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How To: Conquer Dry Rot

dry rot

If you have an old house you’re already dealing with a lot of challenges. There’s lead paint, asbestos, sagging foundations, drafty windows, leaky plumbing, the list goes on and on. Why not add one more thing to the struggle? Two words: Dry Rot

The Serpula lacrymans fungus may be small, but it can single-handedly destroy an older house from the inside out. Prevention is key so we’ll start there, but if you’re already in the throws of the dry rot battle then we’ll also talk battle plans for its ultimate destruction.

What is Dry Rot?

The term dry rot is a little bit of a misnomer because all fungus requires a minimum amount of moisture (at least 20%, but usually 30% or more) to survive and thrive. One explanation of the term “dry rot” circles around boatyards. Back in the day of wooden ships, boats were brought in for dry dock for the season for repair. These wooden boats already had some amount of rot from being submerged in water day after day, but the wood cellular structure was full of water making it hard to notice.

dry rot fungus
The reddish brown Serpula lacrymans fungus is the cause of dry rot

As the wood dried out over the days and weeks in dry dock, the wood would shrink and crumble revealing the damage that was hidden. The shipyard workers would see their boats falling apart now that they were out of the water. The assumption was that the dry air has caused the rot and the term “dry rot” was born.

The truth is there are 4 things that rot needs to occur. Remove even one of these things and it will not survive.

  • Food Source (Wood)
  • Oxygen
  • Water (30%+ moisture content)
  • Warmth (40°F – 105°F but 75°F – 90°F is optimal)

There is not much you can do about removing the food source since we are talking about how to avoid rotten wood, and the only way you can remove oxygen is to keep the wood completely submerged under water or in outer space so we are left with the last two variables. Most of us live in climates somewhere between 40°F and 105°F which narrows it down once again to the number one thing you can do to avoid dry rot which is…

Avoid Excess Moisture

It may have the word “dry” in its name, but the Serpula fungus needs plenty of moisture to proliferate. It occurs over a long period of time in areas where the wood has more than 20% to 30% moisture and poor ventilation. After a while, the wood inevitably starts to soften and essentially turns into a mush that you can think of as pureed fungus food.

While you might think it would be called wet rot since the Serpula fungus only occurs where there’s an excess of water, wet rot is something totally different. In fact, wet rot usually occurs on trees and wood that’s been uncured. The Serpula fungus only develops on lumber that’s been used for construction, i.e., milled, dried, and cured. Wet rot is typically found in forests, not houses.

Look for Vulnerable Spots

The worst part about the Serpula fungus is that it’s never in places where it can be easily seen. Instead, it likes to grow underneath the floors, inside the walls and around the plumbing, and under roofing, where it’s not always immediately visible. That’s why it’s so vital to keep a close eye on your old house and make sure it stays dry and well-ventilated.

You can learn to use a moisture meter to check suspicious areas very easily and know exactly what the moisture content is.

Check These Areas, Too

While it’s essential to look in places like underneath the floor and the walls, you should also check any area where water is close by. Some of these critical areas include:

  • The deck, porch, or patio
  • Around windows and doors, including skylights
  • Floors by the tub, shower, and toilet
  • Shower walls
  • Around plumbing and pipes
  • Exterior house trim
  • Roof, including by the gutters/flashing
  • Wooden siding
  • The wood around your home’s foundation
  • Cabinets underneath the sink
  • Wood in the vicinity of sprinklers

Watch out for Red Flags

When looking in those vulnerable areas listed above, watch out for these major signs indicating the presence of dry rot. To start, check anywhere that smells musty and damp. Keep an eye out for blistering, bubbling, or cracked paint, soft or mushy-feeling wood, or wood that’s splitting. Any area that feels damp or wet should be thoroughly investigated for the root cause and summarily addressed to prevent further water damage.

Peeling Paint
Peeling paint is a sign of high moisture content

A huge red flag to watch out for is water stains. They look like a brown stain that typically has a darker brown ring around the outside. It’s imperative to investigate the source of water stains, no matter how small they are. The Serpula fungus needs a steady source of moisture over time to multiply. By eliminating the moisture source, you eliminate its food source, too.

How to Treat Dry Rot

The Serpula fungus is entirely dependent on moisture as a source of food. The primary treatment method for this damaging fungus is to keep areas that get wet well-ventilated and dry. Remember, timber with a 20% or higher level of moisture is very susceptible. Try rapid drying via heating, lots of ventilation, and dehumidifiers. Fix any leaks or areas where water is getting in or dripping for long-term prevention.


Unfortunately, wood that has been too far damaged by dry rot may require removal. Leaving it in place can alter and negatively impact the structural integrity of the entire house. However, if the timber in your old house is of historical value or can’t be removed, it’s okay to try every method humanly possible to save it.

You don’t need to remove the whole member if there is only a small area that is damaged. You can cut off sections or dig out damaged areas. But be careful to think of rot removal almost like cancer removal. You have to get good margins when removing damaged sections. There is fungus beyond the damage you see, so extend your removal at least a few inches past where the damage appears to end or you’ll face the same situation a few years down the road.

Borate Treatment

If the wood that’s affected can’t be removed, there are other options available. Using products like BoraCare or Timbor can not only remediate existing fungus by killing it with prejudice, but it can also prevent wood from becoming infected in the future.


These treatments are becoming increasingly popular in new construction of wood homes after framing is complete to treat the whole structure to prevent future rot. Check out my previous article all about the process for applying these chemicals. It’s very straightforward and safe for any DIYer.

Don’t Repair Until You Treat

One last tip is that you should not focus on repairing damaged wood until you have dealt with the fungus first. If you attempt to use wood filler without treating or removing all the fungus then you will undoubtedly be stuck making your repair again in short order. Treat first, then repair…always.

When it comes to identifying and ultimately preventing the dry rot, the number one best method is to be vigilant in monitoring the conditions of wood in your historic home. Keep areas where there’s moisture dry and very well-ventilated. It takes a long time for the fungus to develop, so early detection is paramount in preventing dry rot.

2 thoughts on “How To: Conquer Dry Rot

  1. Thanks Scott. Great article! We’ve just moved into a 1903 build in Bristol UK, and the dreaded dry rot has been discovered, both with a rotten lintel above a window on the 1st floor, and then on the ground floor by the front door, which only seemed to have caused minor damage. The rot looks very old, as does the remanence of the fungus/mycelium. It cracks, and almost just falls apart in your fingers. We’ve had “expert” contractors out that have also stated its old, but that it needs all plaster hacked off the walls, timber cut back a meter past it, a new window (that has no sign of damage), chemical treatments, and plaster etc. What I can’t seem to determine is when does old rot not become a concern. This could quite easily be 10+years old (the previous owners were here for 40 years). Of course we’re changing the lintel and any other rotten structural wood, but we need to draw the line somewhere on trying to get all the figures out of the walls etc. Any help would be greatly appreciated.

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