Lime Mortar vs. Portland: Why the Wrong Mortar Is Destroying Dayton's Oldest Brick

September 4, 2026

Walk down Fifth Street in the Oregon District or any block of South Park and look at the brick walls that have been “repointed” in the last fifty years. On a lot of them you will see the same thing: crisp gray mortar joints standing proud and intact, and between them, brick faces dished, flaking, or gone entirely. The joints look great. The brick is dying. That is not weather doing its ordinary work. That is a mortar that was harder than the brick, and it is the single most common way an old Dayton house gets ruined.

Mortar is supposed to lose

A brick wall is two materials, and one of them has to give. Thermal expansion, moisture moving in and out, the slow settling of a foundation on Montgomery County’s Miamian clay: something absorbs that stress every day. In a wall built before about 1930 the mortar was designed to be that something. It was soft, it was porous, it let water vapor out, and when it eventually eroded, a mason came back and replaced it. The National Park Service’s Preservation Brief 2, the standard reference on repointing, states the rule in one sentence: the new mortar must be softer, in compressive strength, and more vapor-permeable than the masonry units. Softer than the brick, or the brick pays.

Lime mortar does that job. Preservation Brief 2 describes it: lime and sand mixed roughly one to three, soft, porous, changing little with temperature, and slightly water-soluble so that hairline cracks re-seal themselves as the lime dissolves and recarbonates. It hardens slowly by absorbing carbon dioxide from the air, which is why old lime joints keep gaining strength for years. Mortar made this way built every brick wall in Dayton from the founding in 1796 through the 1890s and most of them into the 1920s.

When Portland arrived

Portland cement was patented in Britain in 1824 and first manufactured in the United States in 1872, but Preservation Brief 2 notes it was not in common use across the country until the early twentieth century. Up to about 1900 it was an additive to speed set time. By the 1930s most masons had moved to equal parts Portland cement and lime putty, and masonry cement, the premixed bagged product, arrived in the same decade. So the mortar in a Dayton wall built between 1873 and 1930 can be anything from pure lime-sand to a half-and-half blend, and it has to be examined before anyone matches it.

Map that onto Dayton’s brick. The Oregon District, the city’s first locally designated historic district, carries architecture from 1820 to 1915. St. Anne’s Hill dates from 1860 into the early 1900s. Huffman was built between 1870 and 1890 with almost nothing added after 1900. South Park’s 780-plus brick and frame structures run from the 1880s into the early twentieth century. Dayton View, the city’s first planned neighborhood, runs from the late 1880s through the 1930s, and Grafton Hill from the 1880s to the 1920s, both filled in faster after the affluent moved uphill following the 1913 flood. Oakwood boomed in the 1920s and 1930s, right across the transition. Kettering did not become a village until 1952, and Charles Huber laid his first plat of brick ranches in 1956; by then the mortar was Portland-lime and the brick was hard enough to take it. The line is not perfectly sharp, but it is real: pre-1930 brick was made for soft mortar, and postwar brick was made for hard.

What a hard mortar does to soft brick

Preservation Brief 2 explains the mechanism and shows a photograph of the result. A mortar stronger than the brick will not give, so stress is relieved through the brick instead, as cracking and spalling that cannot be repaired. And a dense Portland mortar is less permeable than old brick, so moisture that used to evaporate through the joint is trapped in the brick. Dayton then supplies the freeze: NOAA’s normals put the January high at 37.1 degrees and the low at 21.8, and the Federal Highway Administration counts roughly 110 air freeze-thaw cycles a year in Ohio. Water in a brick expands about 9 percent every night it freezes. The fired face of the brick, its only real weather skin, pops off. Once the skin is gone, the softer core absorbs water faster and the brick crumbles from the face inward.

The Brief’s caption for that photograph could have been written about any of a hundred Dayton walls: hard Portland mortar less permeable than the soft bricks, moisture unable to evaporate through the mortar intended to be sacrificial, water pressure eventually popping the surface off the bricks.

There is a second, quieter damage. Preservation Brief 2 points out that moisture moving through masonry carries dissolved salts and deposits them where it evaporates. If it evaporates at the joint, the salts are harmless efflorescence on the surface. If a hard mortar forces it to evaporate inside the brick, the salts crystallize there and push the face off from behind. Either way the brick, the part of the wall you cannot replace without changing the building, takes the hit meant for the mortar.

How to spot it on your house

• Proud joints, dished brick. Mortar lines standing above a recessed, flaking brick face. The mortar won.
• Face-spalled brick in a band. A run of damaged units at a chimney top, under a sill, or along a parapet, where a past repointing job concentrated.
• Bright gray joints on a buff or cream wall. Nineteenth-century Dayton mortar was usually lime-colored, matching its sand; a uniform gray Portland joint on a Victorian front was a later job.
• Fat joints. Preservation Brief 2 shows the “before and after” of improper repointing that visibly widened the joints; a grinder used carelessly overcuts the brick edges and the mortar fills the damage.
• Damage on the wall, not in the joints. If the joints are sound and the brick is failing, the mortar is the suspect, not the weather.

What to do instead

1. Sample the original. Preservation Brief 2 describes the simple field method: take a few unweathered samples from different parts of the wall, crush them, and examine the binder color and the sand. The sand is what you see in a joint, so matching its color and grain is most of a visual match.
2. Pick a mortar softer than the brick. For soft, hand-made brick the Brief’s exposure table recommends a straight lime mix in sheltered spots, Type K in moderate exposure, and Type O in severe. For harder late-19th-century brick, Type O or Type N. Springfield’s own historic guidelines, adapted from the same Brief, publish a default of 12 parts sand, 4 parts hydrated lime, and 1 part Portland for most historic masonry, which lands at Type O. Type S, the hardware-store default, is the wrong answer for any of it.
3. Remove the old mortar without wounding the brick. To a minimum of two to two and a half times the joint width, with hand chisels on soft brick and never a grinder on the vertical joints.
4. Pack in lifts, tool at thumbprint hard, and cure. Quarter-inch layers, each allowed to harden before the next. Lime mortar is then misted and covered for the first days so it carbonates instead of drying out; the Brief is specific that a high-lime mortar left to dry too fast chalks and fails.
5. Replace what Portland already killed. Spalled units come out and are replaced with matched brick, or turned so the good face is out, in the new lime mortar. See brick repair.

Done this way, Preservation Brief 2 says a repointing job should last at least 30 years and preferably 50 to 100, and it is the only repair that stops the loop of replacing brick that the last repair destroyed.

The paperwork in Dayton

Inside one of Dayton’s 13 locally designated historic districts, the city requires a Certificate of Appropriateness before exterior repairs; there is no fee, the Preservation Officer can issue minor ones, typically the same day, and the seven-member Landmark Commission handles the rest at its meetings on the Thursday after the second and fourth Tuesday of each month. In Springfield’s two districts, repointing is a staff-level review that wants a mortar specification and a sample. Neither process is a burden if the mortar is right, and both exist in part because it so often was not.

If you own pre-1930 brick anywhere from the Oregon District to Oakwood, the most valuable question you can ask a bidder is “what mortar, and why?” The answer should include the word lime and a reason. Our tuckpointing page covers the process, the pricing page covers what lime work costs against Type N, and the FAQ covers the rest. Send photos of the joints and the brick faces and we will tell you which one is winning.