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I've read that one should use a special type of mortar with old brick houses because they expand more - portland mortar vs soft lime.

The recommendation from the NY Landmarks Commission states:

Use a soft, lime-rich mortar that will be elastic and allow for movement, which will help prevent spalling and breakage of historic brick due to expansion caused by water absorption.

https://www1.nyc.gov/assets/lpc/downloads/pdf/pubs/App_Guide_Restoring_Arch_Features.pdf

I might possibly need to submit a permit to do this small repair.

If I need to repair some small sections of mortar to prevent water and ice getting in - should I use commercial regular mortar repair tubes or should I get an acrylic-based or some other special mortar repair product? Is the section small enough that I don't need to worry about matching expansion rates and sand color? It looks like this small section was already repaired more recently and doesn't have matching color mortar. Was using the wrong type of mortar the reason it's cracked?

The cracks are not huge - pictures shown below. Could I even use non-sanded grout repair material?

enter image description here

Cracks around mortar at knee level around the 1st-floor main exterior doorway (up a set of stoop stairs)

Cracks around mortar at knee level around the 1st-floor main exterior doorway (up a set of stoop stairs)

Front on shot of the cracks near doorway Front on shot of the cracks

Some mortar cracks between my house and the next adjoining house. Should I use different mortar for the cracks between my house and the next door house? Some mortar cracks between my house and the next adjoining house

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  • Is that brick facing? Or foundation, trying to understand if you may need need a stronger mix.
    – Ed Beal
    Sep 16, 2019 at 13:57
  • Not sure what you mean by brick facing - but it's facing front on in the 1st and last pictures, while the 2nd picture shows it around a door jamb. The whole location is on the 1st-floor entryway above grade at the top of the stoop. Sep 16, 2019 at 15:01
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    @EdBeal Added a wide shot picture with a circle around the affected area for reference Sep 16, 2019 at 15:09
  • It should be brick facing Sep 16, 2019 at 21:55
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    Grout is just Portland cement with a color additive. 100yo building: you need actual mortar with hydrated lime. @EdBeal, +1.
    – Mazura
    Sep 17, 2019 at 2:32

3 Answers 3

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Yes, historic buildings need mortar that matches the older brick strength; N (psi 750) and S (psi 1800) could eventually cause the bricks to crack. Type K (psi 75) is a bit too weak though. If you can find a store that carries type O (psi 350), that should do the trick. Big box stores only carry N and S, so try to find a locally owned masonry supply store in your area. They should be able to help.

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    The problem is that older bricks were case - hardened, rather than hardened all the way through. This means that they cannot take the compressive force that modern bricks do, since modern concrete expands as it cures, this can apply enough Force to cause the outer shell to crack, and that cost you most of the strength of the brick. With historic houses, the correct mortar is often lime-based, which is what was used when the house was first built. This maybe a bit harder to work with, but not that much. I've done it when repairing my own foundation.
    – keshlam
    Sep 28, 2023 at 23:01
  • Just read an article about not necessarily using type O mortars lancasterlimeworks.com/learning-center/lime-mortar-basics Nov 6, 2023 at 11:46
  • @Liz I've updated the answer with the article from lancaster lime works Nov 6, 2023 at 11:53
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With a facing I would clean all the loose out and mix some but I have the lime and bags of clean sand and pick up some fresh cement. Since you are new to this I would suggest a pre mixed repointing mix. It is a little more expensive than mixing it up your self. Pre mixed is basically Portland cement, clean fine sand, hydrated lime, and water, I will use an add mix it helps to seal but I don’t know if it is necessary, I started using addmix after learning how to tile. The reason I asked about foundations is then a stronger type (n or s ) would be needed. For exterior above grade any grade n, s or m will be fine. But purchasing a premix just go to a big box store and find the pre- mixed repointing bag. Add water according to the instructions better to start with not enough water and work up. There are acrylic? Premixed that you put on and let dry but I have not used these some one else may have advice on them.

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I found 2 products, Lime Putty Mortar from Lancaster Lime Works and Ecologic Mortar from LimeWorks.

Looks like Type O mortar is not recommended: https://lancasterlimeworks.com/learning-center/lime-mortar-basics/

Because the load is evenly distributed over this wide footprint, the actual pounds per square foot (PSI) load at the base of the wall, even on a five-story Georgian structure including the load the floors carry into the walls as well, is easily under 200 PSI (and probably closer to 100 PSI) – with a generous safety factor added in. In other words, the load is minimal and the mortar should therefore be lightly rated.

Many masons and the general construction industry’s response is to provide a “weak” cement mortar, like Type O, with a maximum compressive strength of 350 psi. This certainly makes the mortar softer than most masonry units and historic bricks. While this type of mortar is consistent with the PSI characteristics, there are other criteria to consider.

Remember that in a load-bearing masonry building, we want the mortar to act as a cushion to evenly distribute the load. Type O mortars are hydraulic (portland cement-based mortar), meaning as soon as you add water, they have largely hardened to full depth within 48 hours. In contrast, non-hydraulic Lime Mortars resist point-loading almost indefinitely.

While Type O mortars reach a similar final strength, they will not accommodate movement and allow for stress relief, especially near the exterior of the masonry envelope. There is no autogenous healing from Type O mortars, whether of Portland or “natural” cements. They won’t have the breath ability of a Lime Putty Mortar, which is important when historic masonry is often very porous. Typically, Type O mortar also doesn’t last longer than fifty years.

Plus, Type O risks leaching. The type of lime added to this type of portland cement makes the mortar workable. This also assists with water retention during working. However, the result is free lime that will leach out and cause staining later; it is lime that doesn’t carbonate.

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