If you own a building in Chicago built before 1950, the single most important thing you can learn about it fits in one sentence: the mortar between your bricks is supposed to be softer than the bricks themselves. This guide explains why that rule exists, what happens when a repair crew breaks it, and how to tell which mortar is actually in your walls.
What’s the difference between lime mortar and Portland cement mortar?
Lime mortar is the traditional mix – lime putty or hydrated lime blended with sand – that masons used for centuries, including on virtually every Chicago building constructed before the 1930s. It cures slowly, stays relatively soft, flexes with the wall, and is vapor-permeable, meaning moisture inside the masonry can escape through the joints.
Portland cement mortar is the modern standard: fast-setting, hard, rigid, and far less breathable. It’s the right material for modern brick, which is fired at high temperatures and dense enough to handle it. On concrete block or a 1995 townhouse, Portland cement mortar is exactly what you want.
The problem starts when the two eras meet – when modern cement goes into a historic wall.
Why must mortar be softer than historic brick?
In a masonry wall, something has to absorb the stress of seasonal movement, settlement, and freeze-thaw cycles – and something has to give moisture a way out. Historic walls were engineered (by tradition rather than calculation) so that the mortar joints did both jobs. The soft lime joints flex, breathe, and slowly sacrifice themselves over the decades, while the brick stays intact. Joints can be repointed every generation or two. Original hand-fired brick cannot be replaced.
Fill those joints with Portland cement and the roles reverse. The mortar becomes the hardest element in the wall, so all the movement and stress transfer into the brick. Moisture that used to evaporate through the joints gets trapped behind them – and in a Chicago winter, trapped moisture freezes, expands, and pops the faces off the brick. Masons call it spalling, and unlike a failed joint, it’s permanent damage.
This isn’t a contractor’s opinion – it’s the core guidance of the National Park Service’s Preservation Brief 2: Repointing Mortar Joints in Historic Masonry Buildings, the reference document that preservation commissions across the country, including Chicago’s, base their review standards on. Its rule of thumb: replacement mortar should match the original in composition and appearance, and should always be weaker than the masonry units around it.
How do I know which mortar is in my Chicago building?
Your building’s construction date narrows it down before anyone runs a test:
- Before ~1880: pure lime putty and sand. The softest mortar there is – typical of post-fire workers cottages in neighborhoods like Pilsen and Bridgeport, and of Joliet-Lemont limestone foundations.
- ~1880β1930: lime mortar with growing additions of Portland cement. A 1905 greystone and a 1925 bungalow both read as “old,” but carry noticeably different recipes.
- After ~1930: Portland-dominant mixes become standard. If your building dates from here onward, modern repair mortars are usually appropriate.
A simple field clue: original lime joints tend to be soft enough to scratch with a key or scrape with a fingernail, with a sandy, matte texture. Hard, glassy gray joints that ring under a screwdriver – especially ones that sit proud of eroded brick around them – are almost always a later cement repair. A professional brick mortar repair starts with an actual sample analysis: dissolving out the binder to measure the sand, color, and strength of the original mix.
What does cement damage look like on a historic wall?
Walk down any Chicago gangway and you’ll see the pattern. Look for:
- Brick faces flaking or “popping” right next to hard gray joints – the classic signature of cement repointing on soft common brick;
- Smeared, oversized joints where cement was buttered over the brick edges instead of tooled into the joint;
- White, crusty deposits (efflorescence) – salts carried by moisture that can no longer escape through the joints;
- Cement patches still intact while the brick around them crumbles – the repair outliving the wall it was supposed to protect.
The bitter irony: cement repointing from twenty years ago is often failing right next to original lime joints that are still holding after a hundred. Harder does not mean more durable – it means the damage moves somewhere more expensive.
Is lime mortar repair more expensive?
Per square foot, yes – usually. Mortar analysis, custom mixing, hand tools around fragile brick edges, and slower curing all add labor compared to a crew packing one bag mix into every job. But the math flips over time: properly matched lime repointing on a historic wall is measured in decades, while a bad cement job commonly starts failing – and destroying brick – within ten to twenty years. You either pay once for the right repair, or repeatedly for the wrong one, plus the cost of replacing spalled brick that can never truly be matched.
And if your building sits in one of Chicago’s landmark districts, the choice may not be yours anyway: exterior masonry work on designated buildings goes through the Commission on Chicago Landmarks permit review, which expects replacement mortar to match the historic original in composition, color, and joint profile. Hard cement smeared over lime joints is precisely the kind of work that fails review or has to be redone at the owner’s expense.
Frequently asked questions
Can I just use “Type N” or bagged mortar mix from the hardware store on my old brick?
For a pre-1930 wall, no – most bagged mixes are Portland-dominant and harder than historic brick. Softer designations like Type O, or custom lime-rich mixes, are the appropriate range for historic masonry restoration, and the exact recipe should come from matching your original mortar, not from a label.
My wall was already repointed with cement. Is the damage done?
Not necessarily. If the brick hasn’t spalled yet, the cement can be carefully removed – by hand where grinders would damage brick edges – and replaced with a compatible lime mix. The sooner it comes out, the more original brick survives.
Does this apply to stone buildings too?
Even more so. Limestone – the material of Chicago’s greystones – is acid-sensitive and delaminates where water sits, so stone mortar repair follows the same softer-than-the-masonry rule with even less margin for error.
How can I tell if my repointing was done correctly?
Good historic repointing disappears: joint color, width, and profile match the original, brick edges are crisp and undamaged, and nothing is smeared over the brick faces. If you can spot the repair from across the street, something about the match is wrong.
Sources & further reading: National Park Service, Preservation Brief 2 – Repointing Mortar Joints in Historic Masonry Buildings Β· City of Chicago, Landmark Permit Review, Commission on Chicago Landmarks
Not sure what’s in your joints? Send us a few photos of your wall – we’ll tell you honestly whether it’s a lime wall, a cement problem, or nothing to worry about at all. Learn more about our brick mortar repair and masonry restoration services, or get in touch for a free assessment.