Historic bricks are too soft for modern mortar

Recently, while visiting a client’s house we looked at these side facade. The houses is an end lot so there is a yard beside the house and that side wall is a largely contiguous uninterrupted brick wall. Along the bottom of the wall there are a few spots where bricks have spalled. Upon closer examination, we could see that the fracture line in the brick was directly coincident with where the brick had been pointed in reason years by some other contractor before the client of mine bought the house. The previous contractor used a modern mortar instead of a softer motor.

The bricks are historic and they are a common brick so they are particularly soft and have a low PSI strength. In this case the new modern mortar is much harder then the bricks and it in the case of the picture below you can see that the brick has spalled directly in line with where the new mortar entered into the mortar joint.

Buildings move. Sometimes that movement is very small and just the tiniest about. Imagine over a 30′ tall wall the building moves just a millimeter in the time that it takes the building the heat up an additional 50ﾟ. It doesn’t have to be that much but the wall is so hard and heavy overall that when the soft bricks move with new hard mortar between them it can cause the face of the brick to crack and in this case the brick face spalled or cracked directly off. It’s a perfect example because the remainder of that face of the brick stayed completely in tact.

Pediments, Crossheads, and Pilasters

The door in the photo below has an elaborate cement pediment, crosshead, and pilaster.

Ceramic granule covering masonry rowhome brick facade

Vertical brick cold joint

Alternating brick bands

Some examples of historic buildings built with alternating brick bands follow

One example of an old worn brick chimney

The chimney the pictures below is probably well over a hundred years old and looks as if it’s never been repaired or pointed.

The mortar joints are deteriorated so bad that the bricks themselves are almost loose.

Also, you can see this chimney has no spark arrestor or cap. So essentially rain water or precipitation will just go straight down inside of the inside of the house and there’s nothing to keep rodents out such as a rodent guard, and leaving the top flue is missing.

The remarkable part is that if you know Capitol Hill and DC houses well then you know that this is actually not uncommon. This particular chimney is overdue for some restoration though at this point.

Wood grain left in a concrete wall

The formboards used to build the wall in the photo below left a natural wood grain, just like the original historic formboards did about 50 to 100 years ago.

Concrete is poured into a form in a wet consistency similar to a slushy wet mud. The concrete hardens quickly and within a few days to about a month the country forms can be removed from the new concrete construction.

In modern times we have several materials we use to build our form and walls, including: plywood, melamine veneers, PVC, and more. But in the old days, in historic construction wood planks were used.

This building wall, shown above was built to resemble the Historic style and methods of construction, but in reality there is a sauna to that indicates the Historic effect is actually artificial and not actually historic:

1. If you look very closely you can see there are round spots in the concrete of the wall face that are approximately the size of a quarter or half dollar, about 1.25 inches in diameter. These spots look like a snap ties used in modern forms, not used in historic forms. The snap ties have a plastic collar that prevents the form sides from being squished together.

2. The building above the base wall shown is a high rise building. It’s very unlikely that historic foundation would have been built to the standards required for a modern high rise building.

Sawtooth skylight

This building has a good example of a sawtooth skylight.

Here is a look from the inside.

The sawtooth style layout allows for a large roof of a factory, warehouse, or gathering space to be installed without the use of modern flat roofing products. This type of roof allows for a large area of covering without the need for interior drainage systems and large span structural framing elements.

This type of roof is very similiar to a clerestory roof.

Clerestory roofs have been used in building architecture going all the way back to Greek and Roman architecture. Sawtooth and clerestory roof systems allows for passive fenestration and increased ventilation.

This type roof was commonly built in the large factory and warehouses in the 19th century and has recently made a resurgence with an increased emphasis on green construction which values the passive airflow and luminance.

Soft (control) joint in an ashlar masonry wall

In a masonry assembly the masonry units are held together by mortar joints. That is pretty obvious, but what a lot of people don’t know is that in a masonry wall it’s also normal and correct to have some joints wall not be built with mortar but instead be built with a resilient or elastomeric sealant. For example in most residential walls, every 20 or so linear feet around the facade there will be a vertical control joint. That control joint will be cut directly through every other brick so that the control joint runs in a straight line.

In the photo below you can see a control joint that was intentionally installed in the knee wall.

In the case above, the soft joint has failed, but that works out perfect for us to see the inside of the joint, just for us to examine and understand. Also, it’s ok that it has failed, it should be expected that control joints fail many times within the life of a masonry wall. They require a comparatively advanced maintenance schefule. Typically a masonry wall should last over 50 years without much maintenance. However a control joint for only last approximately 3 to 10 years before need to be reworked and replaced.

Also, it should be noted that in this open during, since the sealant has failed, you can see there is a backer Rod or expansion board inside of the space of the joint between the stones. That is intentional and done correctly in this photo. That expansion board does a couple things to benefit the joint. It allows for movement of the stern without having a hard pinch point and simultaneously provides a backer so that the ceiling does not have a deeper opening which needs to be filled. Sealants have special limits to which they can be applied. For a sealant to work properly it can’t be applied beyond a certain Dimension, in this case having a opening that is roughly 3/4 of an inch x 3/4 of an inch, that is inappropriate space.

In the photo below you can see a regular hard mortar joint in the same wall. We show this here just for comparison.

And also, here for comparison in the photo below you can see a control joint which has not failed. The sealant used has not been applied very smoothly, but all the same functionally this control joint is still working the way it should. It is an impermeable membrane which keeps water or moisture out of the joint. And also, because this is soft and has much higher flexibility then a regular mortar joint, it allows the wall to move a little within the 20 foot long section without breaking the mortar or stone the way would otherwise if the control joint was not there.

In this context when we say hard mortar joint we mean a regular mortar joint and when we say ‘soft’ joint we mean a joint that is filled with a sealant and no mortar at all.

Cross section of a retaining wall, ruins on the ocean

The ruins of this retaining wal show a good view of the cross section of the construction. It’s hard to see the base at grade and the footing is underground. Yet you can see exactly how the watertable and stone coping attach to the rubble stone construction of the wall.

In this case the water table is in the stretcher position. For residential homes and commercial buildings, often where you find the water table it will be coincidence a damp proof course, for example at the top of the foundation.