paint flakes off and the wood begins to crack and crumble. If you find this spread throughout large areas of the house or in any load-bearing parts of the wood structure, you have a serious problem and need to seek expert help.

Specialists will:

  • Use fans and blowers to dry things out
  • Apply fungicides to stop the rot from advancing
  • Carefully remove and repair all the damaged wood without compromising the structural integrity of your home

On the other hand, if you find a few areas of soft crumbly wood in a door, siding, exterior trim, floorboards or other non-structural parts of the house, you can proceed with the repair methods outlined below.

Tools and Materials

hardware store tools

Gather these tools and materials to get started.

  • Fan
  • Saw
  • Putty knife
  • Wire brush
  • Wood hardener
  • Replacement wood
  • Wood glue or epoxy
  • Clamps
  • Sandpaper
  • Paint or other sealant

Step-By-Step Repair

The exact method for repairing dry rot will depend on the nature and scope of the damage, but there are several basic steps involved in the process no matter where the rot has occurred.

sawing wood
roll of sandpaper
  1. Dry out the wood. Any repair is useless until the source of moisture is eliminated and the wood is allowed to dry. It can take many months for wood to dry in cool humid conditions, so you may consider setting up a fan to speed up the process. Expose any concealed wood to fresh air, if possible.
  2. Remove the rotten wood. Larger areas may be cut out with a handsaw; for small areas simply scrape the loose wood away with a putty knife and use a stiff wire brush to remove all the little loose bits.
  3. Apply wood hardener. If you aren't able to cut back into solid wood with a saw, use a chemical wood hardener to firm up the surface of the wood after you scraped away the crumbly part. This should be applied liberally with a brush and left for several hours to dry.
  4. Fill the void. Here is where the approach used will begin to differ based on the material you are working with and the quantity of rotten wood that was removed. If the area where the wood was removed was less than an inch in thickness, it's feasible to use wood filler to fill it. This is a thick paste that can be packed into the void and shaped with a putty knife to reconstitute the original form of the wood; it can be applied in layers to fill larger areas. Otherwise, you'll need to cut a piece of wood to fit the shape of the void. Glue this into place (or use an exterior epoxy if it will be exposed to the elements), using clamps to secure it until the wood dries.
  5. Finish the surface. Once the bulk of the void is filled, there are usually several additional steps to make the wood look as good as new. The wood filler can be sanded down so it is smooth and blends seamlessly with the adjacent surface. If there are small gaps between where a wood block was inserted and the original wood, these can be filled with wood filler for a smooth transition. For either technique, the new surface should be painted, stained or sealed to match the surrounding wood and protect it from further damage.

There are many types of wood fillers, epoxies and hardeners on the market, so the exact steps may vary slightly. Follow the instructions on the container to ensure good results and for personal safety, as many of these substances are toxic.

Stopping the Spread

Though professional knowledge and expertise is needed to keep severe cases of dry rot from advancing, it is feasible for homeowners to use a natural fungicide as a preventative for future dry rot infestation. Boric acid, the common household pest repellent, has the same effect on the dry rot fungus as it does on insects.

old wood cabin

There are commercially available preparations that are premixed at the right concentration for use against dry rot, such as Tim-Bor and Boracare; these can be rolled or brushed on or applied with a sprayer. Though boric acid is a naturally-derived substance, it is still toxic and should only be used in accordance with the instructions on the label.

Reviving and Maintaining Old Weathered Wood

The simple truth is that all old houses have at least a little rotten wood here and there. By repairing it as you find it, you can keep a small problem from erupting into a big, costly one.

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How to Repair Dry Rot