advice needed for porch floor over basement/crawlspace

13 years ago

We have a 1930's house with a block foundation and crawlspace that extends to the outside of the front porch, so that the porch is over the crawlspace. Some friends have a similar issue with their 1894 house where their basement extends to under the porch.

Both of our porches have some damage (consistent with the age of the house) and we both have some water that's getting through the porch and into the space below. My friends are left with a leaky basement, I'm left with a crawlspace that I can't seal until I can address this problem.

So far between the two of us we've been told that our only choices are to put down a solid-surface porch, or create so many layers that we'd never be able to open our porch doors, or even (in my case) get rid of the foundation under the porch entirely except for the sections under the porch pillers in order to properly ventilate it. Um... no.

We both inventoried our neighborhoods and see that at least half of the houses have foundations that go around the outside of the porches, and so have porches over basements or crawlspaces. My previous house (another 1890's house) had a porch over the basement and in 16 years I never had problems with water coming through. All of these houses have survived a lot of years with just the porch floorboards over the joists.

Help! Was there some special magic to the floors from 80+ years ago that were put above basements or crawl spaces, that kept them from flooding the space below? Any solution for us now?

Featured Answer

Robert Hengelfelt

2 years ago

I have had to deal with a few of these porches in historic districts over the years. The system I devised and I am the most comfortable with is to remove the old floor and, if necessary, re-frame to lower the floor joists. I then install 3/4" plywood, and cover with EPDM rubber. In order to keep the historic look of the old porch flooring -which is typically tongue and groove fir installed perpendicular to the house wall- I install a ribbed "G-Floor" mat oriented to direct water away from the house. I then I set floating deck panels on top of the mat. These I construct with 3 1/4" x 3/4" fir (or other naturally rot resistant wood) with 3/4" sleepers glued and screwed from below. The "G-Floor" allows water to flow freely under the sleepers with minimal additional height, and the decking can still be oriented perpendicular to the house, as it would be traditionally. I have not tried this with tongue and groove flooring because I think it's better to have gaps between the planks to allow air movement, but he 3 1/4" boards are a pretty good substitute. The 1 1/2" finished thickness of the floor panels helps to minimize the weight, which makes removal of panels easier for the occasional cleaning, and also keeps the deck surface below the typical 2" threshold height.

Comments (7)

sombreuil_mongrel
13 years ago
As I said in another discussion, the old way was canvas, painted. It's very high-maintenance. I read an old builder's guide which called for porch floors to have white lead putty caulked into the tongue and groove joints as it went together. This could be attempted with a modern sealant, but I can tell you right now that it would fail- eliminating the space between the floor boards would only cause buckling when the wood expanded. The double floor is lower maintenance, but takes 2 1/4" more thickness than the single floor you have. If that comes above the door sills, there would no doubt be some way to lower the framing members so that it ends up only slightly higher, but then I suppose you must maintain the existing headroom below, so sorry in advance for another disappointment.
The alternate approach would be to stop the water at its source; why does so much water fall on the porch floor? Could guttering be improved or awnings or other devices added that would divert it?
Then there's the first and most obvious question, which I have saved for last (LOL): does the porch floor fall away from the house reliably and evenly to the edge? There's no way it can shed water if it channels the water toward the house.
Casey
old_house_j_i_m
13 years ago
Maybe think of your porch floor a little more like a roof ...

I, too, have an old house with incorporated porches (front and back) that sit over top of a full basement foundation (brick with original stucco.) We restored the back porch as a "sunroom" (it was closed in some time in the 30's) and I noted that roofing tar paper was used between the sub floor (plank) and the original T&G porch floor. There are shreds of roofing tar paper hanging on the edges of the bottom of the front porch floor where I can find it, but the old wooden floor was replaced with plywood and then the porch floor was tiled by POs (a bad DIY with 1 inch grout lines that leak like crazy, making my repair job that much more difficult.) The pitch on both porches is good (away from the house)

There are low shingled walls all around the porches so water would be trapped without some sort of system to remove it. The front porch uses a pitched trough with 2 exit tubes at the foot of the short wall but I found no evidence of this on the back porch.

These 2 devices (the roof paper and the trough) surely would have worked well if they were maintained, alas, the metal trough is rotted thru and leaves/critters were getting into the basement till I discovered the holes and band-aid-patched them till I can address repair of the trough properly. (interestingly, the front porch foundation was hidden behind a finished wall in the basement and was only found when we had a termite inspection done.)
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Custom Craft Builders
3 years ago
Neither have a solution ?
kats737
3 years ago
Custom Craft Builders this is a 10-year old post. Do you have a solution or are you looking for one?
Kaci Billings
3 years ago
Same question! Looking at:
https://econodek.com/
https://greenbaydecking.com/duxxbak-decking/
https://tufdek.com
Trex with waterproof liner under
Stacey P
2 years ago
Has anyone found a good solution? I have basement under a screened front porch in a 1920s house. It doesn't get much moisture since there are brick knee walls, but want to have a good flooring option to make sure the basement stays dry. Just took up the outdoor carpet and found it was adhered to the original tongue and groove decking. Thanks!