Lime mortar vs Type N vs Type S: which one belongs on your brick

The mortar in your joints matters more than the mason’s hourly rate, because the wrong mortar does not just wear out, it destroys the brick around it. The rule is one sentence: new mortar must be softer and more vapor permeable than the masonry units. That comes straight from Preservation Brief 2, the National Park Service’s guidance on repointing historic masonry, and it decides which bag comes off the truck on an Evansville job.

The five types, by strength

Modern mortars are specified under ASTM C270, and the five types are usually remembered by every other letter of the phrase MaSoNwOrK.

There is also a straight lime and sand mix, designated “L” in Preservation Brief 2 at proportions of 0:1:3, and lime-rich blends such as Type O at 1:2:9 and Type K at 1:3:11, where the numbers are parts of Portland cement, lime, and sand by volume. More lime means a softer, more permeable, more forgiving joint. More Portland means a harder, faster-setting, less permeable one.

Why the pre-1930 date keeps coming up

Preservation Brief 2 lays out the timeline plainly. Portland cement was patented in Great Britain in 1824 and first manufactured in the United States in 1872, but it was not in common use across the country until the early 20th century, and up to the turn of the century it was treated mostly as an additive to speed the set. By the 1930s most masons were using a mix of equal parts Portland cement and lime putty. So mortar in a building put up between 1873 and 1930 can be anything from pure lime and sand to a wide range of lime and Portland blends.

That is most of Evansville’s historic brick: Riverside, Bayard Park, Haynie’s Corner, Goosetown, Lamasco, the West Side, and the older cores of Newburgh, Boonville, Mount Vernon, and Vincennes. Those walls were built to breathe through their joints. Preservation Brief 2 describes historic mortar as a bedding material, closer to an expansion joint than a glue, with moisture migrating out through the mortar rather than through the units.

What the wrong mortar actually does

Water gets into every masonry wall. The question is where it leaves. When the joint is softer and more permeable than the brick, water evaporates out of the joint, and salts crystallize on the surface as harmless efflorescence. When the joint is harder and tighter than the brick, the water has to leave through the brick face instead, and the salts crystallize just under the surface as subflorescence. Preservation Brief 2 is explicit that salt crystallization inside a unit creates pressure that can spall or delaminate the outer surface. That is why hard mortar on soft brick produces the classic Evansville failure: neat grey joints standing proud of brick faces that have popped off. Once the fired skin is gone, that brick is a $20 to $40 replacement, and the spalling brick page covers what to do next.

Freeze-thaw multiplies it. NOAA’s 1991-2020 normals for Evansville Regional Airport count 92.6 nights a year at or below 32 degrees against only 15.5 days that stay frozen, which is roughly 77 freeze and thaw cycles a winter, under 45.31 inches of annual precipitation. A wall that cannot dry out gets 77 chances a year to break.

How to pick, in practice

• Built before about 1930, soft hand-molded brick: a lime-based mortar, matched for color, sand, and texture, in the Type O, Type K, or straight lime and sand range. Hand tools on the joints, not a grinder.
• 1930s to 1970s brick: Type N is usually right. Check the existing mortar rather than assuming, because the transition years vary house to house.
• Post-1970s brick and concrete block: Type S is normally correct, and it is also the right family below grade and in retaining work.
• Chimneys and parapets: match the unit, not the exposure. People reach for a harder mortar on a chimney because it takes more weather, and on a soft-brick stack that is exactly how the brick gets destroyed.
• Foundations: old rubble stone and soft brick foundations want a lime-based mortar too, for the same permeability reason, even though they are below grade.

What a proper spec says

A quote you can trust names four things: the depth the joints will be cut, the mortar type or mix, the joint profile it will be tooled to, and the curing plan. Preservation Brief 2 sets the standards behind each. Old mortar comes out to a minimum of 2 to 2.5 times the joint width, roughly half an inch to an inch on most brick. Joints are rinsed clean and left damp with no standing water. Mortar goes in in layers of about a quarter inch, each one allowed to reach thumbprint hardness before the next. The joint is tooled to match the historic profile when the final layer is thumbprint hard, since tooling too soon lightens the color and tooling too late leaves dark tool burning. High-lime mortars are then misted, as often as hourly at first and tapering to every three or four hours, and the wall is covered with burlap for the first three days.

Temperature is part of the spec too. Preservation Brief 2 puts working wall temperatures between 40 and 95 degrees Fahrenheit to prevent freezing or excessive evaporation. With an average last spring freeze of April 6 and an average first fall freeze of October 29 in the Evansville area per NWS Paducah, that is the April to November season the published prices assume.

Done right, this is not a repair you repeat. Preservation Brief 2 says a good repointing job is meant to last at least 30 years and preferably 50 to 100. That is the whole argument for paying $12 to $20 a square foot for historic lime work instead of $8 to $14 for the wrong bag: not craft for its own sake, but not doing it twice and not replacing the brick in between.

Ranges are on the pricing page, the process is on the tuckpointing page, and the material side of Evansville’s walls is on the Indiana limestone and Ohio Valley brick page. If your house sits in a preservation area, the historic district masonry rules explain what has to be approved before any of this starts.