Finding the Source Before Touching the Wall
The first thing I told Dave was not to start cutting into the drywall until we had a theory about where the water was coming from. In a wood-frame house, the options are usually a roof leak, a plumbing leak, or outdoor water getting through the foundation. In a brick row house, there's a fourth option that's very common in St. Louis: the brick itself.
St. Louis has a lot of brick buildings that are 80-120 years old. The original mortar in those walls gets porous with age. The flashing at window sills and where the brick meets window frames can fail. And unlike wood-frame construction where there's often a clear water path you can trace, in solid masonry construction water can migrate horizontally through the wall assembly and show up inside several feet from where it entered.
We walked the outside of the building along the wall that corresponded to the damaged area. There was a window directly above the damage, and when I looked carefully at the brick around the sill, the mortar joints in that section were noticeably deteriorated. The brick itself was slightly stained below the sill, darker than the surrounding area.
What Was Behind the Wall
Once we had a theory, Dave cut a small exploratory opening in the damaged area — about 6 inches square — to look behind the drywall before committing to a larger repair.
What he found was drywall installed over the original plaster, which was installed on wood lath attached to the brick. The drywall was 1/2 inch, the plaster behind it maybe 3/4 inch. The moisture had come through the brick, saturated the plaster, and worked its way through to the drywall. The paper on the back of the drywall was brown and soft. The gypsum core was starting to crumble at the very bottom of the damaged section.
The plaster behind it had a white mineral deposit — efflorescence — which is salt left behind as water evaporates through masonry. It looked almost like white frost. That told us the moisture had been infiltrating slowly for a while, not a single acute leak. The visible wall damage had just been the point where it finally accumulated enough to damage the drywall.
The Exterior Fix Came First
Dave's contractor, a guy named Russell who'd been doing masonry work in St. Louis for about 20 years, looked at the exterior and confirmed what we suspected. The mortar around the window sill had failed in several places, and the caulk between the brick and window frame was cracked and peeling. Water had been running down the brick face and finding a path through the degraded mortar into the wall assembly.
Russell repointed the mortar joints around the window opening — removing the deteriorated mortar and filling with new — and re-caulked the perimeter of the window frame. That cost about $350.
His advice was to wait four to six weeks after the exterior repair before doing the interior work. The goal was letting the wall assembly dry out completely. Any trapped moisture left in the plaster and brick cavity when you close the wall up will continue to cause problems. Dave put a dehumidifier in the room and left the exploratory opening uncovered for six weeks.
The Interior Repair
After the waiting period, Dave opened the wall fully from the baseboard to about 18 inches above the visible damage line — a larger patch than the visible damage suggested was necessary, because moisture-damaged drywall often extends further than it appears.
The efflorescence on the plaster got treated with a diluted masonry cleaner and a stiff brush, then sealed with a masonry waterproofing primer. This step matters: if you just re-drywall over it, the efflorescence can continue working through the new material.
The new drywall went in as moisture-resistant board, not standard. On an exterior brick wall in a Soulard row house, that's the right call. The repair was finished with paper tape, three coats of compound, and sealed with shellac primer before painting. Total interior repair materials: around $85. The contractor time to do the finish work was about $400.
The full cost: exterior masonry repair ($350), waiting six weeks, interior repair labor and materials (~$485), plus Dave's original exploratory cutting and the dehumidifier rental. Around $1,100 once everything was done. The $1,800 estimate I mentioned at the start includes the first patch that had to be redone when we found the moisture hadn't fully dried — a lesson in patience.
What St. Louis Brick House Owners Should Watch For
The warning signs that got past Dave — gradual paint bubbling near the floor on an exterior wall — are easy to miss until they're already a problem. In a St. Louis brick home, a few things worth checking periodically:
Mortar joints around windows, doors, and at the base of the brick. Mortar should look solid with no visible gaps or crumbling. Use a utility knife to probe questionable areas — it should meet solid resistance, not crumble or fall away.
Caulk around window frames. Old caulk shrinks and cracks, especially where brick and wood meet because they move at different rates. Cracked caulk at window frames is one of the most common water entry points in older St. Louis homes.
Water staining or slightly darker patches on brick faces after rain. That darkening is moisture absorption — normal to a degree, but if specific areas stay dark significantly longer than others, that section may be absorbing more water than it should.
The National Trust for Historic Preservation's guidance on repointing is useful for St. Louis homeowners since it covers how to match original mortar composition — using too-hard modern mortar in an old brick building can actually cause more damage than the failing original mortar it replaces.