Split-face block in Chicago, explained

Split-face block: the one modern wall every Chicago buyer should know

Between the mid-1990s and the mid-2000s, thousands of Chicago condo buildings and townhomes went up with walls of split-face concrete block: gray or tan blocks with a rough, rock-like face. It was fast and cheap, and it is the one wall system in this guide with a deserved reputation for water problems. The buildings are everywhere, and plenty of them are fine. The difference between fine and moldy is detailing and maintenance, so here is how to tell which one you are touring.

What it is

Split-face block (you will also see it written split face block, or split face concrete block) is an ordinary concrete masonry unit that is cast as a double block, cured, then run through a machine that fractures it down the middle to expose the aggregate inside. The rough face you see from the street is the block's interior. It has no dense molded skin anywhere on it, and it drinks water. That is not a defect in the product; ASTM C90, the standard these blocks are made to, permits a normal-weight unit to absorb up to 13 pounds of water per cubic foot, and up to 18 for a lightweight unit. A perfectly compliant block is still a sponge.

Swipe the drawing sideways ›

During the condo boom it was the developer's dream: it reads as masonry from the street, it goes up in a fraction of the time brick takes, it costs meaningfully less, and one block is both the structure and the finished exterior. Whole stretches of infill construction in Bucktown, Wicker Park, Logan Square and West Town went up this way.

Where the water goes

Tap any numbered point on the wall to see what happens there. The short version: the face of the block is not usually the main leak path, the top of the wall is, and once water is inside a single-layer block wall there is nowhere for it to go but down through the hollow cores. If you are wondering why a hundred year old brick building on the same block gets rained on just as hard and does not do this, that comparison is a little further down the page.

The part most people get wrong

Almost everyone assumes the water soaks through the rough face of the block. Some does. But the industry's own guidance is blunter than that: water usually enters at the interface of improperly filled mortar joints and where the mortar failed to bond to the block. Head joints, the vertical ones, are the usual culprit, and they are supposed to be filled the full thickness of the block's face shell. Joints are also supposed to be tooled concave when the mortar is thumbprint hard, which compacts the surface; raked and struck joints shed water far worse and you can spot them from the sidewalk. There is a practical catch specific to this block: the rough fractured face makes a joint harder to tool cleanly than a smooth block does, so the detail that matters most is also the one this wall makes hardest to get right.

The second thing people get wrong is thinking of the wall as a barrier. A brick veneer wall is a drainage wall: there is a one inch air space behind the brick, flashing at every interruption, and weep holes so anything that gets in runs back out. A single-wythe split-face wall has none of that. The only drainage space is the hollow cores of the block, which is why flashing and weeps matter more here than on a brick building, not less. There is a cruel twist in the industry literature on this point: adding an integral water repellent to the blocks and mortar cuts absorption, but it also means the water that does get in cannot wick back out, which makes proper flashing and weeps more necessary.

Third: Chicago freezes. A saturated block goes through freeze-thaw cycling all winter, which is what spalls the split face and crumbles the joints. It is also the reason the coating question below is not a matter of taste.

How this is different from an old Chicago brick building

This is the fair question, and it is the one buyers ask us most often once they have seen the diagram above. A greystone or a brick two-flat from 1910 stands in the same weather, has mortar joints a century older, and has usually never been sealed in its life. So why is the split-face building the one with the reputation?

The answer is not that the block is bad material or that the brick is tougher. It is that there are only three ways a masonry wall can deal with rain, Chicago built all three, and split-face uses the newest of them with the pieces that make it work left out. Here they are drawn at the same scale, cut vertically, outside on the left.

Swipe the drawing sideways ›

If you want one sentence for the tour: a solid brick wall's water control is built into the wall and does not wear out. A split-face wall's water control is a coating and a set of sheet-metal details, and coatings wear out. That, and not the age of the mortar or the quality of the block, is the actual difference. It is also why the maintenance question on a split-face building has a date attached to it and the same question on a greystone does not.

Two smaller differences are worth knowing because they cut against what people assume. The first is movement. Concrete block shrinks as it cures and keeps shrinking for years afterward, which puts the wall in tension and is what opens up mortar joints; that is what control joints are supposed to relieve, and split-face buildings often do not have enough of them. Clay brick does the opposite, expanding slightly with moisture over its life. The second is freeze damage. The common assumption is that old brick is the hardier stuff, and on this specific point the research runs the other way: modern fired brick generally tolerates freezing while saturated considerably better than soft historic brick. Old Chicago walls come through the winter because there is so much of them and because they dry to both sides, not because each individual brick is tougher.

None of which makes a 1910 building automatically the better buy. Solid masonry has its own list, and it is not a short one: deteriorated joints that need tuckpointing, lintels rusting and lifting the brick above the windows, parapets, chimneys, and the exact same coping problem at the top of the wall. Meanwhile a split-face building that was flashed properly and gets recoated on schedule is a perfectly dry building. The practical takeaway is just that you ask each one a different question. On the brick building you are asking about the condition of the mortar, the lintels and the roof. On the split-face building you are asking whether the water control that was supposed to be built into it ever got built, and when it was last renewed.

Sources for this comparison: the Brick Industry Association's Technical Note 7 and its notes on mass and cavity wall design, the Concrete Masonry and Hardscapes Association's TEK 19-2B on water repellents for single-wythe block, and Building Science Corporation's work on solid masonry walls.

What to look at, and what to ask

None of this needs a ladder or a moisture meter. Most of it you can do from the sidewalk in five minutes on your way in, and the rest is questions for the seller and the association. The point is not to grade the building. It is to know which two or three things to push on before your inspection window closes.

From the sidewalk, before you go in

• Look up at the coping first. The cap on top of the parapet is where most of the water gets in on these buildings, and it is the one thing you can assess from across the street. You want to see a thin metal line under the cap and a lip that projects past the face of the wall. If the cap sits flat on the block with a bead of caulk at each joint and nothing underneath it, that is the single most useful thing you will learn all day.
• Find the weeps, or notice that there are none. Small open gaps in the vertical joints, just above the base of the wall and above the windows, roughly every couple of feet. They are how water that gets into the block cores is supposed to get back out. On a lot of these buildings there are none anywhere, and on some they have been caulked or painted shut by a well-meaning contractor, which is worse than never having them.
• White chalky deposits on the block. Efflorescence, and it is not cosmetic. It is salt left behind by water that moved through the wall and evaporated on the outside. It means water is moving through that wall now, not once, years ago.
• Chipped, spalled or loose blocks, especially high up. That is freeze-thaw damage, and it shows at the top of the wall first because that is where the wall is wettest.
• The mortar joints. Sound, or crumbling and sandy and receding? Scratch one with a key if you can reach it. Also look at the shape: joints that were pressed into a concave curve shed water; joints that were raked out square, or struck flat, do not, and you can tell them apart from ten feet away.
• Any painted or coated finish that is blistering, bubbling or peeling. A failing film means moisture got trapped behind it, which means either the wall was wet when it was coated or the coating was not breathable enough for a Chicago winter. Either way somebody has already tried to fix this.
• Where the downspouts end, and which way the dirt slopes. Boring, cheap to fix, and a wall standing in wet soil wicks. If a downspout dumps at the foundation of a wall with no base flashing, that is a direct path in.
• The underside of the lintels over the windows. Rust, and cracking in the joint just above them. Same rust-jacking problem as a vintage brick building, and in block it shows up faster.

Inside, in the unit you are touring

• Top-floor ceiling corners and closets. Water arrives here first. Stains, bubbling paint, a musty smell, or one patch of suspiciously fresh touch-up paint in exactly the spot water would reach.
• Floors and outlets along the exterior walls. Cupped or warped hardwood tight to an outside wall, rust on the outlet screws, corrosion in the panel.
• Know what a dry-looking interior does not prove. The worst outcome on these buildings is water sitting in the joist pockets where the floor framing bears on the block, rotting concealed structure for years with no stain anywhere on any drywall. A clean interior is good news. It is not evidence.

Questions to put to the seller and the association

• "When was it last sealed, with what product, and at what coverage rate?" A date alone is not an answer. Ask for the product name and then ask for the data sheet. A split face takes 30 to 85 square feet per gallon against 150 to 200 for a smooth wall, so a job that used the right product at the wrong rate fails early and looks identical on the invoice.
• "Is resealing a budgeted line item, with a next date?" In the reserve study or the annual budget, not in somebody's memory. A building that has scheduled the next one has understood the problem.
• "Does the parapet coping have continuous through-wall flashing under it, and when was it last opened up?" If the answer is a shrug, that is your answer, and it is the most expensive shrug on the list.
• "Has anyone ever water tested the wall, or brought in an envelope consultant?" There are standard field tests for exactly this, on the wall field and separately at the windows. A building that has had one and then did what the report said is in a completely different category from one that never has. Ask for the report, and then ask which of its recommendations were carried out.
• "Has there been a special assessment or litigation related to water?" Then read the last three years of minutes yourself. What the assessment paid for matters more than that there was one, and whether it worked matters most of all.
• "Has this unit ever had a leak?" Ask it plainly, and get the answer onto the disclosure rather than into a conversation.

Sealers and coatings: what each one does

"When was it last sealed?" is the right question, but "sealed with what?" is the one that separates a maintained building from an expensively damaged one. There are two kinds, they work in opposite ways, and putting the wrong one on a Chicago wall causes the damage it was meant to prevent.

Penetrating water repellents (breathable)

Silanes and siloxanes are small molecules in a carrier that soak into the masonry and line the pores, making the surface hydrophobic without forming a film. Water beads and runs off; water vapour still passes through. Silanes are the smaller molecule and penetrate deeper, and they need an alkaline, cementitious substrate to react with, which is exactly what concrete block is. Siloxanes and silane-siloxane blends are pH independent and work on a wider range of surfaces. Neither one changes the look of the wall, and neither one will do anything about a crack wider than about 0.02 of an inch.

Film formers: acrylics, elastomerics, cementitious coatings

These sit on top and bridge hairline cracks. An elastomeric wall coating is a high-build acrylic applied three to five times as thick as paint, formulated to stay flexible when it is cold. It changes the colour and texture of the wall permanently, and it is the only category that can span the crazing and hairline cracking an older split-face wall develops.

Products specified for split-face block

We do not sell any of these and we do not take anything from anyone who does. This list exists because "we sealed it" is a meaningless sentence without a product name, and because a couple of these have a specific split-face rating that a nearly identical sibling product does not.

Named because their published data sheets address concrete block or split face specifically, checked against the manufacturers' own current pages. Products get renamed and discontinued; if a bid names something not on this list, ask for the data sheet and look for the substrate list and the perm rating. Nothing here is a recommendation of a contractor or a product for your building.

What sealing alone will not fix

This is the part contractors selling a sealing job will not lead with, and it is straight out of the industry's own literature: a water repellent is rarely effective and is not recommended in place of the repairs below. It cannot fix an ineffective sill, cap or coping, and it cannot fix incompletely filled mortar joints. Put another way, a repellent may reduce how much water gets through the block, but it does nothing about the wall's ability to handle the water that does get in.

A sane order of operations on a split-face building looks like this.

1. Find out where it is getting in before anybody bids on fixing it. That means water testing, at the wall field and separately at the windows, plus infrared or moisture readings, and sometimes opening a small area at a joist pocket to see the concealed condition. Starting with a sealing bid is starting at step nine.
2. The top of the wall. Coping and cap flashing, continuous, lapped, sealed, with a drip edge that projects past the wall. Every Chicago practitioner who works on these buildings says the same thing about where the water comes from, and this is the cheapest large improvement available.
3. Roof drainage, downspouts and grade. Water dumped at the base of a wall with no base flashing is a direct path in.
4. Repointing. Joints cut out to two to two and a half times their width and repacked properly, not smeared over. New mortar should not be harder than the units around it.
5. Sealant at joints and penetrations. Window and door perimeters, control joints, hose bibs, dryer vents, light fixtures, railing anchors, lintel ends. A proper backed and tooled elastomeric sealant, not caulk run over a wet joint.
6. Window head and sill flashing. Heads of all openings, sills sloped at least fifteen degrees, end dams turned up at each end. Retrofitting head flashing usually means taking out a course of block, which is why it rarely happens and why the windows keep leaking.
7. Through-wall flashing and weeps at the base and at each floor line, so the block cores can finally work as the drainage cavity they are.
8. Let it dry before you close it up. Sealing a saturated wall traps the water already in it.
9. Then choose a treatment. A penetrating repellent if the wall is otherwise sound, a high-perm elastomeric if the face is crazed enough that a penetrant will not get you there.

There is a genuine minority view among Chicago inspectors worth hearing: that sealant-and-caulk remediation on masonry can trap moisture and accelerate concealed decay, and that no masonry standards body approves caulk as a masonry repair. We think that overstates it, but it is a useful counterweight to a bid that consists entirely of caulk and sealer.

Sources for this sequence: BIA Technical Note 6A and Technical Note 7; repointing depth from the National Park Service's Preservation Brief 2.

What it costs, with the caveat it deserves

There is no free authoritative cost dataset for masonry restoration, so treat every figure below as a range from lead-generation sites and contractor marketing, not as data. Nationally, sealing or waterproofing a masonry wall is commonly quoted around 0.50 to 4.00 dollars a square foot, repointing from roughly 3 to 25 dollars a square foot depending on height and access, and tuckpointing a bit above that. Chicago restoration contractors publishing 2026 numbers put residential tuckpointing in the 8 to 22 dollar range per square foot, two-flats and row houses higher, and anything needing a swing stage higher again. An elastomeric coating sits at or above the top of the sealing range, because the job includes washing, patching, a block filler on porous units, and two coats.

For a whole small condo building, one Chicago firm publishes 16,000 to 21,000 dollars to seal a three-story building, though the same firm gives two different figures on two different pages, which tells you how firm the number is. Get three bids that each name a product and a coverage rate, and compare those rather than the totals.

The "Chicago banned it" story, and what happened

The reason this matters to a buyer is not legal, it is practical. Nobody is going to hand you a violation because a building is split-face, and no inspector is required to look at it on a schedule. Whether the wall is a problem is entirely a question of how it was detailed and how it has been maintained, which is what the rest of this page is about.

The questions buyers ask

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