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What Causes Wood Rot?

What Causes Wood Rot?Anyone who has owned an old house for more than a couple months will tell that wood rot is a major frustration. In this post, I’ll show you what causes wood rot and how to stop it.

Wood is an amazingly versatile building material. It can molded and shaped to almost any form. It is an excellent insulator. Its structurally abilities are incredible. Its ability to accept paint, stains and varnishes is fantastic. Its one weakness is rot.

I’ve written about the best ways to prevent rot in my previous post My 5 Secrets to Prevent Wood Rot.

Preventing rot is important, but understanding it will give you a better sense of why things rot and why my preventative measures work like they do.

Wood rot is caused by only one thing and I’ll get to that in just a moment, but first, there are 4 conditions that must be present in order for rot to occur. Remove any one of these conditions, and you stop rot in its tracks.

The 4 Conditions That Rot Needs to Occur

  1. Substrate (Wood)
  2. Oxygen
  3. Warmth
  4. Moisture

For you northerners, freezing temperatures slow down the rate of rot, but it doesn’t stop it completely. Since we don’t have much control over any of the conditions except moisture, that’s where we’ll focus our energies.

So what is the one thing that causes rot?

Fungi. The simplest of all plant life. Microscopic fungus spores are all around us floating on the breeze and landing all over our homes. Wherever they land, if the 4 conditions above are present, then you will have rot and that rot will continue as long as those conditions are present.

Even if you remedy the conditions, the fungus is still present and will resume growth (aka rot) when the conditions are more favorable.

How to Stop Rot

So, how do you stop wood rot? Well, you can always remove wood from the equation, but for most of us, that would mean remove our entire house.

And here on earth, there isn’t much we can do about eliminating oxygen or warmth.

On a side note, the lack of oxygen is why wood that is submerged underwater for decades or centuries can remain in completely pristine condition. The organisms that cause rot need oxygen to survive and the underwater environment is decidedly oxygen free. The city of Venice, Italy and the famous Brooklyn Bridge are a couple examples of structures built of wood foundations that have survived hundreds (and in Venice’s case thousands) of years with no trace of rot.

Back to how to stop rot. If we can’t get rid of wood, oxygen or warmth, then the only thing we’re left with is moisture.

So then, how can we stop the wood elements of our house from excess moisture which causes rot?

Let it Dry

It is going to rain or snow and the wetting of your house is not a cause for concern. The concern is when wood isn’t able to dry out. There are a few things you can do to help the wood on your house dry out.

1. Keep it Painted – Keeping your house painted is the easiest way to keep the water out. Paint is a great layer of protection to help the surfaces of your house shed water and dirt. Though it may get wet, the paint keeps the water (and fungus) from attacking the underlying wood.

2. No Standing Water – If any part of your house gets standing water after a rain storm, then that area is much more likely to rot. Standing water will find its way into joints and cracks in the paint and seep into the wood giving rise to perfect conditions for rot. Redesign these elements to allow water to shed off of them.

3. Allow For Air – Good airflow helps everything dry out faster, and the faster things dry, the less chance of rot. Trim back shrubs and trees from your house so that there is a enough room for some airflow between the two. Wet shrubs directly against siding are a major cause of rot on many houses.

If you can keep it dry, you’re leaps and bounds ahead of the rest of us. Yes, the cause of rot is fungus, but if you focus your energies on keeping the exterior surfaces of your house dry you’ll stop it in its tracks. And preventing rot is a lot easier than stopping it once it really gets rolling.

Products to Stop Rot

boracareWhile I’m a big believer in intelligent exterior design to shed water efficiently, there are times when rot has begun and you need to stop it.

The main product that I use to treat rotten wood is also the same product that works great for preventing termites and other insects that destroy wood.

BoraCare is a borate treatment that is easily applied to wood and over time it migrates throughout the entire mass of the wood. It doesn’t just sit on the surface, but treats the whole thing making the wood unappetizing for insects and uninhabitable for fungi.

If you have rot, this is the way to treat it to make sure the spores are killed and won’t come back.

Understanding how and why rot works is a big part of the battle that old house owners fight. Hopefully, this bit of knowledge will better arm you in your battle. Now get out there and fight!

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11 thoughts on “What Causes Wood Rot?

  1. I have found that in my porch decking, rot always occurs where the screws are driven in near the board edges. Is their any way to prevent this? Can I put some grease on the screws when replacing the boards to prevent this? Brush the treated boards with Bora Csre?

  2. I recently found a single floor joist in my basement that has a small spot of rot right in the middle of it. There’s no leaks, no pipes nearby, and it’s not close to the exterior wall of the home. We’ve got a dehumidifier in the basement to keep humidity at 45%. Any idea of what could be the cause?

  3. We have dry rot from a dead trees roots imposing through the masonry in our garage and under our walkway. We are struggling to find someone to address this. Do you have any recommendations? Anne

    1. Oh goodness, that’s no good! Without being able to see the damage in person, there’s not a really solid recommendation we can confidently offer aside from using this directory to find someone in your area who may be able to help you. Good luck!
      -Alyssa at The Craftsman Blog

  4. can wood that is between the outside wall and sheet rock rot in 5 mos if constant drip from outside is coming in?

  5. I just bought an old queen anne, and a broken gutter plus 15 years of vacancy have left some serious challenges for me to address.
    Along one corner, where the bay window meets the side of the house, rain has eroded the pink stucco and gotten into the side of the house. Thankfully, most of the serious rot damage was to the lathe in the dining room wall and ceiling, not the structural support beam in that corner, however there’s still some minor rot, mold, and mildew going on in that wall. I haven’t seen any ants or termites… yet… but it’s January. I expect to get a few more nasty surprises this spring.
    Can Boracare kill mold? Or should I treat the wood with an ammonia mold killing spray first? Will it stop new mold from forming if I do pretreat the inside of the walls? Will it kill existing rot, or just prevent new rot? And can I apply a wood filler epoxy over top of it? Lastly, can I apply it in subfreezing temperatures? The old boiler is broken, and I don’t want to replace the mechanicals until I’ve had the radiators and water pipes checked for cracks and rewired all the electricity.
    Thanks for the website, it’s been a huge help.

  6. True. Dealing with rooted wood is very hassle. In my case, it’s more of a moisture because of a stove in our dirty kitchen but I have to agree about keeping it painted as this one of the things we normally do to keep it protected. Thanks for sharing other tips and keep it up. Happy new year!

  7. Dealing with rotted would in your home is not fun. These are great steps to follow to help prevent wood from rotting and creating a big problem for your home.

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