Base Wall Flashing

Remodelling provides a unique opportunity to learn what can go wrong on a home.  On a home we remodelled a few years ago, after we had demoed an existing closet and bathroom that sat a couple of  feet below grade, we found water damage on the sill plate and the bottom of the studs.


The foundation had a raised stem wall that carried the brick ledge and stepped down with the grade. This room is on the corner of the house, which sits on a hilltop, and the grade begins to slope significantly at this point.  Note the water damage to studs and sill plate, and to the osb sheathing.

A rough section view from the side

A rough section view from the side

Another view of the exterior wall

Another view of the exterior wall

Close-ups of the rot and mold on the sill plate and bottoms of studs

Close-ups of the rot and mold on the sill plate and bottoms of studs

Upon inspection, we found that the base wall flashing, which in this case was standard 6-mil polyethelene, was not turned back under the brick, it was simply draped behind the raised stem wall and terminated between the stud wall and the concrete.


There is a layer of tyvek over the face of the osb sheathing, which you can just make out here under my finger, and the poly is draped behind the OSB.

So there is effectively no base wall wall flashing at this point in the wall. Any water that hits the brick ledge from outside will drain right in. Since brick is a mass wall system and is highly porous, it takes in a lot of water. This house had another problem: the grade had been  built up over the stepdowns in the beam and brick ledge, meaning that soil and mulch were covering the brick ledge by about 12-18”so that ground water was draining against the unflashed brick ledge. Over time this water rotted out subfloor, sill plate, and the floor trusses below, and caused a major mold problem in the vented crawl space.


Once we had demod and assessed the problem, we had three things to do:

  • Remediate the mold

  • Rebuild the floor system

  • Install a new base wall flashing


We installed a galvanized through-wall flashing with a drip edge, and primed and flashed it with a membrane and PolyGuard primer.


These repairs were completely unplanned, and added 3-4 weeks to the project timeline, and were very expensive to boot. My guess is that the raised stem wall condition threw off whomever was installing the 6-mil base wall flashing, and he didn’t leave enough of a layer to lap over the brick ledge. The builder just missed it.  On a house of this caliber, I would also like to see a more robust base wall flashing, but 6-mil could have worked if it had been installed correctly. We will be demoing an advanced base wall brick ledge flashing detail on an upcoming build, so stay tuned for a post on that.

As for the grade outside, he may have installed it correctly, and a landscaper or owner may have come behind him and raised it up against the brick - we don’t know.  But owners have to maintain their properties, or hire someone who will do routine inspections. For homes as large and as complex as this one, paying for a routine annual inspection by the builder or his representative would be a good idea.

This home had other problems, and it became apparent as we worked on it that the original builder had not been paying proper attention. The home was around 20 years old when we did the work, and probably 15% of the work we did was spent correcting the original work. The original owner had long gone, as had the second owner. The problems had been passed down owner to owner, and the humidity and moisture conditions in the home (there were others) contributed to poor air quality.

My remodelling experience has no doubt saved me tons of headaches like this; learning from someone else's mistakes is far better than learning from my own (which I've also done more than I'd like to admit).