I have a below grade 2 car internal garage.  Basically the house is built into a slope that rises toward the back yard with the garage doors on the front of the house.  So the rear wall is completely below grade with the left side going from partially below grade (door side) to completely below where it meets up with the back wall.  When we first purchased this home in 2002 there was evidence of water penetration on the back wall.  We did some grading on the rear-outside that showed some improvement, but we still were getting some water in the garage with heavy rain.  Next was the downspout.  I dug up the pipe that ran down to the street and found that it was collapsed in places and full of mud.  So there was not much water getting through to the street.  So we replaced that as well.  I thought my problems were solved until the next deluge.  There was still some water building up on the walls and eventually trickling down toward the floor drain in the center of the floor.  As far as home-owner performing tasks, I believe that I exhausted things I could possibly do myself to remedy the water problem.  I got some estimates from a few basement waterproofing companies.  They all wanted to put an interior weeping tile system combined with a sump pump.  I was reluctant to go this route because I wanted to prevent the water from entering my house.  Allowing it to enter and then pumping it back out seemed kinda silly.  When I asked them about excavating on the outside they would balk at that solution.  I figured they would since their bread and butter seems to be putting a band-aid on the problem instead of solving it at the source.  So I hired a local equipment operator to excavate.  He got me down to below the footer so I could lay down a perforated PVC pipe.  With the earth removed I pressure washed and put a few coats of a liquid rubber foundation sealer on the walls.  When the last coat tacked up a bit we put a 6mil vapor shield plastic sheet over the wall.  With the pipe socked I opted to also make a giant filtration sock for the gravel layer.  For this I used a 8 oz geotextile fabric.  So after backfilling all but about the last 15 inches with gravel I have a socked perf pipe surrounded by a giant gravel sock.  This has worked wonderful for the past 18 years.  I get no water penetration on the foundation walls since the time of completion.  The water exiting the pipe is still coming out clean during heavy rains.  The problem I still have is mold on the walls in the garage.  Mostly in the corners where the block walls meet.  Every spring/summer I pull workbenches and tool and storage boxes away from the walls, spray them down with a bleach mixture, followed by a scrub and a rinse.  Last year I paid extra for a solution from Bactronix hoping this would keep the mold at bay a bit longer.  I also run a dehumidifier 24-7 in the garage.  Neither helped as I still see mold growing on the bottom courses of those blocks.  I'm not sure what else to do, or if there is anything I can do to keep mold off of those walls in a non-climate controlled section of the house.  (no vents or returns in there obviously).  I've also done my best to seal the garage doors with new side and door bottom seals.  I assume moisture just simply permeates through the block floor causing my continuing mold issues.  Is there anything I can do to keep the mold from coming back?  

  • Can you please use the edit button to add some paragraph breaks to your wall of text? Mar 25, 2021 at 23:16
  • Are you sure your "mold" is not actually efflorescence? They are commonly confused. It would be rather uncommon for the concrete walls to contain anything that mold could actually live on.
    – kreemoweet

2 Answers 2


Get some hydrogen peroxide, dilute to 7% with water and spray or roll it on the blocks. Within 24 hours the mold is gone any that is still left reapply and that usually takes care of mold on concrete block sometimes a 3rd round then an occasional treatment keeps the concrete bright.

No chemical residue to worry about the hydrogen peroxide breaks back down into water and oxygen.

If you get strong hydrogen peroxide on your hair it will bleach it, gloves and glasses should be worn.

I purchase my hydrogen peroxide online at ?30% in gallons but I live in Oregon and some places have year round mold problems I kill mold prior to working with it because I don’t want to breathe it. Just a note I am not sure how long it lasts but if not kept in a dark cool area it turns back to water after a few years so far mine gets used before that long as it is a great sanitizer with no bleach stink (learned that working in a hospital years back).


Your bleach mixtures are not effective against killing mold on porous surfaces such as cinder blocks. Bleach will kill mold on non porous surfaces like counter tops but it doesn't seep into porous material to kill the mold roots.

Try spraying straight vinegar or a 3% concentration of hydrogen peroxide on the blocks and let sit for 10 minutes. Saturate the area. Scrub the area and wipe off area to remove residual mold and spores. Then re-saturate the area and let it dry. Vinegar and hydrogen peroxide are great at seeping into the pores and killing the mold roots.

  • I love 3% hydrogen peroxide but on concrete block I use a much stronger solution so I don’t have to scrub.
    – Ed Beal
    Mar 25, 2021 at 14:56
  • I'll give the hydro-perox a try. The block pores are pretty well sealed with drylock and a couple topcoats of gloss Zinnser Permawhite latex. The most is actually growing on that topcoat. I used that product because it claims "mold and mildew resistant"...I guess no-so-much.
    – Greenyone
    Mar 26, 2021 at 15:41

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