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Community and Q&A

Waterproofing a Porch on a Below-Grade Slab

FrankCili | Posted in General Questions on

I bought a home which an attached sunporch. The sunporch was built on a concrete slab and has water damage. The slab is below grade and water is finding it’s way in between the wall studs and the concrete slab. I started digging out around the foundation and my plan is to do a French drain around the foundation and send it to the edge of my property. The plywood on the bottom of the sunporch is all rotted and needs to be replaced. There’s is a small 6 inch flashing that is sandwiched between the stud wall and the plywood and extends below the siding into the ground. I’ve been researching options I have to waterproof the walls and I can’t find any similar structures that are built this low. Has anyone had similar issues or have any idea for what I should do. All ideas welcome. Thanks

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  1. Expert Member


    All flashings will eventually succumb to water if it sets up against the surface and pools. A picture would help, but otherwise I'd start with a drain of some kind to get the water away before it is ever an issue. You may need to cut the grade back and build a small retaining wall for this to happen.

    1. FrankCili | | #3

      Here are some picturess

  2. walta100 | | #2

    One has to wonder if your “sun room” started out in life as a slab on grade patio with no foundation.

    If as you said “The slab is below grade “ is lower than the ground around it something will need to change you can lower the grade or raise the building.


  3. Expert Member

    That doesn't look substantially below grade, and should be easily remedied by a good drainage system. The rot you see appears to me like it has been caused by that window leaking.

    1. FrankCili | | #5

      I appreciate the quick responses. Yes that picture shows the window leak but around the whole bottom edge the plywood is rotten. My thought was to cut it out and replace it then flash it. Do you think that’s a good idea

  4. walta100 | | #6

    Thanks for the photos.

    It is hard to tell what the grade is doing in photos. I think you need to remove enough dirt so that the siding is 6 inches above the new grade and the new grade falls away from the house with ¼ inches of drop per foot for the next 10 feet Do you have the 9 inches for something like that?

    Given the amount of water on the floor it seems unlikely that much water came from a window leak.

    What is the building permit situation for this addition? I am guessing they never got one and there is no foundation under the addition.


    1. FrankCili | | #7

      Walter, Thanks for the replies. I have about six feet on the one sided until my neighbors yard, The yard is pitched slightly toward the structure. the other sides I might be able to remove enough dirt to get the yard down. I'm in the process of doing some landscape removal around it as you see in some of the photos. You are right, the water on the floor is defiantly not from the window. I've watched it when we've had heavy rain this summer and all of the water is coming from in-between the stud and the concrete. What happened was the flashing was installed in-between the studs and the plywood leaving the plywood exposed to outside conditions. Then landscaping was added bringing the landscaping even higher than the siding. The landscaping had one of those plastic tube edging which allowed for water to pool, especially in the area the down spot from the gutter exhausted water into the landscaping. With continual pooling of water the lower 4 inches or so of plywood rotted out. The structure was then flooding and at that point the interior was ripped out. The house was my grandmothers so I have a decent understanding of the history before me. the structure was recognized as an addition in 1970 about 6 years after the house was built as shown on my appraisal. I believe the original addition was a screened in porch and my uncles finished the walls and did the siding years after. I have not dug enough to know how deep the slab is.

  5. Expert Member
    PETER G ENGLE PE | | #8

    The best repair is to get the grade level lowered at least a few inches below the siding, with the grade sloping away from the house as mentioned in #6 above. Second best is probably to replace the base of the wall with waterproofed masonry extending above grade. When that's not possible, this is my approach:

    Dig a trench around the building for installation of a French drain that drains to daylight, ideally 6"-8" below the top of the slab. Remove the sheathing at least 8" above the finished grade and replace it with pressure treated plywood sheathing. Install peel & stick rubber flashing over the sheathing, lapping it onto the edge of the slab. Use primer for best adhesion. Install a vinyl or metal flashing to protect the rubber from sun and trimmer damage. Bring gravel up to grade level and/or use a dimpled drainage mat to drain surface water into the French drain system. Absolutely divert roof gutters away from this area, depositing water on the surface as far from the building as possible.

    IMO, the biggest issue with this approach is that you have a wrong-side vapor barrier on the wall below grade. Depending on interior humidity, the base of the wall can get damp from condensation and it has very little potential for drying to the inside, even with vapor permeable interior finishes. This leaves the interior base of the wall somewhat at risk for dampness, mold growth, etc. But short of replacing the bottom foot or so of the wall with masonry, this has been the best approach I've seen.

    1. FrankCili | | #9

      Thank you Peter, this is super helpful. I will do some research on some of the product you mentioned. My main concern has been how am I going to seal up the structure at grade level. You touched on all of this. My question is how high and how low into the ground should the rubber flashing extend? If you have a specific product that you use for the peel in stick rubber flashing along with the metal flashing let me know. I'm not too familiar with the product. Appreciate the help!

  6. walta100 | | #10

    My opinion in the long run it is next to impossible the make and keep a water out of the building if the water is allowed to collect on the exterior and build up to a level above the top of the slab. The pressure of the water will find every crack or pin hole and seek it own level. Any sealant over a large area is a temporary victory.

    If you decide to seal resign yourself to the fact you are building a boat and everything below the water line must be a 100% impervious.

    Fix the grade and be done with the problem not ease but permanent.

  7. FrankCili | | #11

    I completely agree with that. The French drain is my best option to keep water away from the room. I do want to make sure I replace all the rot and make sure the building is sealed. I do live in an area with snowfall in the winters so sealing the structure is important as well

  8. FrankCili | | #12

    UPDATE: this past week I’ve been working on replacing the rot of my 3 seasons room and working on the water issues. My plan is to seal the structure of the room first and lower the grade of the yard. In the spring I will install a French drain to fully eliminate all water around the structure. When I started this project the landscape stone covered the last row of the siding and water would pool on the outside of the structure then find its way in. The builder used a metal flashing on the studs that covered the transition of the slab but this left the sheathing as the last line of defense. Since then the landscape has been removed and the concrete slabs that it is sitting on is now visible. My strategy for replacing the rotted plywood was to remove the bottom two rows of siding. I cut the bottom foot of sheathing out. I opted for zip sheathing. Tapping the bottom edge of the board and the transitions from one board to the next. I then used an all purpose outdoor sealant at the bottom edge of the sheathing to eliminate Any air gaps. I then cleaned the side of the slab with a wire brush and used a tar like product meant for exterior walls to seal the slab and the transition from sheathing to slab. I would like to see what you guys thinks. Always looking to learn. I’ve attached pictures.

    Moving onto the interior I did want to ask a question about insulation. I plan to insulate the walls but I’m not sure the best method of insulating the dead space between the roof and the drywall ceiling I will be installing. Is it safe to use faced bat insulation on top of the drywall? I just worry moisture will be an issue. Right now there are 1 inch holes at the eves I between the studs and no roof vent. What do you guys think?


  9. FrankCili | | #13

    Here are pictures of current state

  10. FrankCili | | #14

    Here are some more pictures

  11. user-2310254 | | #15


    It would be helpful to create a swale that helps the water to move away from the foundation. I also would avoid putting any foundation plantings back into that space.

  12. FrankCili | | #16

    Thanks for the reply Steve. I will definitely not put another landscape bed around the structure. The plan is to install a French drain bringing the wash stone up to the surface. Most like will turn into a swale based off my yard pitch. Good news is the French drain that will be installed in the swale will deposit the water 30 feet at the back of my lot where I have another swale

  13. user-2310254 | | #17

    While you have the walls open, it wouldn't hurt to air seal as much as possible with caulk and canned foam.

    1. FrankCili | | #18

      That's a good idea. I will caulk the seam between the 2x4 and the slab and I will use spray foam to seal the gab between the sheathing and the same 2x4 on the floor. I will also use spray foam at the seam of the old sheathing and the new.

      Thanks Steve

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