Dry rot is the result of wood-rotting fungi weakening the roof beam by the natural process of decay. In short, it's a fungal infection eating away at your roof beam — and it will become widespread if ignored. Like most living creatures, a fungus requires food (wood), water (usually from rain), oxygen, and a temperate climate (not too hot or too cold). But unlike most living creatures, the wood-rotting fungi have the ability to become "zombie-like" by never quite dying; they can form a protective coating and hibernate until the right conditions re-emerge.
The Purpose of Fungi
Mother Nature wastes nothing — not even death. Death is the inevitable end of living things, but it is also the beginning of the next lifecycle. Fungi's job in the great scheme of things is to be Mother Nature's little recycler.
In the forest, when a tree falls, fungi spores take up residence and get to work chomping down the remains into soft, small spongy particles that ultimately become part of the soil — giving life to yet more generations of trees. Without fungi, there would literally be no room for new life. The earth would become one large pile of dead stuff.
(Learn more about the role of fungi in nutrient recycling.)
Your roof beams are the food source that wood-rotting fungi are designed for — dead wood. Given half a chance, fungi will leave you with rotten beams.
The Purpose of Your Roof Beam
Leaving aside the decorative roof beam, the majority of exterior roof beams are functional, load-bearing beams. Their strength and integrity are required to maintain the load, i.e. hold up your roof. In fact, in some Streng and Eichler homes the beams span the entirety of the roof. So, these are actually both exposed exterior and interior beams.
At Cross Purposes
You need your roof beams to maintain their structural integrity. Wood rotting fungi need to eat. There is a never ending battle between these two needs. Once the fungi get the upper hand, the only question left is repair and restoration of what's left of the original roof beam versus extraction and replacement. The answer depends on the extent of decay when you decide to act. The better part of valor in this war is diligent maintenance and prevention.
This post is a short, over-generalized blurb on roof beam dry rot. To get a better explanation, read Spottin' The Rotten.
The Results of Dry Rot and Poor Roof Beam Repair Revealed:
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