What a Mortar Joint Does

What the stone is for

Three jobs, and they are usually listed in the wrong order.

1. Bedding. It makes an even seat for units that are not perfectly flat, so load spreads across a whole face instead of bearing on high spots. This is the main job and the least discussed.
2. Filling. It closes the wall against weather and against air movement.
3. Sacrificing. It is the softest, most permeable, most replaceable part of the assembly, so that movement, salt and freezing damage go there rather than into the units.

Bonding is barely on the list. Mortar does not glue a wall together in the way most people imagine. A masonry wall stands up because of weight, geometry and bond pattern, and mortar makes that work rather than substituting for it.

Lime and cement

Older mortars are lime based: lime putty or slaked quicklime with sand, sometimes with a little natural cement. They are soft, they set slowly, they take up movement without cracking, they let water pass through and dry out again, and they can reseal small cracks over time as free lime redeposits.

Portland cement mortar is a different material. It is strong, sets fast, is far less permeable, and does not accommodate movement. For the structures it was developed for, those are all advantages.

Put a cement mortar into a wall of soft, absorbent, low fired brick and the ranking inverts. The joint becomes the hardest thing in the wall. Water that used to leave through the joint now leaves through the brick, taking salts and then the face of the brick with it. Movement that used to open a hairline in the mortar now cracks the units. The wall does not fail dramatically; it loses its faces, a few millimeters at a time, along every joint.

The principle, stated once

The mortar should be no stronger than the unit it beds. Everything else in this subject is a detail hanging off that sentence.

It is worth knowing because it explains an appearance that reads as neglect and is not. A wall with soft, eroded, slightly recessed lime joints and intact brick faces is a wall that has been working exactly as designed for a century. A wall with proud, hard, sharp joints and crumbling brick faces has had something done to it.

Where this publication stops

Here.

This is an article about a material and how it behaves. It is not an assessment of anybody's wall, it is not advice about any particular building, and it does not tell a reader what to do next. Tallyhouse does not survey, price, specify or repair anything, does not sell material, and does not recommend or name anybody who does.

That line is drawn deliberately and it is drawn narrow. A publication about what walls are made of sits next door to a trade, and the difference between the two is whether anything is being sold. Nothing is being sold here. About says what this house is, and the disclaimer says what nothing on this site is to be used as.