My dad added a porch to our house back in the 1970’s and I must say he did a pretty good job overall. Especially for someone who wasn’t a professional builder or carpenter. However, there was one critical flaw in the construction: the main sill plate running across the front of the porch was made from a non-pressure-treated 2 x 4. And it wasn’t protected from the elements.
Over the years, this board was subjected to considerable moisture from rain contacting it directly or running down the support posts (the porch was a mostly open design). And once the moisture got under the sill, there was nothing to stop it from soaking into the wood from underneath with the porous concrete retaining any moisture not immediately absorbed into the wood. The result was that the sill became extremely rotted.
The wood rot was so bad that large pieces of the board literally disintegrated when it was removed. Yet, the sneaky thing was that the rot wasn’t all that obvious until the last few years. My dad kept the board painted so that there were no visual signs of rotting on the surface. It wasn’t until the board started to shrink and crumble that the severity of the rot became apparent. The amazing thing is that the board had not yet collapsed (or at least not too much) where the roof support posts rested on it. I can only assume this is because the neighboring support posts were bearing the lion’s share of the load and the sill underneath one of the main support posts was still intact.
The end of the sill under the corner post was the worst. It was more powder than wood. The post itself had also started to rot. I’m not 100% sure but I think this rot was of the wet rot rather than the dry rot variety because it lacked the orange fruiting bodies and white mycelium growth that is characteristic of dry rot. Both types of rot are caused by a fungus that takes hold when sufficient moisture is present but wet rot requires a higher moisture content (about 50%) than dry rot (about 20%).
Regardless of what type of rot it was, corrective action had to be taken to prevent the front of the porch from sagging or perhaps even collapsing. This made for a weekend renovation project for my brother and I. In a nutshell, the job involved erecting temporary support posts to take the load off the existing posts, ripping out all the rotten boards, replacing them with pressure-treated ones, and then removing the temporary supports once the new ones were in place.
What did we learn here? Well, for one, it’s important to use pressure-treated or naturally decay-resistant wood in outdoor applications where the wood comes in contact with water, especially if the water is likely to be trapped under the wood or otherwise prevented from draining/drying quickly. That’s rather obvious. But also, I think this experience highlighted the importance of building outdoor structures in such a way that the wood is shielded from the water as much as possible, regardless of whether the wood is decay resistant or not. In this case, I’m sure the sill would have fared much better if the porch was better sealed from the elements and the bottom of the sill was caulked to prevent water from seeping underneath it.