When we trace an unexplained interior stain on a Bay Area property, the answer is a planter far more often than people expect. It is easy to see why it gets overlooked: the planter is outside, the stain is inside, and there is no obvious connection between them. But a planter is a container of permanently wet soil pressed directly against structure, and that is a much harder condition than rain on a wall.
Why planters are a harder problem than a wall
An exterior wall gets wet during rain and then dries. A planter never dries. Irrigation keeps the soil saturated year-round, and that soil holds water in continuous contact with the structure on the other side.
Standard waterproofing products are not rated for permanent immersion. They are rated for intermittent wetting. Using one inside a planter is a specification error that takes a few years to reveal itself.
The three things that go wrong
Almost every planter leak we investigate comes down to one of three causes, and often to a combination of them.
- No membrane at all — the planter was built directly against structure
- A membrane not rated for continuous moisture contact
- Drainage that was never installed, or that has silted up and stopped working
- Root intrusion puncturing the membrane from the planted side
Drainage is not optional
A membrane with no drainage behind it is holding back a standing column of water indefinitely. Even a correctly specified membrane will eventually be defeated by that, usually at a seam or a termination.
A proper planter assembly gives water somewhere to go: a drainage board or gravel layer against the membrane, filter fabric to keep soil out of it, and a weep or overflow drain that actually discharges. Blocked weeps are one of the most common failures we find, and they are also among the easiest to fix.
Block walls have the same problem
Concrete block is porous and full of cells and mortar joints — a lot of potential pathways. A retaining block wall holding back saturated soil behaves much like a planter, and produces the same efflorescence and staining on the dry face.
The fix follows the same logic: seal the wet face with a membrane rated for the exposure, add a drainage layer, and make sure water reaching the base of the wall has a route away from the structure.
Rebuilding it correctly
Repairing a leaking planter means excavating the soil, cleaning the wall face back to sound substrate, repairing cracks and joints, then rebuilding the assembly in the right order: membrane, root barrier, protection board, drainage layer, filter fabric, soil.
It is more work than coating the inside of the planter and hoping. It is also the only version that stops the interior staining permanently, which is the actual goal.