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Community and Q&A

Porch Floor Boards Moisture Problem

FlitchAndCo | Posted in General Questions on

Hi folks,

I’m helping a friend with a moisture issue at her beautiful old house in midcoast Maine.  On the northeast side of the house there is an original screened-in porch with a more recently added deck. It looks as though rain (that’s “wind-driven moisture” to you guys) is causing the screened porch floor boards to rot. A previous carpenter added a (not very attractive) PVC gutter on blocking to attempt to address the issue, but I’m not sure it’s helping.
Additionally, it looks like someone wrapped the ends of the deck boards with Ice & Water Shield and added a kind of sole plate made of composite decking for the screen wall to sit on. The result is that the whole thing stays damp after it rains. While the Ice & Water Shield might be protecting the ends of the deck boards for now, it’s not meant to be a finish material–it’s ugly and it’s going to deteriorate.
There are several issues that could be the cause of the problem– what do you guys think?
  • While the gutter is catching most of the rain coming off the roof, it’s proud of the roof’s drip-edge, allowing water to travel back in the house.
  • The sole plate made from composite decking is trapping moisture against the fir deck boards.
  • The deck boards were installed too close together, trapping moisture. When they swelled up, the problem was exacerbated.
  • Water landing on the more recently added exterior deck is splashing back into the decking of the screened-in porch, causing it to rot.
  • The new deck was attached directly to the house with no space, preventing water from draining
  • It’s Maine. It’s damp all the time here. Perhaps boards of a shaded north-facing porch just need to be replaced sometimes?
Curious what you guys think is the problem/solution. What is the best practice for attaching a screen wall to decking? I’m tempted to add a proper slanted threshold to the doorway and something like a water table to replace the composite sole plate, but I’m not sure if there’s an easier/better solution. Should I just adjust the gutter and replace the rotted deck boards with properly spaced material and see if that solves the issue?
Pics attached.

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  1. charlie_sullivan | | #1

    I'd start by spraying a hose onto the roof and seeing how much of the runoff is actually caught by the gutter, and adjusting that as necessary.

    Also clean out the cracks between boards on the deck to allow it to drain more easily.

    How much further to go with other repairs or modifications is a harder question.

  2. Expert Member
    Michael Maines | | #2

    Brian, I think everything you mention is contributing to the problem. Basically it's poorly designed. I've designed a lot of screened porches in coastal Maine and as far as I know, none have rotted out. We have a screened porch at our camp, built in 1930, still in good condition because it has good details. (For others reading, in some northern states, "camp" is similar to rustic cottage. Ours is in Augusta--I think Maine is the only state where your summer cottage may be in the capital city.)

    Near the coast, onshore winds and a tall house create negative pressure on the lee side, so even if your gutter was able to catch dripping rain, it won't do much when the wind is blowing, which is most of the time when it's raining here. So while gutters can be a good idea, they aren't an ultimate solution.

    The deck boards look like vertical grain fir. That's a traditional choice for porch floors in New England because they are durable and have some rot resistance. They are usually sloped toward the exterior for drainage, and accumulated water needs to be able to drain. Here is looks like the composite sill will block any drainage, if the floors are sloped at all. Even if they are sloped, having a gap between boards helps them dry, but can also allow bugs to get in.

    The composite sill is sort of a good idea, because it gets the vulnerable end grain of the wall system away from standing water. But it also traps water.

    The projecting deck boards, whether or not they are wrapped in I+WS, are not a good design, because water will wick back under the composite sill. I have used a similar detail on occasion (without the I+WS) but only when including a way for things to dry, usually with gapped deck boards, and window screening at the floor perimeter to keep the skeeters out.

    A basic building science principle is that assemblies need to dry to one side or or the other, or preferably both. There is likely damp ground under the porch, so not much drying potential in that direction. The composite sill, I+WS and enamel floor paint are all limiting drying up and out.

  3. FlitchAndCo | | #3

    Thanks Michael! Would you recommend replacing the composite sill with a piece of fir decking? Is there anything I can reference to see what a proper assembly would look like?

    1. Expert Member
      Michael Maines | | #4

      Brian, I can't find any relevant details on this computer, but if you can't rip everything out and start over, I would cut spacer blocks to replace the composite sill, then run moldings on both sides to cover the gap. Keep the moldings 1/8" to 3/16" off the decking as well. That will prevent most wicking and allow the deck boards and screen frames to dry.

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