Most concrete needs a vapor-open sealer, not a film. That covers the usual cases: a driveway, a garage slab, an above-grade wall, anything dry to the touch and damp to a meter.
The answer changes the moment water arrives with pressure behind it. A wall holding back saturated soil near the bottom of a slope is a different job from a damp patio, and the products sold for the patio fail on that wall.
Two questions sort the job out. Is the water moving through the pores as vapor, or is it being pushed against the concrete, and which face of the concrete can you reach?
The Water Decides Before the Product Does
Waterproofing concrete starts with the water itself, because vapor and pressed water need opposite assemblies. One wants the pores open so moisture can leave, and the other wants a membrane held against a head of water that never stops pushing. Same concrete, two different answers.
Concrete is porous by nature, a connected network of capillary pores that vapor migrates through. That movement is slow, and on most structures it does no harm at all. It is also what makes sealing possible, since narrowing or lining those pores is enough when nothing is driving the water.
Head pressure is a different order of magnitude. Fresh water weighs about 62.4 pounds per cubic foot, which puts roughly 0.433 psi on every square foot for each foot of depth. Three feet of saturated ground above a footing presses about 1.3 psi against that wall, near 187 pounds per square foot of wall area.
Nothing rolled on from the inside resists that for long. The water has an entire wall to find a way through, and it needs one path.
A slow seep leaves the same picture every time: a pale tide line along the base of the wall after a wet week, paint bubbling in a narrow band just above it, and a floor that stays cold underfoot long after the rest of the basement has warmed.
Two different problems. Which face of the wall can you reach? On a finished basement the answer is the inside, and the inside is the wrong side for water under pressure.
Three things look alike from inside a basement. Groundwater under pressure arrives through the wall or the joint where wall meets floor, and it keeps arriving. Surface runoff shows up after storms and fades within a couple of days. Condensation collects on the coldest surface through humid months and reads like a leak until the air is dried. The grading table further down treats condensation as a ventilation and humidity item rather than a defect, which is the split worth making before calling a waterproofing contractor.
Fix the Concrete Itself First
Every surface treatment sits on concrete whose pore structure was fixed at the batch plant. A low water-cement ratio and a real cure lower the permeability of the concrete body, and that sets the ceiling for anything applied later.
The water-cement ratio is the weight of mixing water divided by the weight of cement, as the Portland Cement Association defines it. A typical mix runs 7 to 15 percent cement, 60 to 75 percent aggregate, and 14 to 21 percent water. Cutting water tightens the capillary network, as long as the mix stays workable enough to place and consolidate.
Curing protects the layer where water actually crosses. A surface that dries out early never finishes hydrating, and the top few millimeters stay more porous than the concrete below them. That is the layer every sealer is asked to work with.
Integral admixtures change the concrete from within. Sika splits its watertight range into pore-blocking products such as Sika WT-100 and crystalline products such as Sika WT-200, which the company says also promotes self-healing of cracks. Water reducers, in Sika’s account, work by shrinking the capillary structure rather than plugging it.
Admixtures cannot close a cold joint or a service pipe, and nobody can add them to concrete placed years ago. Those limits are where surface methods take over.
What Each Surface Method Can and Cannot Do
Four families cover nearly every concrete job: penetrating repellents, film-forming coatings, cementitious coatings, and crystalline products. They differ less in how well they shed water than in what they need underneath and how much pressure they tolerate.
- Penetrating repellents. Silane and siloxane products soak into the pores and line them with a water-repellent layer, the same chemistry that makes treated masonry shed rain. Ghostshield’s Siloxa-Tek 8510 is a siloxane example. The pore stays open to vapor, and the substrate has to be dry, because water sitting in the pore keeps the silane out.
- Film-forming coatings. Acrylic, epoxy, and urethane products build a skin on the face. They take abrasion well and tolerate mild pressure. Applied to the inside of a wall holding back saturated soil, the skin blisters and comes off in sheets you can lift with a putty knife.
- Cementitious coatings. Cement-based products bond to damp concrete and accept water on the far side, which is why they carry most tanking work and most inside-face repairs where nothing else will stick.
- Crystalline products. The Xypex range is the best-known example, sold both as a concrete admixture and as a surface-applied coating. The chemistry needs water to grow crystals inside the pores, and the company markets self-healing as part of the system. Dry concrete slows the reaction rather than feeding it.
Substrate condition settles more jobs than product choice does. Repellents need dry concrete, crystalline products need water, and coatings need a surface sound enough to hold them.
Moisture in a slab gets measured two ways in the trade. The calcium chloride method of ASTM F1869 reports moisture vapor emission, and in-situ relative humidity probes under ASTM F2170 report the moisture state inside the concrete. They answer different questions, and a coating picked off the wrong number peels.
Sika’s own framing is blunt about this. Watertight construction is a system, and the paths that carry water include the joints, the embedded items, and the cracks, not only the concrete. When one of those three is open, a sealer is being asked to do repair work.
Joints and penetrations carry their own products for that reason. Sika pairs waterbars at movement and construction joints with profiles that swell on contact with water and seal the joint line, and the same swelling material wraps pipe entries where a coating cannot reach. A homeowner chasing a single horizontal wet line on a poured wall is usually looking at one of those joints rather than at the whole face.
Application detail decides the rest. A repellent goes on in thin passes with enough dwell time for the pores to drink it, then gets back-rolled so nothing dries in a puddle on the face. Coatings want a sound, roughened substrate and their own recoat window. Manufacturers publish coverage, dry times, and pass thickness for each product, and those numbers beat any general instruction.
Match the Method to How Dry the Space Has to Be
How dry a concrete structure must stay gets decided before the first drawing, and it is expressed as an environmental grade with a target relative humidity rather than as a product. That target then sets how many layers the job needs.
The Basement Information Centre, run by The Concrete Centre inside the Mineral Products Association, publishes the grading table that UK design follows. Grade 1 covers basic utility space such as car parks and plant rooms, where some leakage and damp patches are tolerable and local drainage may be needed. Grade 2 permits damp areas but no water penetration, at 35 to 50 percent relative humidity. Grade 3 is habitable space, dry with no penetration and held at 40 to 60 percent, with ventilation or dehumidification carrying part of the load. Grade 4 covers archives and landmark stores, down to 35 percent for book storage.
A habitable basement therefore needs a drained and tanked assembly, not a coat of sealer, and the price gap between those two jobs is wide. Excavating against an exterior wall, adding a drainage board and membrane, and reinstating the ground is where the money goes.
The grade is also what a finished job gets measured against, and it is read as relative humidity inside the space rather than as whether the wall feels dry. A finished basement usually aims at the Grade 3 band of 40 to 60 percent, and holding that without a dehumidifier running all summer is the part of the budget that gets underestimated.
From the inside the choice narrows to cementitious or crystalline work. A film-forming product on the inner face usually loses, and waterproofing a basement wall from the inside comes down to picking between those two behaviors with the water already on the wrong side of the concrete.
Waterproofing Concrete FAQ
How do you seal concrete against moisture?
Clean the surface, let it dry, then apply a penetrating silane or siloxane repellent. It lines the pores instead of closing them, so the slab keeps releasing vapor while liquid water beads off. A wet slab has to dry first, because water in the pore keeps the sealer out.
What are the three main methods of waterproofing?
Integral waterproofing works inside the concrete through admixtures and a controlled water-cement ratio, surface waterproofing blocks water at the face with coatings or repellents, and drained waterproofing collects water behind the structure and carries it away. Most serious jobs combine at least two of the three.
What is the best way to waterproof concrete?
On a slab with no water pressure against it, a penetrating repellent or a crystalline coating is the durable default, and that covers basement floors in most houses. Where pressure is present, the floor is a symptom, and the work starts with drainage and a membrane outside or beneath the slab.
Can I waterproof a basement myself?
Inside work is within reach: cleaning, crack repair, and a cementitious or crystalline coating are homeowner tasks. Outside work means excavation, drainage, and membrane installation, which is contractor work on nearly every site.
Before any of it, measure the slab. That number decides whether a repellent, a coating, or a repair goes on the wall, and buying the product before the number exists is how the same wall gets paid for twice.
