Wood rot is decay caused by fungal growth in damp wood. Wood is porous and can easily absorb water allowing for fungal growth to start. When timber becomes damp enough to have 20% or more moisture content and can’t dry out quickly enough or it repeatedly gets wet, it creates the ideal conditions for wood-eating fungi.

Simply stated: Wood + Water = Wood Rot

It depends.

Some cosmetic nonstructural wooden features and trim pieces on your home’s exterior, with only marginal wood rot, likely don’t need to be replaced. They can be filled using a putty knife to apply a high-quality exterior wood filler and painted. Then the damage is repaired and will look great for years.

However, sometimes it’s easier, and less expensive in the long run, to just replace the wood. For example, if the rotten parts comprise 15 percent or more of the item, they should be replaced. Additionally, it’s best to replace the wood if the item is structural. To do otherwise can be dangerous and could cause part of the structure to collapse.

For wood that’s inexpensive and easy to find, it’s usually best to replace it rather than repair it with an epoxy wood filler and wood hardener.

The simple answer is: Stop the wood from getting wet and give it time to dry out. Basically, wood rot is caused by undue amounts of moisture getting into the wood. When speaking of fascia, it typically results from gutter overflow and insufficient drip edge flashing behind the gutter. The constant exposure of the wood to overflowing water causes the wood to rot. If enough wood rot occurs in the fascia, then water continues further back into the wood until it starts to rot out things like sub-fascia, soffit, and rafter tails.

Roofing

The roof protects the interior of the house from rainwater and gets the water safely into the guttering system.

Guttering

A properly functioning guttering system plays an integral role in preventing rotted wood. The gutters channel the water away from the windows, trim, and siding. Then the downspouts, extensions, and underground drains keep water away from the foundation, basements, and crawl spaces. If these areas have too much moisture content it can lead to structural damage, more rotting wood, and respiratory illness from the accumulation of mold.

Proper flashing, caulking, and exterior painting

Even with a good roof and properly maintained gutters, when it rains the entire structure will get some moisture on it. That is why you should use flashing where needed, high-quality paint, and caulking to protect the structure.

Ground/Yard sloping

The ground around the home needs to be leveled to move water away from it rather than sloping towards it.

The above factors are true of all homes, but they can be especially important in homes with large mature trees. Many times, these trees shade the house, keeping the sun from fully drying the home after a rainstorm.

Additionally, trees often drop huge amounts of debris and other organic matter causing the gutters to overflow, hold nasty water, and slowly drip. That uncontrolled water keeps the wood wet and the fungus growing, contributing to rotting wood.