DMV Waterproofing
June 23, 2026

The Mold Followed the Downspouts: A CMU Basement in Olney, MD

Case file — Olney, Montgomery County · DMV Waterproofing, Rockville branch. One of 79 projects we completed across the DMV in 2025. New here? Start with our open letter to DMV homeowners and our guide to DMV foundation types.

At a glance

Location: Olney, MD (Montgomery County)
Foundation: CMU (concrete block)
The problem: Mold returning in three exact spots, stepped cracks, cracked slab
Real cause: New downspouts dumping roof water right at the foundation corners
The fix: Buried/redirected downspouts + 124 ft interior French drain + dual sump
Timeline: 8 days
Warranty: Written lifetime · 5-star review

What the homeowner saw

The owners kept finding mold along the base of the block wall — and it kept coming back no matter how many times they cleaned it. There was a musty smell in the basement, and the wall felt damp at the bottom.

What I found

The first thing I do on any inspection is read the wall. Walking down the stairs, the mold wasn’t random — it was sitting in three specific places: directly across from the stairs at the base of the CMU wall and slab, then the far right corner, then the far left corner. The same story in three spots tells you the water isn’t coming from inside — it’s being delivered to those exact points from outside.

The block itself confirmed it. There were classic stepped cracks running through the mortar joints on the right and left, and a cluster of cracking in the middle of the wall. The concrete slab had cracked from the corners — the spots where the foundation had been working hardest. I photographed everything, took my measurements, and went outside.

I was right. The downspouts had been redone — brand new, oversized 4″×6″ — but they were dumping roof water right at those foundation corners. Every storm, for years, all that roof water was being poured straight against the block. CMU is hollow and porous: it doesn’t just leak at the cracks, it soaks the water up and holds it inside the block like a sponge that never dries. That’s why the mold always came back in the same places.

Before — a gutted Olney, MD basement with water staining and mold across the CMU concrete-block foundation walls — Before: water staining and mold across the block walls of this Olney basement.

Before — a heavy black mold band along a CMU block basement wall in Olney, MD, where roof water had soaked into the block — The mold tracked exactly where roof water had been hitting the block for years.

Before — the home's old corner sump pit beside a mold- and efflorescence-stained concrete-block basement wall — The original corner sump and a block wall covered in efflorescence and mold.

What we did

The fix had to work on two fronts — outside and below.

First, outside: we buried and extended the downspouts to carry the roof water well away from the foundation, so we stopped feeding the problem at the source.

Then, below: the right system here was a full interior French drain. Our own crew installed 124 feet of drainage to intercept groundwater before it reaches the basement floor and route it to a sump. We set the home up with a dual-pump system — a primary plus a battery backup, so it keeps pumping during the storm-driven power outages this area gets, and a secondary pump for the heavy-volume days. We scraped out the existing mold, ventilated the space, and finished the wall with an 8-foot vapor barrier to keep moisture off the living area for good. Then we tested the whole system. No subcontractors — every step was done by our Rockville crew. The project ran 8 days.

A downspout buried and tied into exterior drainage with a French drain to carry roof water away from the foundation — Step one outside: the downspout buried and routed away from the foundation.

After — interior French drain, dimple-board drainage and a new white vapor barrier installed along the basement walls — After: interior French drain, dimple board and vapor barrier along the walls.

The result

Bone dry. Through testing and the storms that followed, the basement stayed completely dry — no dampness, no return of the mold. The homeowners left us a 5-star review.

After — the new sump pump with battery backup that keeps the Olney, MD basement dry during storms — The new sump with battery backup, sized to keep pumping through storm outages.

The takeaway

On a block foundation, mold that keeps coming back in the same spots is almost never a cleaning problem — it’s a water-delivery problem. Read where the mold is, follow it outside, and you’ll usually find a downspout or a grade pushing water at the wall. Fix the source above ground and relieve the pressure below, and the wall finally gets to dry out.

Seeing mold or damp block that won’t go away in your Olney or Montgomery County basement? Schedule a free inspection — I’ll read the wall, find where the water is really coming from, and tell you honestly what it takes to fix it. Learn more about our interior French drain & sump systems.

FAQ

Why does mold keep coming back on my block (CMU) basement wall even after I clean it?
Because concrete block is hollow and porous — it absorbs and holds water inside the wall. Until the water source outside is stopped and the groundwater below is drained away, the block stays wet and the mold returns. Cleaning treats the symptom; drainage treats the cause.

Written by Selcuk Altan Atasoy — civil engineer (University of the District of Columbia) and licensed waterproofing inspector (DC, MD & VA), in the field since 2005. I started in waterproofing at ECS in Chantilly, VA, inspecting foundations across the DMV — from single-family homes to apartments and parking structures to hospitals — and I now inspect residential basements for DMV Waterproofing.