Why old brick needs a mortar softer than the brick

In a brick wall, something has to give. Walls move a little with temperature and moisture, and water gets in and has to get out. In a well built old wall, the mortar is the part that gives. It is softer than the brick, it lets water dry out through the joints, and over time it wears away. Then it gets replaced, and the brick is still there.

That is the whole idea. The mortar is the cheap, sacrificial part. The brick is the part you want to keep.

Much of the brick in older Illinois houses was fired softer than brick is today. It is more porous, so it soaks up and gives off water more easily, and it is weaker. The mortar it was laid in was mostly lime and sand, which is softer still and very good at letting moisture move through it.

Put together, that is a wall that breathes. It gets wet, it dries out, and as long as the joints are kept up, it can last a very long time.

Find a spot on the wall that has clearly been repointed at some point, and a spot that looks original. Scratch each joint with a key. Original lime mortar on an old house will usually scratch fairly easily and feel sandy. A hard cement repair will barely mark. If the brick around the hard repair is flaking and the brick around the soft original joints is not, you are looking at the problem in this guide.

The trouble starts when an old wall is repointed with a strong modern cement mortar. That mortar is harder than the old brick and much less willing to let water pass. Now the wall cannot dry through its joints, so water dries out through the brick instead, carrying salts with it and leaving the brick wet longer.

When that wet brick freezes, the faces pop off. When the wall moves, the hard joints do not give, so the brick cracks instead. You end up with sharp, solid looking joints and brick crumbling around them. It is one of the most common kinds of damage on old houses, and it is caused by repair, not by age.

Look for brick that is flaking or crumbling while the mortar around it is still hard and standing proud. Look for a bright gray, very smooth mortar that does not match the softer, sandier joints elsewhere on the wall. Look for cracks that run through the brick instead of along the joints. Any of those suggests someone used the wrong mortar at some point.

If the damage is already there, the fix is to carefully remove the hard mortar where it is causing trouble, replace the damaged brick, and repoint with a mortar that suits the old work.

A good match is about more than color. The new mortar should be similar in hardness, in how it handles water, in the size and color of the sand in it, and in how the joint is finished. A lime based mortar is the usual choice for old soft brick and stone. A repair that matches looks like it was always there and works with the wall instead of against it.

On a newer house with hard modern brick, a modern mortar is the right choice. The rule is the same in both cases: the mortar should suit the brick.

• Softer than the brick
• Lets moisture dry out through the joint
• Sand color and size close to the original
• Joint finish matching the rest of the wall
• Lime based for old soft brick and stone

If you have an older house and someone is coming to repoint it, ask what mortar they plan to use and why. It is a fair question and a good mason will have a clear answer. Brian Lee Smith can look at your brick and tell you what suits it. Send a close photo of the brick and joints to (217) 549-2230 with your town.