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Mortar Bees Holes – Examples from the field

Learn what mortar bees holes are, why they occur, and how to mitigate them

Mortar Bees or Mason Bees will burrow into relatively soft mortar, right on the face of a brick masonry wall and create little burrows.  Historic brick mortar happens to work particularly well, for them to burrow into because historic brick mortar is generally much softer than modern mortar.

Historic brick mortar is made from sand as an aggregate and lime as a binder.   Lime mortar such as the brick mortar used in the construction of the vast majority of historic brick buildings in Washington DC is relatively soft compared to modern brick mortar. Modern brick mortar generally has a PSI or pounds per square inch of compressive strength resistance over 3,000 pounds. Historic mortar by comparison is generally less than 1,400 PSI and. on the low end, historic brick mortar can be as soft as just a few hundred pounds per square inch of compressive resistance. That’s very soft, compared to modern mortar.  As historic mortar ages and deteriorates, the cohesive binder will degrade and deteriorate and the outside face of the brick mortar joints will become extremely soft, in some cases so soft that mortar can be removed from a brick joint by bare hand.

This sort of condition provides a perfect substrate for bees to burrow.   In past articles such as the article with the following links, we’ve examined and discussed similar conditions which also provide a perfect substrate for Biocolonization.

How Rain Affects a Brick Facade Part II

Clinker Bricks in a Unique Architectural Masonry Style

Top 10+ Prevalent Threats to Masonry Structures – PART II

It’s no coincidence that the same conditions described are also perfect signs that it’s time for a wall to be restored and tuckpointed. Brick restoration most often involves joint repointing, aka tuckpointing.

Brick pointing (or tuckpointing, repointing) is the process of raking loosened deteriorated old mortar from the outer surface or face of a brick facade wall between and surrounding each brick and then replacing that mortar with new mortar.  In the context of restoration of historic masonry buildings, it’s very important that the wall is pointed using historic appropriate mortar, a special type of brick mortar made to match or closely model key characteristics of historic mortar.  The main characteristics of importance are compressive resistance strength, modulus of elasticity, and permeability.

We often discuss mortar and brick compatibility in our terminology encyclopedia and in many of our individual blogs on historic masonry restoration.  If pointing is completed improperly as is so common in Washington DC,  it’s detrimentally destructive to the building.  It’s one of the most prolific types of types of construction defects in the entire industry. Using improper mortar is harmful and extremely expensive to repair, once completed incorrectly. We’ve said it many times before and here again as we underscore that it’s extremely important to hire a contractor for pointing work who is thoroughly experienced in historic masonry restoration. The principles of historic masonry can be complex, but the simple essential of ensuring compatibility cannot be overemphasized.

The picture below shows a wall of a freestanding residential home in Northwest Washington DC. This particular building was constructed around the mid 20th century.   This particular building happens to be much newer than the vast majority of buildings that our company typically restores.

mortar bees

This type of building facade is very common, across many neighborhoods in the United States.   When you look closer though, you can see many different and small holes bored into the brick mortar.

what are mortar bees holes

These are fine details, almost difficult to notice even when you’re standing in front of the home, but when you look closer at the brick, right at the mortar joints, you can see these small holes all over the mortar.  In other cases, particularly at historic building, borings can made even into the surface of very soft and/or deteriorated masonry units such as brick or stone.  (By comparison, in this case only the deteriorating mortar joints have been susceptible to Mason Bees.)

why do mortar bees holes are happen

These holes are made by Mason Bees or also otherwise known as Mortar Bees. They’re average size bees that literally bore holes into the mortar.

causes of mortar bees holes

The picture below shows a deep recess into a bed joint between the individual bricks of the wall. At the top and bottom of the boring hole, you can see a deposit, almost like a stalactite and stalagmite, mineral deposits common in underground caves, created over centuries of slow dripping water, leaving minerals behind, in miniscule amounts with each drip.  Here the deposit is created by the bees themselves, using the same brick mortar material.


The next pictures below show a different material where a hole in the mortar was filled with an organic material.


The picture below shows a similar bore in a prepend joint. Prepend joints are vertical, whereas bed joints are horizontal. Perpend joints typically deteriorate faster than bed joints, for a variety of reasons, both starting with a typical higher level of inconsistency in installation and because of the less compacted nature at the head joints, at the sides of each brick in a stretcher position.


Naturally, in the process of brick construction, as individual brick units are laid into a masonry facade, the bed joints are set more more consistently full and are compacted by the brick. Comparatively, the perpend joint is often, even when attempting a full head joint, filled less than the respective bed joint.

To properly maintain, repair, and care for these historic buildings, a knowledge, interest and understanding of historic building principles is required.  Here in Washington DC, historic masonry buildings are extremely expensive and the amount of financial loss caused by improper repointing

and low quality construction is staggering.   However, in addition to the direct financial value of the property, there is also a cultural loss when historic buildings are damaged. By comparison, consider neighboring poor cities, when historic buildings are damaged, it’s not just the loss of value to the property owner, there’s also a loss to all inhabitants and visitors of a city, present and future, who care about architecture, history, and culture.

We encourage all of our clients, and all readers of this article and to our blog in general, to prioritize the historic built environment of Washington DC and neighborhoods such as Capitol Hill, Dupont Circle, and Georgetown and become educated on on the difference between proper historic preservation versus improper work which leads to significant damage to the historic fabric of a building.

From a conservation and preservation perspective, several approaches can be taken to improve conditions related to deteriorated historic brick masonry. Primarily, lime mortar brick joints and low temperature fired soft red clay bricks should be inspected and checked on a routine maintenance schedule, either seasonally or at least annually.   If brick masonry is kept in good condition, the life of embedded wood elements can be significantly extended.  Hire a professional contractor which specializes, understands and appreciates historic construction elements and buildings.

In this article we talked about the terminology and  concepts of historic masonry restoration, follow the links below for more related information from the IDS website:

  • Binders in mortars and concrete
  • Brick burns 
  • Butter joint
  • Capillary action
  • Cementitious siding
  • Cheek wall, masonry — Draft
  • Chemical testing
  • Code, building — Draft
  • Cold joint
  • Cold weather masonry work — Draft
  • Damp proof course
  • Downspout
  • Electrical distribution panel — Draft
  • Fenestration
  • Ferrous metals
  • Great Chicago Fire
  • Green bricks 
  • Gutter, roof
  • Lime mortar
  • Lintel
  • Oxidation
  • Parapet coping
  • Plug, clay
  • Pressed bricks
  • Raking, of mortar joints
  • Raggle, aka reglet
  • Rectilinear
  • Roman bricks
  • Roman arches
  • Roof eave
  • Roof termination 
  • Row buildings and row homes
  • Sedimentary rock
  • Scratch coat
  • Sprung arch
  • Strike, or striking of mortar
  • Tapestry bricks
  • Tooth-in, interlocking masonry connections
  • Vitreous
  • Water diversion systems
  • Zipper-joint

These concepts are part of the fundamentals of historic masonry restoration, tuckpointing, and brick repair.

The links in the list above will take you to other articles with more information on defects, failures, preservation and repair of historic masonry.  You can learn a lot more on our blog.  Feel free to check it out.  If you have questions about the historic masonry of your building in Washington DC, fill out the webform below and drop us a line.  We will be in touch if we can help.