Why Old St. Louis Brick Needs Lime Mortar
Mortar has to be softer than the brick around it. St. Louis common brick fired before about 1930 is soft and porous, so a portland-heavy modern mortar is harder than the brick it sits between. When the wall moves or freezes, the stress lands on the brick face instead of the joint, and the face spalls off. A lime-rich mix or an ASTM Type N mortar keeps the joint sacrificial.
Walk down almost any block in Soulard and you will find two kinds of wall. One has crisp brick faces and slightly recessed joints. The other has bricks with the outer quarter inch shaled away, sitting between joints that stand proud of the wall like ribs. The second wall was usually pointed in the last forty years with the wrong mortar. Understanding why is the most useful thing a St. Louis brick owner can learn.
What makes pre-1930 St. Louis brick different?
St. Louis was built out of the clay under it. Local common brick was fired in beehive kilns where temperature varied across the load, so a single batch produced everything from hard, well-burned brick to soft, underfired brick. The hardest units were sorted out as face brick for the street elevation. The softer ones went into side walls, rear walls, party walls and interior wythes, which is why the alley face of a building often looks so much worse than the front.
That soft common brick is porous by design. It takes water in, and it gives water back up as it dries. Compressive strength on a soft historic common brick can be a fraction of what a modern extruded face brick delivers. Modern brick is fired hotter and more evenly, comes out dense and hard, and has a much lower absorption rate.
So a wall built in 1895 and a wall built in 2005 are made of materials that behave nothing alike, and they need mortars that behave nothing alike too.
Why does mortar have to be softer than the brick?
A masonry wall is always moving. It expands in July heat, contracts in January cold, takes up moisture and gives it back, and settles a little as the ground under it moves. All that movement has to go somewhere.
In a properly built wall it goes into the mortar. The joint is the weak, replaceable part. It absorbs the movement, it cracks first, it carries the salt and the damp out of the wall, and after fifty or eighty years you rake it out and put fresh mortar in. That is a maintenance item.
Flip the relationship and the wall works against itself. If the mortar is harder than the brick, the joint refuses to give. The brick is now the weak element, so the compressive stress concentrates on the face and the edges of each unit. The face pops off in flakes. That is spalling, and unlike a failed joint it cannot be undone. A spalled brick has to be cut out and replaced.
There is a water side to it as well. Soft brick relies on the joints to let moisture escape. A dense portland mortar reduces that path, so water leaving the wall exits through the brick face instead. When it freezes on the way out, it takes the face with it.
What do the ASTM mortar types mean?
ASTM C270 sets out four mortar types by proportion and by property. The short version, from strongest to weakest:
| Type | Typical minimum compressive strength | Where it belongs |
|---|---|---|
| M | 2,500 psi | Below grade, foundations, retaining walls in hard units |
| S | 1,800 psi | Modern structural masonry, high lateral loads |
| N | 750 psi | General exterior above grade on modern brick |
| O | 350 psi | Interior and low-stress work, mild exterior exposure |
The trade shorthand for remembering the letters is MaSoN wOrK. The number that matters for a historic wall is the bottom of the list. Type S at 1,800 psi is routinely harder than a soft St. Louis common brick. Point a 130-year-old wall with it and you have installed a grid of stone around units that cannot resist it.
Type N is the usual starting point for exterior work on older brick. On genuinely soft units, a lime-rich mix weaker than Type N is often the right answer, closer to the hydrated lime and sand mortars these walls were built with. The Park Service guidance on repointing is explicit that new mortar should match the historic mortar in strength, permeability and appearance, and should never be stronger than the masonry units.
Can you test what the original mortar was?
Yes, and on a significant building it is worth doing. A mortar analysis takes a sample of sound original mortar and separates the binder from the aggregate, usually with a mild acid digestion. What comes back tells you two useful things: the binder-to-sand ratio, and the color, shape and grading of the original sand.
The sand matters more than most owners expect. Mortar color comes mostly from aggregate rather than from pigment, so matching the sand gets you most of the way to a match without dyeing anything. Grading affects the texture of the finished joint.
On a normal house you can get close without a lab. Take a sample of sound original mortar, crush it, and look at the sand under good light. Then mix test boards, let them cure for a week or two, and hold them against the wall in daylight. Cured mortar always reads lighter than it does wet, which is why judging a fresh joint misleads people.
How do you specify the right mix?
Put these in writing before work starts.
- Mortar type named, including whether hydrated lime is in the mix and in what proportion
- A statement that the new mortar will be no stronger than the existing units
- Sand source named, and a cured sample board approved by you in daylight
- Joint cut to roughly twice its width, with hand tools used on head joints
- Joint profile matched to the original: concave, weathered, flush or grapevine
- No sandblasting or high-pressure cleaning on the brick face
- Work suspended when the wall is below about 40 degrees, or protected and heated
In a City of St. Louis local historic district, the Cultural Resources Office publishes standards for repointing that cover exactly this ground, and the district ordinance can make the mortar mix and joint profile a condition of approval rather than a preference.
What if the wall was already pointed hard?
This is common here, and the answer depends on how much damage is done. If the joints are hard but the brick faces are still sound, the usual plan is to leave it alone and watch it. Cutting out a hard portland joint risks more brick damage than the joint is causing.
Where faces are already spalling, the work is to cut out the failed brick, replace it with salvaged common brick of similar hardness, and repoint that area with a compatible mix. Over time the wall gets converted area by area. We cover how that is done on the brick repair page.
Where to go from here
If your building predates 1930 and someone has quoted you tuckpointing without naming the mortar, ask the question before you sign. The mix is the whole job. Our estimates state the mortar type on the page and we mix cured sample boards for you to approve, and you are welcome to hold ours against any other bid you are considering.