What is Dry Rot?

by Crown Roofing

Posted on March 12, 2020 at 9:00 AM

Dry rot has destroyed part of the roof.

The day finally comes to receive an estimate for replacing your old roof. The roofing contractor climbs his ladder and carefully inspects the condition of the roof and takes measurements. The verdict comes, and because this roof is more than 20 years old a roof renovation is recommended. The roofer goes over specifics and gives you a written estimate. Later you look over the estimate and see that one of the problems mentioned was dry rot of the deck and fascia. After thinking about this a moment the question comes to mind "what is dry rot?" Normally we associate rot with moisture so how is it that something rots when dry? It's a good question so let's have a look at that in more detail.

Understanding Dry Rot

Brown rot decay with cubical like cracking.

Dry rot seems to describe decay of a substance without the presence of water. This is actually a misnomer and usage of this term dates back to the 18th century as a general description of damage present in cured or dried timber of ships and buildings, thought to be caused by internal ‘fermentations’ rather than by water moisture. Also referred to as "brown rot decay" dry rot is actually caused by certain fungi that deteriorate timber without an apparent source of moisture, even though it is a fact that all wood decaying fungi need a certain amount of moisture before decay begins. Dry rotted lumber takes on a dark or browner crumbly appearance, hence the name brown rot decay, with cubical like cracking or checking, that becomes brittle and is easily crumbled.

How Does Wood Rot When Dry?

What dry rot looks like in nature.

"Dry rot" appears to have received it's paradoxical term by these 18th century ship builders through observing that wood undergoing perpetual saturation in water, as in ships or wooden water tanks, does not rot. But wood cannot rot from conditions of perpetual dryness either. If rot in lumber does not progress in conditions of either high moisture saturation or total dryness, what, then causes the condition of dry rot in lumber? Dry rot advances in the transitional conditional cycles from moisture state to dry state. The fungi responsible for dry rot are inhibited in an environment of moisture saturation and dormant in completely dry conditions. The fungi are most active when wood becomes slightly damp as it cycles from wet to dry. As damp wood drys out fungi will advance rapidly through he wood creating the most damage to lumber. The rate of dry rot progression is also slowed in colder weather and accelerated when it's warm.

Stages of Dry Rot

The life-cycle of dry rot can be broken down into four main stages"

• The Migration stage
• The Hyphea stage
• The Mycelium stage
• The Sporing stage

In the spore stage, the microscopic spores can resemble a fine brown dust that is transported in the air. Spores which have landed on damp or moisture laden wood will begin to grow fine white strands known as hyphae. As they grow they will eventually form a large mass known as mycelium. The forth and final stage is a fruiting body which pops our new spores into the surrounding air. It is during the mycelium stage that the fungi have done the most damage to lumber by digesting the parts of the wood where the mycelium have penetrated, which becomes evident when the wood fully drys out.

How the fungi responsible for dry rot migrate through air born spores. Here shown are hyphea as they would appear on wood infected with dry rot fungi. From hyphea mycelium will grow and creep over the surfaces of damp lumber. At the peak of the growth cycle the fungi produce spores.

The Costs of Dry Rot

With sever roofing damage all the roofing lumber will also have to be replaced

Once dry rot takes hold and attacks the wood the resulting decay results in significant loss of stiffness and rigidity of the lumber. Unfortunately, dry rot is an infection which is extremely difficult to eradicate and requires replacing all lumber infected with the fungi. This is why roof leaks are so damaging to your roof. If you have a long time roof leak the moisture will eventually rot the felt underlayment and allow water to saturate the roofing deck and rafters. Summer then comes and the warming of the damp wood promotes the rapid growth of the fungus responsible for dry rot. Longterm exposure of roofing lumber to moisture usually means extensive dry rot decay and serious damage to a roof. The longer you let dry rot go the more costly the roofing renovation becomes because of the cost replacing the roofing lumber which has been dry rotted.

What To Do About Dry Rot

Bulging paint surfaces can reveal dry rot taking place underneath.

Fungi exist to break down dead wood products. It does the necessary work of breaking down dead wood so that it can be returned to the earth as soil. In the same way, this fungus growing in the roof of our home since the result will be the gradual breakdown of roofing lumber. Dry rot, though, can be prevented by making sure there are no leaks in the roof. Visually inspecting the external roof for shingle damage during the summer, and attic areas for leaks during winter are necessarily part of a proactive strategy in maintaining a healthy and long lasting roof. Preventing dry rot is a lot easier than trying to get rid of it once it gets started. For more information on homeowner roofing tips and strategies please see our blog on the subject of roof preventative maintenance, and if you are minded to tackle minor roof repair yourself check out our blog for tips on DIY roofing repair. As always please reach out to us if you have questions or feel it's time to replace your roof.

Crown Roofing

Your Local Neighborhood Professional

Completely rotted lumber must be replaced if infected with dry rot fungi. If dry rot has gotten out of hand on your roof, never hesitate to call your trusted local neighborhood professional, Crown Roofing. Over time dry rot will cause damage which requires replacing all lumber which harbors the fungi responsible dry rot. We're the roofing professionals and have the experience and equipment necessary to take care of your roofing needs. If you are ready for a roof renovation please contact us for a straight and honest roofing estimate. Remember, the sooner you take care of problems like dry rot the less expensive the cost of roof replacement when that time comes around.

Leave a Comment:


Roofing Talk May 12, 2020 at 10:24 AM

Makes sense to me. It's easy to pass on looking at roofing issues but now that summer is nearly here I'm going to get up there on the roof and have a look.

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