Question about Waterproofing a Porch Floor

Question from Wayne S.: My question is about waterproofing a porch floor on a two story porch. I rebuilt a two story porch last year on a 120 year-old Victoria. I put 3 coats of porch and deck paint on it.

The upper porch is leaking to the lower porch. We get a lot of wind, rain and snow here.

The floor is 5/4 tongue and groove fir parallel with the house and has a 2% slope. The boards are parallel with the house. Before putting down the flooring, we primed all the wood on 6 sides and all boards were tight fit.

I suspect the boards are shrinking a little. Is more paint the answer or any other finish? Or is it still part of being new?

Two story porch
Two story porch
Answer from Dave: You ask a very good question about waterproofing a porch floor.

I think you should be able to minimize the leakage but unless you have a totally different flooring system it will leak. The tongue and groove has to move based on weather conditions, moisture, and humidity or it will cause you other issues.

When it moves it obviously creates gaps. Let me also ask Gregory, owner of Vintage Woodworks, for his thoughts as he has years of expertise with porch floors.

Response from Gregory about Waterproofing a Porch Floor

Gregory, owner of Vintage Woodworks: I agree with you, Wayne, that running the porch boards parallel to the house is part of the issue, as any water that gets into the cracks between boards has no place to go but down.

However, even if the boards were installed perpendicular to the house, it is likely the problem would persist, as it is not feasible to get a waterproof seal between painted boards.

I believe the solution to waterproofing your porch floor (as much as possible) is to seal the surface with a thicker-than-paint coating that has fibers in it, thereby (hopefully) preventing moisture from getting into the cracks between boards.

Unfortunately, I do not know any specific brand or coating to point you to, but I'm sure a bit of Internet research will provide multiple choices.

Most of these types of fiber coatings that I've seen installed are on commercial buildings. They are also used on metal and other types of roofs and could be described as a thick coating sometimes applied with a mop.

They are usually black or silver.

Because of their thickness when dry and their fiber content, coatings of this type resist the hairline cracks that paint typically develops, and it is those tiny cracks that allow moisture to penetrate the surface.

I personally doubt caulk and repainting will be much of a solution, as the space between boards is typically too small to permit good caulk adhesion, and at best, caulk and paint cannot be expected to waterproof a horizontal or slightly sloping surface.

However, caulking the seam between baseboard at walls and the flooring might be worthwhile, as this crack is often wider in places.

Please let me know what your research uncovers for fiber coatings so we can share with our readers.

Update: We found a product that enables you to waterproof your deck by channeling water from the underside. Take a look.

Close look at upper porch
Upper porch floor
Waterproofing a deck or porch floor is not necessarily simple but there are measures you can take to make it as leak proof as possible as noted above. We will add to this article as we learn more.

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