Waterproofing Cinder Block Basement Walls
A block wall is a stack of hollow boxes, and once water gets into the boxes, paint on the inside face is not going to hold it back. Here is how to tell what your wall is doing, and which fix it actually needs.
Alex Rivers
Home Improvement Editor
Last Updated
September 23, 2026
In This Guide
Most advice about sealing basement walls is written for poured concrete, and block walls punish people who follow it. The cores fill, the mortar joints wick, the salts push coatings off from behind. The good news is that the cheap fixes work more often than contractors admit, as long as you do them in the right order and know when to stop.
1. Why Cinder Block Walls Leak Differently From Poured Concrete
A poured concrete wall is one monolithic slab. When it leaks, it almost always leaks at a specific place: a shrinkage crack, a tie-rod hole, the cold joint where the wall meets the footing. You can point at the spot, and a targeted repair has a fair chance of ending the problem. A concrete masonry unit wall, which is what most people mean by cinder block, is a completely different animal, and treating it the same way is the root of most failed DIY jobs.
A standard 8 by 8 by 16 inch block has two or three hollow cores, and in most residential basement walls those cores are left empty unless the wall was reinforced with rebar and grout. Stack the blocks and the cores line up into vertical chimneys running from the top course down to the footing. Water that gets through the exterior face does not have to reach the inside face to cause trouble. It drops down the cores and collects in the bottom course, sitting on the footing, sometimes several inches deep.
Then there are the mortar joints. A block wall has more than two linear feet of mortar joint for every square foot of wall, and mortar is more porous than the block and far more prone to hairline separation. Water wicks through joints by capillary action even without a crack. That is why a damp block wall often shows a grid pattern of darker lines before the block faces themselves get wet, and why stair-step cracks follow the joints rather than cutting through the blocks.
The minerals come along for the ride. As water moves through the block and evaporates at the inside face, it leaves dissolved salts behind as a white, chalky crust called efflorescence. On a block wall this is often heavy, because the cores give water a long residence time in contact with cement. Efflorescence is cosmetic on bare block, but behind a coating it is destructive: the crystals grow in the pores and lift the coating off from underneath.
Finally, block walls resist sideways soil pressure worse than poured walls. They are strong in compression and comparatively weak in bending, so saturated or frozen soil pushing on the outside can produce a horizontal crack along a mortar joint, usually a third to half of the way down, and eventually a visible inward bow. That is a structural issue wearing a water problem as a disguise, and no coating addresses it. Our explainer on hydrostatic pressure in a basement covers where that force comes from.
2. Diagnose the Water Before You Buy Anything
Every fix on this page solves a different problem, so the first job is working out which problem you have. The four common ones are condensation from humid basement air, capillary dampness through the block, seepage at specific cracks or joints, and water under pressure pushing through the wall or the joint at the floor. Spend a weekend and a roll of plastic on diagnosis and you can avoid spending a month and several hundred dollars on the wrong coating.
Start with the plastic sheet test. Tape a piece of clear plastic, about two feet square, tightly to the wall on all four edges, and leave it for 24 to 48 hours. If moisture collects on the wall side of the plastic, water is coming through the block. If it collects on the room side, the air is humid and the cold wall is condensing it, which is a dehumidification problem covered in our guide to basement moisture control. Many basements show a little of both.
Next, map the wet areas against the weather. Dampness that appears within hours of heavy rain, concentrated near one corner, almost always traces to a downspout or a low spot in the grading on that side. Dampness that builds slowly over days of wet weather and appears along the whole base of the wall points to a high water table and hydrostatic pressure. Water that shows up where the wall meets the slab is its own diagnosis, covered in detail in our guide to a basement leaking where the floor meets the wall.
Then check the cores. Tap along the bottom course with a hammer handle and listen, or drill a single small hole into one core an inch or so above the floor in an out-of-the-way spot. If water runs out, the cores are holding water and any interior coating will be fighting a reservoir. This one test changes the whole plan, and it is the step most homeowners skip.
Last, look for movement. Hold a long level or a taut string line against the wall from top to bottom. A straight wall with stair-step hairlines along the joints is normal settlement. A horizontal crack along a joint, a wall that bows inward, or blocks that have shifted relative to each other at the crack need measuring. Note the numbers, photograph everything with a tape measure in frame, and keep reading before you touch a brush.
| What You See | Likely Cause | Where to Start |
|---|---|---|
| Water on room side of plastic | Condensation | Dehumidifier, air sealing |
| Water on wall side of plastic | Capillary dampness | Grading, then coating |
| Wet corner after rain | Downspout or grading | Outside fixes |
| Water runs from a drilled core | Full cores, pressure | Weep holes and drain |
| Wet line at floor joint | Hydrostatic pressure | Interior drain |
| Horizontal crack or bowing | Lateral soil pressure | Structural engineer |
3. Fix One Is Outside: Grading, Gutters and Downspouts
The cheapest waterproofing work on a block basement happens outdoors, and it is the work that makes every indoor fix more likely to last. A coating on the inside is trying to hold back water that has already soaked the wall. Reducing the amount of water that reaches the wall in the first place does more than any product you can brush on, and the tools are a shovel, a level and a few lengths of downspout extension.
Begin with the roof, because it is a huge collector. One inch of rain on 1,000 square feet of roof produces roughly 620 gallons of water, and a typical gutter system funnels all of it to four or five points at the corners of the house. If a downspout dumps at the foundation, that corner of the basement is receiving a concentrated flow every storm. Extend every downspout at least four to six feet from the wall, farther if the ground slopes back toward the house.
Clean the gutters and watch them in a real rain. Overflowing gutters drop a curtain of water straight onto the soil along the foundation, and on block walls that is precisely where you do not want it. A clogged gutter or a downspout disconnected underground at a buried drain line is a common, invisible cause of a single wet wall, and it costs nothing but a ladder and an afternoon to find.
Then fix the grade. Building codes call for the ground to fall at least six inches within the first ten feet away from the foundation, and older houses have usually settled flat or even back toward the wall. Add compacted soil, ideally a clay-rich fill rather than loose topsoil or mulch, and shape it so water sheds away. Keep the finished grade several inches below any siding and below the top of the block.
Give these changes a season before you judge the basement. In our experience a significant share of damp block walls, especially those with wet corners, dry out enough after grading and downspout work that a single coating is all they ever need. If the wall is still wet with the outside sorted, you have learned that the water is coming from groundwater rather than surface runoff, which points you toward drainage rather than paint.
THE RULE
Fix the water at the surface before you fight it at the wall.
Downspout extensions and a properly sloped grade are the least expensive waterproofing work on this page and the most commonly skipped. Doing them first makes every indoor fix cheaper and longer-lasting.
4. Preparing a Block Wall for a Masonry Waterproofer
Interior masonry waterproofing coatings, the cement-based and acrylic products sold under names like DRYLOK and similar masonry waterproofers, genuinely work on block walls with dampness and light seepage. They fail when they go on over dirt, salts, paint or running water. Nearly every coating failure we see on block traces back to preparation, not the product, so this section matters more than the one about the brush.
Start with a stiff wire brush, by hand for small areas or on a drill for whole walls. Scrub the entire surface, paying particular attention to the mortar joints, until you are looking at sound, bare block. You are removing loose mortar, dust, old whitewash, mildew and the surface layer of efflorescence. Vacuum as you go. If the block face crumbles under the brush, it has been degraded by years of moisture and salt, and coating it is building on sand.
Efflorescence needs more than brushing. After the dry scrub, treat the wall with a masonry cleaner or efflorescence remover made for the purpose, or a diluted acid solution mixed and handled strictly per the label with gloves, goggles and ventilation. Scrub, rinse thoroughly with clean water, and let the wall dry. Our guide to efflorescence on basement walls covers the removal methods and why the white bloom will come back if water keeps moving through.
Next, check for paint. Sprinkle water on the wall: bare block darkens and absorbs it within seconds, while paint or sealer makes it bead. Masonry waterproofers need to bond into the pores of bare masonry, and they will not stick to latex or oil paint. On a painted wall you either strip it back to bare block, which is slow, messy and often means abrasive blasting, or accept that a penetrating coating is off the table and look at drainage instead.
Finally, plug the leaks. Any hole or crack that is actively weeping water needs to be keyed out and filled with hydraulic cement before any coating goes on, because no coating will cure against running water. Our step-by-step on how to use hydraulic cement covers the undercut channel and the ninety-second working window. Dry hairline cracks and gaps around pipes can take a masonry sealant instead; our guide to caulk for basement wall cracks covers which products suit which crack.
5. Applying the Coating: Two Coats and Honest Coverage Math
Buy for two coats from the start, because every masonry waterproofer we know of requires two to perform as claimed and the warranty usually depends on it. Labels typically quote around 75 to 100 square feet per gallon per coat on smooth masonry. Two coats halves that to roughly 37 to 50 square feet of finished wall per gallon, and rough or porous block pulls the number lower still, because the first coat disappears into the surface. Our DRYLOK coverage calculator guide walks through the measuring.
Check the conditions. Most masonry coatings want the air and the wall above roughly 50 degrees Fahrenheit through application and curing, and a surface that is damp but not wet is often acceptable while standing water is not. Read your product's own label on this, because cement-based and acrylic coatings differ. Ventilate if the product is solvent-based, and plan the job for a stretch of dry weather so the wall is not being actively recharged from outside.
Use a stiff nylon masonry brush for the first coat, not a roller. The goal is to force the coating into every pore and pinhole, and block is full of them. Work it in with a scrubbing motion, going over the mortar joints in two directions, and look at the wall from an angle with a work light: any pinhole left open is a path for water. A roller with a thick nap can apply the second coat on smoother block, but brushing both coats is the safer habit.
Wait the recoat interval the label specifies, commonly a few hours, then apply the second coat with the same attention. Once it has cured, test it the same way you diagnosed the wall: the plastic sheet goes back up for a day or two. A properly applied two-coat system on a wall with dampness and light seepage should show no moisture behind the plastic. Some residual darkening at the base after heavy storms is a sign the wall needs drainage, not a third coat.
Be realistic about what you have bought. A good masonry waterproofer on a prepared block wall commonly lasts years, but it is a barrier on the inside face of a wall that is still wet on the outside. It does not stop water entering the cores, and it does nothing about pressure. If you want a product comparison, our roundups of the best cinder block sealers and basement waterproofing paint cover the options.
The Numbers That Matter
2
Coats, minimum
37-50
Sq ft per gallon, finished
50°F
Typical minimum temperature
6 in
Grade fall in first 10 ft
6. When the Cores Are Full: Weep Holes Into an Interior Drain
If your drilled test hole ran water, or the wall is wet along its base even after the outside has been fixed, a coating is the wrong tool. Sealing the inside face of a block wall with full cores traps the water inside the wall. It builds up, pushes salts and moisture against the back of the coating, and in cold climates it can freeze in the cores. The professional answer is the opposite of sealing: let the water out, deliberately, into a drain.
The method is simple in principle. A trench is cut in the slab along the inside of the wall, down to the footing. A perforated drain pipe or a drain channel is laid in washed gravel, sloped to a sump pit. Then holes are drilled into the bottom course of block, one into each core, commonly around half an inch to an inch in diameter, so every core drains into the gravel bed. A dimpled plastic membrane goes against the wall to guide any seepage down behind it, and the trench is backfilled with fresh concrete.
This is how most interior waterproofing systems sold by basement contractors work on block foundations, and the reason is that it accepts the water rather than fighting it. The wall stays damp on the outside, but the water inside the cores never builds enough head to push through the inside face or the floor joint. Our guide to the basement drainage system covers the pipe, gravel and sump side of the job in detail.
It is a legitimate DIY project for someone comfortable renting a concrete saw and a jackhammer and breaking up a few tons of slab, but it is hard, dusty work and the sump pump becomes a critical system. Most homeowners hire it out. When comparing quotes, confirm in writing that the contractor drills every core in the bottom course and ties into a sump with a battery backup, and compare their system against the options in our pillar guide to the best basement waterproofing systems.
A cheaper variant drills weep holes into the bottom course just above the slab and covers them with a baseboard-style channel that carries water to the sump without cutting the floor. It can work on walls where water collects in the cores but the slab joint is dry. It will not help if water is coming up under the slab, and it depends on the channel staying clear. Treat it as a middle step, not a substitute for a full perimeter drain. Our overview of interior basement waterproofing compares both.
7. Exterior Membranes, and When No Coating Will Ever Hold
The only fix that stops water entering a block wall at all is on the outside. The soil is excavated down to the footing, the block is cleaned and parged with a cement coat to smooth the joints, and a true waterproofing membrane is applied: a rubberized asphalt, a sheet membrane, or a thick liquid-applied product. A dimpled drainage board goes over it, a perforated footing drain runs along the base to daylight or a sump, and the trench is backfilled with free-draining gravel.
It is the most effective option and by a wide margin the most expensive, because the cost is mostly digging. Decks, patios, driveways, plantings and utility lines along the wall all add to it. It also makes sense in specific circumstances: when you are finishing a basement and want the wall dry rather than drained, when the wall needs excavating anyway to be straightened, or when there is no room inside for a drain. Our guide to waterproofing basement walls from outside covers the process and the questions to ask.
Know what is already on your wall before you pay for it. Many houses built with block foundations received only a sprayed bituminous damp-proofing coat on the outside, which is thin, brittle with age, and was never meant to resist water under pressure. That is not waterproofing. If a contractor proposes exterior work, ask what membrane and drainage board they install and whether the footing drain is new, because re-spraying damp-proofing over old block is barely an improvement.
Now the hard limits. A coating will fail on any wall with active hydrostatic pressure, meaning water visibly pushing through under force or flowing from the cores. It will fail on a painted wall that has not been stripped to bare masonry. It will fail on block that is crumbling or heavy with salt. In each case the coating is not wasted money only if it is part of a larger plan that includes drainage, and on its own it will peel or blister within a season or two.
Bowing is the limit that matters most. If your string line shows the wall bowing inward by more than roughly half an inch, or a horizontal crack is widening, stop all waterproofing work and get a structural engineer to look at it. Repairs such as wall anchors, steel or carbon-fiber bracing, or rebuilding a section are engineering decisions. Waterproofing a wall that is moving hides the evidence, and sealing the cracks can make lateral pressure worse by keeping water in the soil against the wall.
| Fix | Solves | Will Not Solve |
|---|---|---|
| Grading and downspouts | Surface water at the wall | High water table |
| Interior coating | Dampness, light seepage | Full cores, pressure |
| Weep holes and drain | Full cores, floor joint leaks | Water entering the block |
| Exterior membrane | Water entering the wall | Structural movement |
| Engineer and bracing | Bowing, horizontal cracks | Water on its own |
8. Common Mistakes to Avoid
Coating over efflorescence
Brushing a masonry waterproofer straight onto a wall with white mineral bloom is the single most common failure. The salts keep growing in the pores and push the coating off from underneath within months. Wire-brush, clean with an efflorescence remover, rinse, and let the wall dry fully before the first coat goes on.
Sealing a wall with water in the cores
If a test hole in the bottom course runs water, an interior coating traps that reservoir inside the wall. The pressure builds behind the coating, it blisters, and in cold climates the trapped water can freeze. Full cores need weep holes and a drain to let the water out, not a better sealer.
Skipping the outside work
People spend a weekend coating a basement while a downspout still dumps hundreds of gallons per storm at the corner. The coating then fails at exactly that corner. Extend downspouts, clean the gutters and fix the grade first; it costs less than the paint and makes the paint last.
Using one thin coat on rough block
Stretching a gallon to the label's single-coat figure leaves pinholes across porous block, and each pinhole is a path for water. Masonry waterproofers need two coats worked into the surface with a stiff brush. Budget from the finished two-coat coverage, not the headline number, and expect rough block to use more.
Waterproofing a bowing wall
A horizontal crack or an inward bow means lateral soil pressure is moving the wall. Sealing it hides the crack an engineer needs to see and does nothing about the load. Anything bowing by more than roughly half an inch, or a crack that is widening, needs a structural assessment before any waterproofing money is spent.
9. Frequently Asked Questions
What is the best way to waterproof cinder block basement walls?
Work from the outside in. Extend downspouts, clean gutters and grade the soil away from the wall first. If the wall is still damp, prep it properly and apply two coats of a masonry waterproofer. If water sits in the cores or leaks at the floor joint, weep holes and an interior drain are the real fix, not more coating.
Can you seal cinder block walls from the inside?
Yes, for dampness and light seepage. A masonry waterproofer brushed onto clean, bare block in two coats works well on walls that are not under active water pressure. It will not hold on a painted wall or against water flowing from the cores, and it does not stop water entering the block from outside. It only stops it at the inside face.
Why do cinder block walls leak more than poured concrete?
Block walls are hollow and jointed. Water that passes through the outside face drains down the cores and collects in the bottom course, and the mortar joints wick moisture by capillary action even without cracks. Poured concrete is solid, so it usually leaks only at specific cracks or joints, which are easier to find and repair individually.
How many coats of waterproofer does a block wall need?
Two, at minimum. Most masonry waterproofers state that two coats are needed for the product to perform and for the warranty to apply. On rough or porous block the first coat soaks in heavily, so plan on roughly 37 to 50 square feet of finished wall per gallon across both coats, and less on very coarse block.
Should I drill weep holes in my basement block wall?
Only as part of a drain system. Weep holes in the bottom course let water trapped in the cores escape, but that water has to go somewhere, normally a perimeter drain in gravel that runs to a sump pump. Drilling holes without a drain simply puts water on the floor. Do the test hole first to confirm the cores actually hold water.
Does DRYLOK work on cinder block?
It does, within limits. DRYLOK and similar masonry waterproofers are designed for block and bond into bare masonry well when the wall is cleaned, free of efflorescence and paint, and not actively leaking. Applied in two brushed coats, they handle dampness and light seepage. They are not designed for walls with full cores, strong hydrostatic pressure, or structural movement.
When should I call a structural engineer about a block wall?
Call one if you see a horizontal crack along a mortar joint, an inward bow of more than roughly half an inch, blocks that have shifted out of line, or any crack that measurably widens over a few months. Those are signs of lateral soil pressure. An engineer's assessment usually costs less than a single wasted waterproofing job and tells you which repair the wall needs.
Related Basement Waterproofing Guides
Buyer's Guide
Best Basement Waterproofing Systems
Every approach compared, from coatings to full perimeter drains.
How-To
Basement Drainage System
How an interior drain and sump handle water that coatings cannot.
How-To
How to Use Hydraulic Cement
Plugging active leaks before any coating goes on the wall.
The Bottom Line
A cinder block basement is not waterproofed by the product you choose. It is waterproofed by the order you do things in: get surface water away from the wall, find out whether the cores are holding water, then pick the fix that matches what the wall is actually doing.
For a damp wall with dry cores, two properly prepped coats of masonry waterproofer are cheap and effective. For full cores or a leaking floor joint, spend the money on drainage instead of paint. And for a bowing wall, spend it on an engineer first. Everything else is detail.
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About the Author
Alex Rivers, Home Improvement Editor
Alex has spent over a decade working on residential and light commercial property maintenance, and now tests every tool, coating and machine that appears in these guides personally.