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Waterpoofing on-grade sill plate/foundation of wall built on existing patio

Waterpoofing on-grade sill plate/foundation of wall built on existing patio


Old 03-21-17, 05:27 AM
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Waterpoofing on-grade sill plate/foundation of wall built on existing patio

I'm replacing the rotted sill plate/bottom 1' of plywood/studs, etc. of a retrofit room that connects the main house to the garage and was built on top of a former patio concrete slap. (It was built before PT wood was a thing.) Unfortunately, it was built on-grade (possible 2-3" above the ground), but years of neglect have allowed the soil to build up next to the house (which I'm also rectifying). The exterior tiles don't overlap the concrete and instead end ~1" inside the edge of the slab.

I'm going to remove the bottom row of tiles to replace the plywood and sill plate and am trying to decided my best option to further waterproof it before replacing the tiles. I'm thinking of using roll flashing on top of the plywood and bent (twice) over and down the edge of the slab.

I can't decided if I should first use a leak barrier (such as GAF | StormGuard Water & Ice Shield) to overlap the plywood/slab and then install the roll flashing on top of it? Will water be more likely to find it's way "up" between the leak barrier and the concrete as opposed to the concrete and flashing? (If the laid barrier were to come unstuck from the concrete.)

Any of advice, ideas, suggestions? Is the leak barrier overkill?

I realize with this existing mess there are no truly permanent fixes, but I'm just looking for something that'll last as long as possible. Keep in mind, I may not be able to lower the surrounding soil much...I could conceivably build a little trench and fill it with gravel though if that will help

Thank you
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Old 03-21-17, 06:01 AM
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Without a photo it's hard for me to respond with specifics. However, it sounds to me like you should abandon wood and use a PVC product like AZEK.
Old 03-21-17, 06:26 AM
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You mentioned "tiles". What are these tiles and where are they located?

First, I would work very hard to get the ground/soil as far away from wood as possible. Do some serious grading to get the soil lowered and sloping away from the house. Without that you are really fighting a loosing battle especially if snow can pile up against the house.

I would use everything you can. The water and ice shield can help stop water that might wick up under the siding. You'll still have one area of major weakness and that's where it meets the slab. Splashed water from rains and from melting snow that gets into and on top of the slab can still wick inside to soak the sill plate. I would NOT install the ice shield all the way to the top wall. You still want the outside of the wall to breathe so I'd probably only do one roll width high.
Old 03-21-17, 07:29 AM
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@Tony: Thank you, I'll think about it but part of this project is me hoping to use some some existing material I've been endlessly storing. I already have PT plywood.

@PilotDane: Sorry, I'll try to clarify. The exterior of house are ceramic (or probably that one word I'd like to avoid saying) tiles attached to the sheathing. They run all the way down to the slap, and were originally sealed with a large strip of caulking.

I actually may be able to grade the soil but it will be significant work, I live on a slight slope but my backyard has two levels. One wall of this room abuts a 2' alley next to the house between the backyard and driveway, the rear wall is along the higher portion of my backyard (15' wide x 25' deep)

I only intended to do one roll width of the ice shield along the bottom seam of the sheathing and slab. As for it were behind the flashing in this picture: http://stocktonproducts.com/pix/originals/TSF.jpg. Maybe 1' up the sheathing and 1' over the edge of the slab and down. I was then thinking of installing the roll flashing very similarly to that picture, again up the sheathing 6"+ and then down over the edge of the slab 6". Then reinstalling the tiles back down to the "ledge" formed by the flashing.

I realize the picture illustrates a correct drip edge. I was hoping that the ice barrier sticking/sealing the metal flashing and the concrete down below the edge of the slab (and grade) would be sufficient to keep the water running back up into the sill. (As long as the soil is monitored and kept a couple inches below it.) I figured if necessary, a gravel trench could help as well.

Apologies if there are any typos, I'm writing this on my phone.
Old 03-21-17, 08:26 AM
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So you have asbestos shingle siding? That type of siding weathers and paints well. The asbestos is really on a concern if you saw, drill or need to dispose of it.

Regrading might be a lot of work but in the long run it will be worth it!
Old 03-21-17, 10:29 AM
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Not 100% sure but I assume they are. And yeah, thank you, shingles would be the correct term.
Old 03-24-17, 03:45 PM
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Hey, I've been meaning to get back here and ask this but been busy.

Regrading is definitely doable...And really wouldn't be that difficult right now, my entire backyard is just soil (no grass) as it's in the middle of it's own (multi-year) renovation.. I'm attempting to get it under control from the over grown jungle it was a few years ago after years of neglect.

Regardless, I still think I'm going to need leak barrier and some sort of flashing...as you can see from the attached pictures (click to zoom), the shingles still come down flat on the slab, without any overhang.

If I use a single piece of 8"-10" roll flashing (on top of the sheathing, under the last row of shingles), what shape should I bend it into and how far should it overhang the slap?

Thank you


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