Rot due to wetting AND drying?

[Spruce_Goose]

Howdy all.

I have come across the notion many a time that wood subjected to repeated 'wetting and drying' is wood subjected to one of the worst scenarios for decay.

I am having a hard time determining the mechanism for this, or if its even true.

Increasing 'drying potential' of wood is often touted as the best thing to do if it is inevitable that it will get wet. This makes perfect sense since letting wood stay moist is going to lead to hungry fungi feasting.

But this seems to contradict the wetting/drying notion. I do understand that really saturated wood may reach a point where oxygen is restricted and decay conditions are lessened, but I don't see this being the case, save for nearly submerged wood. 

My theories are as follows:
1)That wetting and drying may cause swelling, leading to compression set, and then subsequent shrinkage and the opening of large cracks on the surface. This would allow deeper penetration of water and increase decay.
2) That it is not the 'drying' but perhaps warm sunny temperatures heating damp wood to optimal fungi feasting conditions.
3) That the whole notion is a misnomer and its really only about the wood constantly getting wet.

What say y'all?

[Don P]

I agree with you that constant cycling of wet/dry drives checks deeper which lets the moisture get deeper into the wood. A sun baked deck is a good example, it gets wetted frequently and swells, the surface dries rapidly over a swollen core. When the drying shrinkage strain exceeds the tension perpendicular to grain strength a check develops. This lowers the tension strength in that area so the next cycle the check grows, wet/dry/repeat.

[GeneWengert-WoodDoc]

Decay is an involved process.  Temperature is important as well as availability of water and oxygen.  If wetting and drying create avenues for moisture to get into the wood and then not dry as quickly as surface moisture, decay conditions can be prolonged.  A water repellent on the exposed surfaces does inhibit decay by reducing moisture contents, until there are avenues for getting beyond the surface treatment.