I am replacing a front porch, it currently is an elevated porch with your typical dirt and debris underneath, and it is an open porch. I intend to excavate a new basement room underneath it, cutting a hole in the current exterior basement wall to access the new room. The porch will remain open (not enclosed) - so the choice of floor for the porch is pretty important since it will also serve as a waterproof barrier for the new basement room. My chief concern is making sure this porch is waterproof, ideally with a redundant layer of protection.

It seems a fairly typical choice is wooden joists, steel or pan decking, and then concrete. Very similar to this article except the area below will be a room attached to the house, and I'll use wooden joists instead of metal C-channels.

An alternative my architect proposed is a fiberglass deck. There fairly common down the New Jersey shore, here's a couple of examples. We do get a wide temperature range here though - ranging from 10 degrees F to up to 100, with this porch in direct sunlight for half the day. (My architect isn't favoring one approach or the other, he just let me know of the option.) The fact that these decks are used down the shore - where you deal with not just water but salt - seems pretty good. Some random FAQs I'm finding online is that we could expect it to last 25-30 years (similar to concrete it seems?), another says get a new gelcoat every 3-7 years. Cracks could form from settling, and repairing them doesn't seem too difficult but a large crack would compromise the waterproofness until it was repaired. I think the same is mostly true for concrete - except I think the consequence of a crack is worse, and repairing it is not as reliable?

So my questions are more on the waterproofing and concrete side (although anyone with experience with fiberglass is also helpful!) How waterproof could I expect a concrete porch floor to be? What I'm reading seems to say that concrete by itself is not waterproof, but there is some sort of coating that would make it so? How reliable is that? Also - completely separate from waterproofing the concrete or the waterproof fiberglass layer - what could I put on top of the metal decking or beneath it as a separate waterproofing layer to act as a redundancy?

  • I remember watching a show where they built a deck on top of a first story by essentially putting down a flat-roof-style membrane roof, then building the deck on top of that. At a minimum, that separates the waterproofing and support functions, so you're not having activities right on your waterproof surface.
    – Armand
    Commented Mar 13, 2023 at 7:36
  • If you had a dimensioned sketch showing a profile through the basement and porch, that would help.
    – Huesmann
    Commented Mar 13, 2023 at 12:35

3 Answers 3


You build your basement room, you roof it to waterproof, you build a porch deck on top just like you build a deck on any other waterproofed low-slope roof. And yes, the roof needs to slope.

EDPM rubber is a typical good choice for the actual roof membrane. Concrete is not a roof membrane, it's merely a substrate (or sometimes an overlay.) Felt and hot tar would be the old way, but it's not a good choice these days, as it ages poorly compared to EDPM rubber.

The porch deck is not the roof of the room below. It's above that.

  • +1 One thing to keep in mind is that the roof will need maintenance, so you'll want to consider that when designing the porch i.e.: you'll want to be able to get access to the roof without demolition.
    – JimmyJames
    Commented Mar 13, 2023 at 19:15
  • Also, don't forget drainage.
    – JimmyJames
    Commented Mar 13, 2023 at 19:16

I'd suggest redefining the problem. Rebuild the entire system as if the extended basement is the new foundation line and the "porch" is now an actual extension of the house with real walls and windows and roof built upon that foundation.

That's what keeps rain off the rest of your basement, after all. And it avoids needing to think about how you are going to put a separate ceiling over the new basement area.

Appropriate choice of windows can keep the new "sun room" feeling like a porch, if you like. Or you can call it a mud room/entryway. Or incorporate it into your current front room.

Better use of the money, I think.

  • It's a good suggestion, but I would need to apply for a zoning variance to enclose the porch, which at this point in time is not feasible (partly because my architect is on the zoning variance committee, and took on the job based on the fact I already had gotten a zoning permit and didn't need a variance.) But thank you. Commented Mar 13, 2023 at 15:50
  • So how does your architect propose you solve this problem? This is part of what he should be delivering...
    – keshlam
    Commented Mar 13, 2023 at 17:41

What to waterproof with is dependent on what you use to build the deck of the porch.

If it is concrete, you need to waterproof the surface of the concrete and seal the edges, so water does not get drawn into the concrete. For that you can use a concrete sealer that produces a waterproof membrane on the surface, such as Red-guard or Aqua-Defense. ( these products are used to coat shower walls prior to tiling , and can be the protection from water on a daily basis for years. Therefore they are proven.) Then paint the deck whatever color you desire, or place whatever surface you planned on as a final finish. ( Tile, indoor-outdoor carpet, glue down wood, LVP). As long as the membrane is not compromised with nails or screws it remains waterproof.

I would not have any additional layers under the concrete. Should any water get in would it not be able to be released directly underneath from gravity.

A secondary layer of resistance will result in water migrating to the point of least resistance and weeping through a wall or someplace where it will not be as evident and causing a lot more damage being unseen for possibly years.

If you choose fiberglass it is in itself waterproof. You would only have to be sure the edges are sealed properly.

  • I doubt very much that the interior waterproofing products such as you mention are specified or proven for exterior applications.
    – Ecnerwal
    Commented Mar 13, 2023 at 14:13
  • you doubt...but have you verified the multiple uses?
    – RMDman
    Commented Mar 13, 2023 at 14:28
  • this from just one product: RedGard Waterproofing and Crack Prevention Membrane is suitable for use on exterior concrete slabs. For further assistance please contact us at 1 800 xxx xxxx option 2. by
    – RMDman
    Commented Mar 13, 2023 at 14:31

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