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Table of Contents
- What Is A Parapet?
- Types of Parapet Walls
- Common Uses of Parapet Walls
- Parapet Wall Construction
What Is A Parapet?
Parapets, frequently referred to as parapet walls, are raised barriers at the edge of a roof. Parapet walls are often extensions of the structure’s main wall and act as a barricade or railing on the roof. Many parapets have built in drainage systems called scuppers, making them significant for flat roof design and construction.
Because of the many flat roofs in Massachusetts, any Boston roofing company should have in-depth knowledge of parapets. The various types, uses, and construction materials are all important! A key consideration in a city like Boston, MA is drainage. Because New England gets so much year-round precipitation, installing a flat roof the right way is more important than ever.
These roofing walls have existed long before modern roof replacement practices, so it is valuable to first understand their historical significance.
History of Parapets
One of the earliest known references to parapets is by Moses in Part 5 of the Torah, where he states, “When you build a new house, make a parapet around your roof so that you may not bring the guilt of bloodshed on your house if someone falls from the roof (Deuteronomy 22:8 NIV).”
Their relevance to homeowner safety will be discussed later in this article, but beyond keeping occupants safe from falling off a roof, parapet walls are historically important for protecting inhabitants from outside attackers.
Many ancient castles and military forts, even some as recent as The Alamo in Texas, had defensive parapets, known as battlements, constructed on the roof to provide cover for troops defenting the structure.
Types of Parapet Walls
Parapet walls can be categorized in two ways; design style and operational shape.
By Design Style
The design of these walls often has little to do with their functionality. Different designs can make the look of the entire building feel entirely different.
Considered by some as the original style of parapet wall, embattled parapets are the type we mentioned in our historical breakdown above.
Embattled parapet walls were originally used for the military defense of buildings, castles, and sometimes even entire towns and countries. China’s “Great Wall” has embattled parapets built into the side.
The gaps between the raised portions of embattled parapets are called crenels. From a military perspective, the entire wall provided an archer cover, while the crenel gave them a place to safely shoot arrows through.
You won’t find embattled parapet walls being constructed any more, but they no doubt give older buildings a very beautiful, historic feel.
While the embattled look was most popular during ancient times, plain parapet walls are what you’ll usually see in modern building construction. As the name suggests, this style is architecturally quite boring, but don’t let the plain name make you underestimate their value!
Plain styles can be made from a number of materials, but they’re generally constructed with concrete. Unlike embattled styles, a plain parapet wall does not have gaps, which is why many designers prefer them when roof top privacy is the principal goal.
Perforated parapets serve similar purposes to plain designs, but offer increased aesthetic value to the roof line. A perforated parapet wall can come with or without gaps throughout the wall.
The perforated aspect means the walls themselves have design elements built directly into the exterior. This gives the building owner an opportunity to increase the curb appeal of the structure. Some examples include arches, circles, and various religious symbols
Sometimes called “double walls”, paneled parapet walls are similar to perforated styles in that they have a base “plain” design with cosmetic additions to the exterior to make the outside look different. Where they differ is that the paneled styles are much more basic than perforated walls, consisting simply of extra wall panels to give the wall more depth.
The types listed above are good for describing the general nature of a wall’s design, but do not consider the roof wall’s actual shape. Shape is important because it affects how the parapet works.
Flat parapet walls are the most frequently used shape and are commonly found on flat roof surfaces. They are what most people picture when they consider a wall sitting on a flat roof.
Sloped parapets are found on the edges of sloped roofs, such as gable roofs. The sloped walls follow the slope of of the roof itself, providing architectural depth to the home or building.
Stepped parapet walls can be found on both flat rooftops and pitched roofs.
When found on a pitched roof, they are similar to sloped parapets in that they follow the exact slope of the roof. The primary difference between the two types is that stepped parapets have “steps” instead of a straight-lined slope design.
When used on a flat roof, the parapet’s base remains flat, but the wall is built up in steps to an apex at the top. This blueprint can give the building a grandiose appearance, which is why you’ll see them on older churches and government buildings.
Much like stepped styles, curved parapet shapes feature a curve instead of steps. They follow the slope of a pitched roof (or the roof line of flat roofing) but have a circular curving shape instead of a straight line.
Common Uses of Parapet Walls
Now that we’ve covered what parapets are, what they look like, as well as their shapes and, we can move on to their purposes. What is the point of a roof’s parapet wall?
A nicely assembled rooftop parapet wall can dramatically change the look and feel of a building. One of the more common reasons perforated and paneled styles are used over plain styles is to improve the building’s appearance.
Nobody likes nosey neighbors and if you live in a city, neighbors can live quite close to your space. For some folks, this feels like an invasion of privacy and rooftop plants can only cover so much of your neighbor’s line of sight. For more coverage, you’ll need a tall roof wall parapet.
A parapet exterior wall along the roof can turn your rooftop space from something that’s completely out in the open, to a more secluded hideaway. If you have a roof deck patio where you spend lots of free time, this privacy is critical.
In addition to hiding your activities, these roof barriers can cover up unsightly objects such as HVAC units and service hatches to give your whole home or commercial space a more clean cut look.
As discussed earlier, the original use of a parapet wall was to increase safety.
If a building has a flat roof, its occupants are more likely to visit and congregate on the roofing area. And why not? Doing so is fun and provides the best views compared to anywhere else in the building. All too often, people can get far too close to the edge where they risk falling off by mistake. Having a wall surrounding the perimeter of a roof makes it much more difficult for these accidents to happen.
Protection From The Elements & Water Management
When we say protection we do not mean protection from attackers which was significant in past centuries. Protection nowadays means shielding the roof area from external debris and dealing with adverse weather conditions.
Keep things off the roof
Because they are walls in nature, parapets are great for keeping debris and bad weather off of a flat roof.
When wind blows dust or other debris off the street and up into the air, a wall along your roof keeps the dust, trash, or other rubbish off the roof. This means less maintenance for home and building owners.
Driving rains and water infiltration
Perhaps more relevant than floating debris is heavy, driving rains.
A well-designed parapet wall can help keep water away from the building’s interior because it has less of an opportunity to end up sitting on the roof membrane. Ponding water on flat roofs can lead to deterioration of roof materials, leaks, and many more issues.
Breaking up winds
High winds have been known to cause vortices that swirl around on a roof, and in some cases, do significant damage by blowing off roofing materials. Parapet walls can help reduce this natural phenomenon.
According to HJ Leutheusser at the University of Toronto’s Department of Mechanical Engineering, “…parapets can help reduce high local suctions… If it is of the proper height, a parapet can lift the flow and the separation lines high enough to prevent their re-attachment to the roof surface, so that one large vortex region forms to “absorb” the small, tightly sealed one.”
Parapets are sometimes used as fire walls by condo and apartment builders. The walls are made extra thick between one condo and the next so if a fire breaks out in one unit, the flames have more difficulty spreading the the next unit.
Parapet Wall Construction
Construction of a parapet can be broken down into two categories. The first is design elements; such as how high the walls should be and what they’re created from. The second category encompasses the wall’s features; such as drains, coping, and drip edge.
Parapets come in all shapes and sizes but even though many types can be created, there are only so many materials that can be used for construction.
For the purposes of explanation, the design details outlined below will have to do with proper construction of parapets. For example, of course these roofing walls can be created at any height… you could have a 2 inch wall, but this pseudo wall would not satisfy any of the requirements necessary for a parapet wall to serve the purposes outlined in the previous section.
Slope of Coping
The top of a parapet wall is called coping (more on coping in the next section). It is important for parapet walls to have a sloped top to avoid rain accumulation. In cold climates like Boston, these slopes also serve a duel purpose for keeping snow and added pressure from building up on top of the wall.
The slope of a parapet wall should slope inward towards the roof itself, so water and melting snow does not drip down the exterior wall; potentially causing damage to siding and windows, or falling onto the street below.
The danger of water ponding on top of the coping is similar to the dangers of it pooling up on a flat roofing system. Liquid and water vapor can seep into the materials underneath, which will end up damaging the membrane over time. If the membrane is not installed perfectly, water can quickly navigate behind the membrane and cause structural damage.
How tall should a parapet wall be? According to section 705 of the International Building Code, “The height of the parapet shall be not less than 30 inches (762 mm) above the point where the roof surface and the wall intersect. Where the roof slopes toward a parapet at a slope greater than two units vertical in 12 units horizontal (16.7-percent slope), the parapet shall extend to the same height as any portion of the roof within a fire separation distance where protection of wall openings is required, but in no case shall the height be less than 30 inches (762 mm).”
To simplify; the minimum parapet height for a flat roof is 30 inches. Measured in feet, this is about a 2 ½ feet. Keep in mind that these are minimum requirements and most wall heights should be increased based on the situation.
What They’re Made Of
The material chosen to create these walls is important because it determines their strength and how they will function.
Many original parapets were framed in wood out of necessity. Most buildings were built out of wood, so assembling a stone wall on top of the building could be too heavy.
When wood is used as the primary material in modern parapet assembly, the IBC states that “exterior walls are required to be noncombustible or framed with fire retardant-treated wood.” The use of fire retardant treated wood is important for fire protection purposes.
Stucco Masonry (Stone or Brick)
Natural stone and brick are both used as a parapet material.
Even on walls that are made of steel or metal, stone stucco can still be used as the coping cap for the wall. This is because the stone can be more easily shaped to slope towards the interior of the structure.
Steel and Metal
Contemporary walls are made using contemporary materials. And it doesn’t get much more current-day than smooth steel and fabricated metal.
Using these materials is beneficial because the metal can be molded and shaped into any form that the building requires. The end result is the exact height, thickness, measurements, and image that was desired. Steel offers a great way to get the job done right with less manual labor.
Features & Materials
Creation of the walls is limited to a few materials; namely wood, stone, and steel as mentioned above. However, the actual features of the walls are made from different materials and serve different functions.
Starting from the top, now we here; coping. Coping is a fancy name for the top of a parapet.
The importance of sloped coping was discussed earlier, so we can keep this section brief. Just remember; it is very important to keep the coping watertight. This is coping’s most important job! If the coping isn’t completely waterproof, the interior wall is doomed to fail over time.
Drip edge built into the top of these walls helps prevent the compromise of the entire wall. Blueprints for many parapets include an overhang on the coping, with the drip edge (or other type of flashing) attached to the edge of the overhang. Without any overhang, there isn’t much point to the drip edge so let’s assume the coping does indeed have an overhang.
Overhangs with drip edge are far superior in wicking moisture away from the parapet compared to an overhang without drip edge attached. Because without it, water can run underneath the overhang quite easily. Over time, this water will work it’s way under the coping then back into the interior of the wall. Having a good drip edge system in place helps divert the water away from the wall.
You may be familiar with rubber membranes as the primary material used for flat roofing construction. In addition to tar and gravel, these are amongst the most popular bases for flat roof systems. Having a rubber membrane underneath the parapet’s coping is important, too.
Many parapets fail because they either do not have a membrane underneath, or because the membrane in place is insufficient. There are many great resources online for proper adherence of rubber membranes, so we will not go into extreme detail. However we should note that there are two relevant types of membranes used on a roof wall parapet.
The first and most common membrane type is a simple rubber membrane under the coping. This type protects moisture coming from the top of the wall, and perhaps from under the drip edge, but does not completely waterproof the wall. For that you need a membrane that is fully adhered.
Fully Adhered Rubber Membranes
Fully adhered rubber membranes run from the coping (top) of the parapet wall, down the side of the wall, under the cant, and connects with the roof membrane or waterproof barrier under the roofing material.
Fully-adhered underlayments are the far superior type of membrane, but is hard to implement into a roof design because it is all-inclusive. It needs to be weaved into the roof wall, over the insulation, and into many sections of the roof itself. The original builder of the structure, to a certain extent, must build this type of barrier into the roof-wall in order to have it at all.
We would make a “yes you can!” joke here, but we can’t.
Much like a roof cricket, a cant, is a simple feature built up where the roof and wall meet. Cants help divert water away from the wall and into any drainage located on the roof.
Though they are quite simple, cants play a special role in roofing architecture because they help guard against water pooling against the intersection of the main roof and parapet, which is a very vulnerable area.
Because the area where parapets intersect with the roof are so vulnerable to water pooling, they’re also a great location for drainage solutions.
Drains built into the side of a parapet are called scuppers. A fun name in a boring world. Most flat roof scupper drains are made from aluminum or other metals. Installing these drains along the roof edges can go a long way in improving the overall drainage capabilities of a flat roofs.
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