Exterior wall flashing (C) Daniel Friedman Guide to Siding or Wall-to-Slab or Foundation Top Flashing
Wall & Foundation Flashing for Building Exteriors

  • FLASHING WALL to FOUNDATION or SLAB- CONTENTS: Details on where & how to install flashing at the intersection of building siding or building walls with a horizontal surface such as a projecting foundation wall or a concrete porch or floor slab.
  • POST a QUESTION or READ FAQs about how to install wall, foundation, siding flashing at horizontal surfaces on buildings
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Building wall or siding flashing details at horizontal surfaces:

Wall or siding connection to foundation top, porch floors, slabs: flashing materials, choices, installation details & specifications: this aritcle describes the cause, repair and prevention of leaks & rot damage at a wood framed wall that was not properly flashed and sealed at the wall bottom. A wall or siding needs special detailing to prevent leaks and damage where the wall encounters any horizontal surface such as a foundation wall top or a concrete porch slab.

This article series provides details of foundation tops, building walls, exterior trim, wall corners, and roof-wall intersection flashing to prevent leaks & water damage. We describe the different flashing materials that can be used, how and where flashing should be installed at various building wall intersections and corners, at the intersection of a roof and building wall, at other locations, and how to seal these locations against leaks or insect damage.

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Guide to Siding or Wall Flashing at Horizontal Surfaces

Wall to slab flashing leak repairs (C) InspecApedia MSOur photo of foundation top flashing at page top shows a couple of problems: the foundation extended past the building wall, forming a shelf that invites leaks as rain runs down the building wall. The metal flashing placed on wall top slopes back towards the building, directing water inside the structure.

Details like this risk rot and insect damage to the structure. Maybe, in this case, the flashing extends "upwards" behind the building siding, forming a zee-shape that might reduce this risk. Without further inspecting we don't know.

At left and below are photographs provided by the reader whose porch wall flashing and leak damage repair are discussed below.

[Click to enlarge any image]

At above-left we see aluminum wall siding (probably 1960's vintage) installed on the enclosed wall of a porch on an Indiana home. The intersection of the corner column with the wall and the explosed plinth block at the column base make me think this was originally an open or screened porch later enclosed to add three or four-season-occupiable space to the home.

Reader Question: exterior porch wall-to-slab flashing damage, repair, design details

Here are pictures of my closed in porch, and two sketches of how I was thinking of repairing the wall (or having it repaired). In the last two pictures you can see that in the past someone cut out some of the old sheathing and put in chip board, which rotted along the bottom because water got in. You can see that the flashing, only comes up less than an inch.

Proper wall-to-slab or wall-to roof flashing and sealing deatils are important if we are to avoid leaks, rot, insect damage, mold contamination or other unpleasant problems over the life of a building.

Wall to slab flashing leak repairs (C) InspecApedia MS Wall to slab flashing leak repairs (C) InspecApedia MS

What we don't know from the house exterior is just how much leak, rot, and water damage may be lurking behind the aluminum sided wall, but the fact that this is 1960's vintage-looking siding would lead an experienced home inspector to be alert for rot and damage. Aluminum wall siding is not a waterproof building skin; the bottom edges of each siding course are clipped together, often including vent openings. Wind-blown rain can easily enter the wall at all sides of the windows as well as between siding segments and possibly at the wall bottom near those corner posts.

Reader's remarks continue:

Above where you see the rotten chip board, everything in the walls seems to be fine – studs, drywall, etc. Below that, it’s all rotten. There must be some studs that aren’t all rotten because the place hasn’t fallen down, but in the spot in the pictures its rotten.

So I am hoping that we can fix the water problem by adding some fairly tall flashing, probably with a small board under it to make it slope across the concrete and shed water, and cut out the rotten plate and the bottoms of the studs and replace them with a new plate and short studs screwed to the sound parts of the existing stud. I am not sure what to do with the places at the bottom of the “posts”.

I think the little blocks of wood at the bases of the posts also catch water.

In good construction house wrap and flashing details aroudn the porch windows would have redirected any such water back to the building exterior. But a retroft porch enclosure installed decades ago may hide some surprises

Wall to slab flashing leak repairs (C) InspecApedia MS Wall to slab flashing leak repairs (C) InspecApedia MS

Indeed, as we'd predict, leaks in the wall, particularly around the wall bottom J-channel have rotted the siding. But on a closer look at the photos I have to correct myself - which is a warning about assuming too much when you are an ignoramus: click to enlarge each of the photos above and you'll see that:

The siding looks like vinyl, not aluminum, suggesting a more recent installation

There is a section of OSB run horizontally along the bottom of the wall - that light yellow coloured material; it has rotted from wall bottom leaks.

Above the OSB we see the chopped-off bottom of fiberboard sheathing, telling us that the original porch enclosure may have been as old as I thought, but the wall was repaired more recently, probably within the last 10-15 years or even less.

The absence of housewrap on this wall means that wind-blown rain could have leaked into the wall at windows or at higher wall sections. The wax-impregnated fibergboard is pretty water-resistant but the wall-bottom J-channel trapped water and allowed those basidiomycetes (wood rotting fungi) to go to town on the OSB. You can bet that when the wall is opened we're going to find some rot or insect damage or both at the wall bottom sill plate and at the lower portions of the wall studs.

The rotted OSB as well as deterioration of the plinth block below the corner column (above right) tell us that the more recent repair, probably performed in response to leaks into the enclosed porch, was not the best, and that it too leaked.

Reader's remarks continue:

So my main questions are these:

1. Could my ideas work? Should I do it with the existing siding, which is still pretty flexible and in OK shape, or replace the siding?

2. What should I do about the corner posts? Would it help to have the posts go all the way down straight, with no wider block, or would it be best to remove the posts altogether?

By remove the posts, I mean remove the outside planks; the posts are hollow boxes, maybe with something inside, and maybe by removing the outside planks we could just make a smooth wall right out to the corners. Here are my sketches of proposed repairs:

Wall to slab flashing leak repairs (C) InspecApedia MS Wall to slab flashing leak repairs (C) InspecApedia MS

Any advice you could give me would be great. - S.M. , Terre Haute, Indiana, USA, 24 August 2015 [by private email]


Tite Seal flashing tape (C) InspectApedia CPWhat you propose looks almost exactly right. What remains to be considered includes

  1. How to seal the top of wall-to-slab flashing: We need to be sure that water that can be blown into the upper siding on the wall cannot run behind that wall bottom Zee flashing.

    If you were going to remove all of the siding for this repair you'd install new housewrap and bring it down over the outside of the top of the zee flashing.

    If you are just removing bottom courses of siding, then you might get away with gooey flashing tape (image at left) that is taped over the top of the flashing and whose upper half is well-bonded to the plywood or board wall sheathing.
  2. Avoiding holes in flashing: Similarly we don't want punctures in the zee flashing that might be caused when nailing the bottom siding ocurse. To that end, the use of flashing tape can solve the problem as it should seal around nails.
  3. Sloping flashing over horizontal projections or surfaces: As you are using a small spacer block to give durable slope to the otherwise too-horizontal portion of the zee flashing, I suggest that you either use tapered wood (cedar shingles would be ok) or the heaviest metal flashing you can obtain. I don't want someone to later bend down or punch into the unsupported but sloped portion of the flashing.
  4. Other job repairs to be combined: Going to this much trouble means we don't want to do this job twice. So I'd think about replacing those plinth blocks under your corner columns with pre-fab fiberglass open bottom vented units or at least replacing them with sound, sealed treated wood.
    See PORCH COLUMN REPAIR or REPLACEMENT for some ideas about repairing your porch columns and venting them.

Reader follow-up:

I will definitely replace the plinth blocks with something like what you suggest; I replaced them once already with whatever wood I had around just to keep from having open holes, and they are rotting again. I should have realized I needed to do more then (maybe 5 years ago).

Let me be sure I understand a couple of things.

  1. I'll guess from 1 that if we don’t take off all the siding and do new wrap we should basically cover the sheathing with gooey flashing tape as far up as we can reach? The old wall sheathing, by the way, is some kind of fiberboard-tarpaper combo.
  2. In 2, do you mean I should run a strip of flashing tape over the flashing where I’m going to nail on the bottom course of siding, so that I’m nailing through it and the flashing, and the tape should seal up those holes?
  3. In 3, I think you’re saying get some cedar shingles and cut off the thin ends to make them the right length to go from the wall to the edge of the concrete, and then set a whole row of them down on the concrete so that the flashing can rest on them and not get bent down. Is that right?

Reply: more details about flashing and sealing a wall to a concrete porch edge or projecting foundation wall

Celotex brand Fiberboard wall sheathing (C) Daniel Friedman
  1. You don't necessarily need to remove all of the siding on the walls to be re-flashed, though depending on what structural repairs are needed you may end up doing that.

    Certainly I would not leave in a wall cavity fiberglass insulation that has been soaked. But you do need to be confident that any wind-blown rain that enters the wall siding system will be directed down the wa

    ll and out on the exterior of the wall bottom-to-slab flashing.

    Wind-blown rain can enter at butt joints of wall sheathing if they are not tight and where housewrap was omitted.

    I suspect you may find it difficult to get the wall flashing top sealing flashing tape to adhere well to Homasote (fiberboard) type wall sheathing materials as that sheathing was often wax-coated.

    The decision to pull off all siding and install proper housewrap depends in part on what you find: evidence of a history of leaks through the siding such as water stains on either side of the wall would bear consideration, as will evidence of more insect damage or rot damage to the wall studs than was apparent at the wall bottom


    If your wall sheathing looks like the material shown at left, that's a fiberboard product.

Self adhesive flashing tape (C) Daniel Friedman
  1. The flashing tape runs over the top of the metal flashing and is bonded on its upper half to the wall sheathing and lower half to the metal flashing. There should be no nails below the area covered by the flashing tape.

    At left you can see that the peel-off paper exposes a nice and gooey sticky surface of the flashing tape.

    But if you find that you need to nail the bottom edge of siding (as you may need to install a J-channel to hold the first (lowest) siding course down at its lower edge) in that case I'd cover the entire vertical surface of the metal flashing with flashing tape so that it will seal around any nails.

    Do not seal the outlet end of the flashing - we want any water that enters the wall to run on the outside of the vertical flashing, meet the sloped near-horizontal bottom bend of that flashing and run out over and past the end of the projecting concrete slab (or foundation in other applications).
  2. Supporting the sloped nearly-horizontal bend of the wall-bottom flashing: If you use thin aluminum flashing it may be easily punctured or bent down and then may fail to drain properly. Your support block would be enough if the flashing is thick copper but otherwise supporting it with tapered cedar shingles cut to fit will make for a more durable repair.
  3. At the bottom edge of the near-horizontal flashing make an additional drip-edge lip or bend (that I will illustrate here) so that water runs off of the edge of the flashing and drips to ground rather than running down the slab or foundation wall face.

Porch Slab to Wall Flashing Leaks

Below are examples of another wall-to-porch flashing problem that is more difficult to solve. The house exterior wall terminates at a concrete slab (over a basement) without proper sealing. Though there was wall-bottom flashing, wind-blown rain or melting snow can send water back up under the flashing's near-flat lower extension onto the slab. Water leaked into the "basement" space that extends below the ecxterior slab (below right).

Porch wall to floor juncture exposed to the weather (C) InspectApedia Porch wall to floor juncture exposed to the weather (C) InspectApedia

A better design would have brought the wall down to a raised concrete curb that itself was part of the slab pour or built and sealed onto the porch slab; A repair that might work would be careful sealing of the flashing to the slab surface.

Flashing Siding at the Top of a Non-Projecting Foundation Wall

Flashing the siding and wall sill plate at the top of a foundation wall that does not project out past the upper wall is pretty easy. Steve Bliss' sketches below illustrate flashing at a concrete foundation wall (below left) or at a house wall where a water table trim board is installed (below right).

Figure 1-3 Termite Sheilds (C) Wiley and Sons - S BLiss Water table trim flashing details (C) Wiley and Sons - S Bliss

These less-annoying wall-bottom flashing procedures are discussed in detail at FLASHING WALL DETAILS

Flashing a Wall to a Flat Roof

Similar wall flashing problems occur when an upper wall on a building abuts a nearly-flat roof surface such as the roof shown at below left. At below right is our recommended flashing detail [click to enlarge].

If the veritcal wall is being flashed to a flat roof we expect the roof to be covered with a membrane and we seal both the upper surface of the wall - roof flashing (to the wall sheathing) or we slip the upper edge of the wall flashing under properly-installed housewrap.

Then we seal the outer edge of the flat portion of the roof-wall flashing to the roof surface. This step is necessary on flat and low-slope roofs to prevent water from running back up under the flashing where it can enter the structure at the roof-wall juncture.

Roof wall flashing details (C) InspectApedia CP Roof wall flashing retrofit deatil (C)

This installation is discussed in detail at FLASHING WALL DETAILS.


Continue reading at FLASHING WALL DETAILS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see FLASHING ROOF-WALL SNAFU for examples of roof-wall abutment flashing foul-ups to avoid

Or see RAIN SPLASH-UP SIDING DAMAGE For more details about splashback or splash-up water damage to exterior walls, see

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