Masonry Restoration Blog

17th August 2026

Ignorance is Bliss (Until Reality Comes Knocking)

by Keith Boyd

In all of the years that I’ve been an historic masonry repair specialist in America, I’ve never been more surprised at the lack of knowledge among architectural companies who purport to be restoration specialists. Masonry 101, including the very basic of knowledge about how you do not ever put a Portland based mortar on top of lime mortar, is not common amongst many architectural firms.

In the USA, there is a free set of standards and guidelines from the US Department of the Interior for the rehabilitation and restoration of old buildings and monuments, which is there for all to see. In those very standards it tells you, when replacing old or historic mortars, you should replace like for like. Do architects specify this when drawing up plans for the reuse or restoration of old and historic buildings, no, they often don’t. They’re still cutting and pasting specifications from previous new-build projects, that bear no resemblance to the needs of historic masonry repairs.

A fifteen minute read of the masonry requirements in the standards and guidelines could save America from so much future heartache when the Portland they specify now, becomes the problem of moisture retention once more in the future. You also have to ask, why isn’t historic masonry repair included in any architectural degree course?

Americans love history. They travel the world looking at old monuments and buildings, marveling at their longevity and building techniques. Then at home, they think nothing of putting modern Portland mortars into old and historic masonry expecting a good outcome. It’s not going to happen.

Old buildings were made to breathe. The action of the sun and the wind would dry out a building after rain or snow had hit. The lime mortars that were used back before the 1920’s had a smaller cellular structure than that of the brick and stone they encompassed. This allowed the mortar to suck the remaining moisture from the masonry and take it to the face of the building where the sun and wind would do their drying actions. It’s called the poultice effect.

The moment you place a Portland mortar over the top of an old lime mortar, you immediately stop this interaction as the Portland will allow water into the structure, but because it sets so hard, it will not allow that retained moisture out again. So, you may ask, where does all of that moisture go? Well, it looks for the driest area to migrate to and that usually means the internal part of the wall. After enough time has passed that moisture becomes mold and then more expense, as that moisture retention is dealt with internally instead of externally.

How many times have you walked past a building and been able to pull mortar from the brick joints with a fingernail? I’ve done it too many times to mention in America. It is the second reason you don’t put Portland mortar on top of lime mortar. The Portland traps the moisture, the existing lime mortars reconstitute, or re-slake, and then there is a chemical reaction between the lime and the Portland, which, over time means that the lime mortar will push the Portland mortar out of the wall. The Portland then becomes obsolete as a structural material, and ugly as an aesthetic material.

Architects please listen: Stop putting Portland mortar in any dilution on top of existing lime mortars. They are not compatible and never will be.