Wind Driven Rain Into Deteriorated Mortar Joints

Wind driven rain is one of the many types of weather and environmental factors which wear and deteriorate buildings.   Building deterioration,  in many cases, is natural, but can be managed proactively.    Weather, precipitation, moisture, water intrusions are all types of factors which can damage and deteriorate buildings. This set of items are related to and really revolve around the issue of moisture. Together this subset is a group of items that fits inside of a larger set of types of factors which lead to or cause deterioration in buildings.

Some of the main types of external factors which cause building deterioration are as follows:

  • Precipitation
  • UV
  • Wind
  • Freeze-Thaw
  • Natural organic decay

Buildings will invariably age over time; however,  Protected from the majority of the items in the list above, Buildings can continue to exist in an otherwise mostly unmaintained state, indefinitely.   For example, if you consider a wooden piece of furniture from a 1,000 years ago, if that furniture is kept indoors and out of the sunlight it can function exactly the same as it did a 1000 years ago without noticeable signs of wear or deterioration.   Buildings are exactly the same, in this way, as an ancient piece of furniture.

However, when exposed to precipitation or moisture, the furniture or a building will deteriorate.   Masonry, compared to other building materials, can resist the deleterious effects of moisture very well.   Some types of masonry also resist moisture better than other types of masonry. Brick is relatively good at resisting moisture. Historic brick was fired at a relatively low temperature,  especially when compared to typical high temperature fired masonry of today.

Brick, both modern and historic, is made from clay, very similar to mud.   In modern circumstances, that clay is dried and then ground and filtered and then rehydrated and mixed, then formed into plug.   Plug is a mass of clay that is similar to the shape of bricks but a long extrusion of material from which each brick is cut.

Modern brick is then put into a conveyor and cooked cooked or fired at a high temperature over 2000゚F. Historic brick by comparison was also fired but instead of being fired on a conveyor, it was fired in a kiln stacked in cubes. Historic bricks from which the majority of DC rowhomes and DC buildings were built, were fired in beehive kilns. Beehive kilns have a dome shape which helped convey the heat, to balance the distribution of heat to all areas of the kiln. However, by comparison to modern techniques and modern technology, the historic beehive kilns were very ineffective at evenly firing bricks. Because of this uneven application of heat, not all bricks would be sufficiently fired, within every batch. There was significant inconsistency. On the opposite end of the spectrum, some of the bricks would not be under-fired, they’d be “cooked” at such a high temperature that they would become close to vitreous in nature and discolored. Generally the discoloration would go but go from a range of red to purple. Those “overcooked” bricks were called clinkers because when struck with a hard object they would make a sound similar to glass, indicative of their vitreous nature.

You can read more about kiln firing processes at this link.

Buildings are made to resist water entry from vertical falling rain, roofs are designed with flat, more accurately also known as low-sloped roof systems.  Some roofs have eaves, extending overhangs to keep rainwater from running down walls, in vertically falling precipitation events.  These roof systems keep the brick walls dry in many cases.  Fascia and rake boards terminate in a way to overlap the joints between the roof and brick masonry walls.  Drip edges and flashings are impermeable or low-permeability materials, concentrically built to cover exposed areas susceptible to water entry.

The image below shows examples of a both drip edge and a rake board installed on a historic stone masonry building, similar to how it would be installed in many types of brick buildings. Fascia board is similar to a rake board except a fascia board is installed horizontally. Rake boards, by comparison, are installed on the gable side of a building running at an angle both somewhat horizontal and vertical.

These types of systems work very well, but the big limitation and problem with these types of passive building elements is that they do not resist wind driven or horizontal driven precipitation and rain.    Precipitation and weather events happen in a variety of forms.   Many of the simple rainfall events are just light rainfall without high winds. However other weather events of heavy rain and high winds result in wind driven rain. Directional or wind driven rain like this can cause water to enter parts of the building that are otherwise unprotected.  Historic buildings are generally not well built to resist ice damming or wind driven rain. These are conditions that lead to water entry. Normally these areas are not at risk but in wind driven and even sideways types of rain, water can enter under drip edges, beneath eaves, and run down walls.

In the image below you can see a similar example of a common bond brick wall. In this particular picture the brick joints have been raked to clear mortar joints at the outer ¾” of an inch to 1.25″ of the face of the wall.   The mortar joint raking process is an integral step in the overall process of tuck pointing and masonry restoration.  While the tuck pointing process is intended to specifically treat that outer face of the mortar joint, in this particular example you can see there is a litany of voids and recesses from deterioration and absences at initial construction which allowed for pockets of build-up seepage of water into the brick wall.

When water runs down masonry walls with deteriorated mortar joints, these voids can allow water to enter deep within the brick wythe and brick walls. Water entering these walls will dissipate slowly and evaporate very slowly. In the time that water enters it can seep into more susceptible materials that can be damaged by water and can cause deterioration in the brick itself. Brick mortar while somewhat resistant to moisture are not completely impervious.

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This link will take you to an article with more information about water seepage into mortar joints.

Brick mortar is made of a combination of a binder and aggregate / sand.  In the case of historical mortar, the binder is generally a historic lime, in, in the case of modern modern mortar, a Portland or similar type of cement.   Historic lime mortars are susceptible to moisture because they can break down and over time have lost some of the original cohesive structure.  This is one of the most important reasons that tuck pointing historic deteriorated lime mortar walls is so important.

The picture below shows an example of a historic wall where the mortar has simply deteriorated overtime. You can see the outer 1″ face is  Mostly a recess or void where the mortar has fallen from the wall and now leaves the remaining mortar exposed.

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This link will also take you to another article with more information about mortar binder disintegration and mortar joint deterioration.

In the closer picture below you can see that the mortar which remains, mostly consisting of sand, void of the original deteriorated lime, is now so soft that it is susceptible to plant roots.

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This link goes to an article with further discussion on the damage plant roots can cause in masonry buildings.

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As well, this link goes to an additional article where we look closer at damage ornate or decorative vines can cause to historic brick masonry walls.

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Washington DC is in a geographic region which experiences deep freezes in the winter, hot temperatures in the summer, and cycles of heavy rain.   One of the pressures of the winter is not just the freezing itself, but the repeated cycles of freezing and falling. This freestyle causes water in masonry walls to experience contraction and expansion. That expansion puts massive pressure on brickwork.   Voids and recesses in murder are bad because they allow water to enter and seep into places where the water can cause damage, but those voids and recesses also allow for pockets of water to build-up which can become deleterious and damaging to the building in the winter.

Local weather conditions, see NOAH, etc.

  • Rainfall
  • Wind driven rain
  • Freeze cycles

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Scam pointing is a serious problem, created by low cost or poorly trained masonry contractors (sometimes not just misinformed but also intentionally misleading) in Washington DC. One of the biggest problems with scam pointing is not only that it can cause further damage to the bricks by applying mortar that is incompatible with the remainder of the existing residual moisture, but it also covers up voids that remain active points of water entry.

The two pictures below show examples of scam pointing where the thin layer of mortar applied to the surface of the joint peels right off without application of force or pressure, that that thin application of mortar is not compatible with the substrate and allows allows water to find its way around the fake application on the surface and into the core core of the brick wall.

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The photo below shows another example of voids in masonry mortar. This particular void happens to be at a perpend joint. The perpend joint is the vertical joint between bricks.


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 following terminology and  concepts, follow the links below for more related information from the IDS website:

  • Plug
  • Beehive kilns
  • Roof eaves
  • Roof gable
  • Passive building elements 
  • Wind driven precipitation 
  • Historic lime mortar

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.