A Comprehensive Guide To
Commercial Roofing Systems:
Flat Roofs, Sawtooth Roofs, & Living Roofs
Commercial roofing systems come in a variety of styles and unlike the typical residential roof, they often function as more than just a way to keep a building dry. Because most commercial buildings have a large footprint, a conventional pitched roof isn’t really practical as it would have to extend extremely high to have an adequate pitch. So other than traditional churches, most buildings that size will have either a Flat or Sawtooth roof design. Each of these have advantages and disadvantages. In recent years the popularity of Living Roofs has been growing as well (no pun intended). In this article we will cover the pros and cons of each of these popular roofing systems as well as some of the material and drainage options a commercial building owner will need to consider.
Flat Roofs Aren’t Flat
A majority of commercial roofing systems are what we in the roofing industry call ‘flat roofing’, so we will cover this extensively in this comprehensive guide to commercial roofing. The term Flat Roof is a bit misleading because if it were really flat the roof would pool water and the roof would fail much sooner. Generally a roof is considered to be a “flat roof” when it has less than a 10-12° slope. Flat roofs have some advantages for larger retail and commercial buildings.
So just because something looks flat, don’t assume it is. We think everyone could agree on a basic principle like this. Just ask the internet!
Pros and Cons of Flat Roofing
The Pros
Cost of installation:
Because less materials are needed and the installation process is faster, a flat roof system is generally less expensive to install compared to a pitched residential roof. The average cost of a flat roofing system will vary based on size, complexity, and the materials used. We will cover the options of flat roofing materials shortly, but the average flat roofing process is fairly fast and straightforward.
Durability:
Depending on the design and materials used in a flat roof, they can be quite durable and long lasting with proper inspections and maintenance. Of course there are always variables and if you’ve read any of our other roofing guides, you already know that we recommend using quality products and a top notch professional roofing contractor that guarantees their workmanship. Having said that, flat roofs have one advantage over pitched roofs, which is that they don’t suffer from wind damage anywhere close to a pitched roof with asphalt shingles.
Rooftop Access:
Anyone that’s ever tried to work on their residential roof knows how daunting even simple jobs can be on a steep roof pitch. Because regular inspections are the key to increasing the lifespan of a roofing system, having the ability to simply walk around without needing a safety harness is a huge benefit. While some single story retail buildings may still require a ladder to reach the rooftop, taller buildings will have easy access via a stairwell or roof hatch.
Additionally, larger residential buildings take advantage of this accessibility by using some of the large open spaces to create rooftop decks or even swimming pools in highrise hotels.
Utility Space:
While residential houses make use of their pitched roofs for attic storage and have utility rooms, a large flat top roof essentially acts as the utility room and home to many of the air conditioning units and other mechanical systems of a commercial building. This use of space is incredibly efficient as it allows for larger mechanical units to be placed close to the source and hides them out of view from the general public.
The Cons
Drainage Issues:
As we already pointed out, flat roofs aren’t flat, but because their slope is so gradual, they can start to develop low areas. When this happens water will start to pool in one area. This can cause the materials to break down faster which can lead to even more pooling and eventually become prone to leaks. This process is hastened when the temperature falls below freezing in overnight cold snaps. In cases like this special care should be made to give the water a path to find its way off the roof through surface mounted drains or a special drainage channel leading to a scupper.
Maintenance & Upkeep:
Remember when we said that flat roofing systems were easier to inspect? That’s good news as they need to be inspected and maintained more frequently than residential roofing. Ideally, a flat roof should be inspected at least twice a year. The older a flat roof is or when problem areas are identified, the inspection rate needs to be more frequent.
The sad reality is most roofs do not get inspected nearly as often as they should be and when they do, it’s by an untrained professional who goes up there, looks around for 10 min and says “looks good to me boss”.
We could (and should) probably write a whole guide on how to inspect a commercial roof, but this isn’t the time or place to go into all of the things to look for when evaluating potential problems. For now, we will just strongly suggest you set up an inspection schedule with a roofing contractor that is experienced in commercial roofing projects. Their trained eye and methods are more likely to spot small problems and offer inexpensive solutions, before they become bigger issues which will require large scale renovations and repairs.
Lifespan:
One disadvantage of any flat roof is they will generally need to be replaced more often than a roof that is designed to shed water like a pitched roof. While some membrane systems have a longer average timeline, they too will eventually fail and allow water to cause damage to the decking below. So while a flat roof is typically cheaper to install and maintain, they will need to be replaced sooner. What can we say, there’s no ‘perfect solution’ when it comes to keeping a building safe from the elements.
Other Considerations
Aesthetics:
It’s almost not worth mentioning, but since this is a comprehensive guide we should point it out the obvious. Most flat roofing materials have a limited range when it comes to visual aesthetics. Then again, most of the time a flat roof will never be seen by the general public because the building’s exterior walls extend upwards beyond the roofing surface. So with the exception of rooftop decks or mixed use areas, this isn’t much of a consideration when compared to cost and ease of maintenance.
Ventilation:
Flat roofing systems that are enclosed directly below the support beams of the roof create a challenge when it comes to ventilation. With a very limited space for air to circulate the trapped air can become excessively hot and then moisture will condense when it cools down. This creates the perfect conditions for mold and eventually rot to take hold. We will cover this topic in greater detail below.
Flat Roofing Materials
MBR (aka 'Torch Down')
The majority of commercial roofing we encounter is MBR (Modified Bitumen Roofing), or what’s more commonly called “Torch Down Roofing”. The obvious advantage of MBR is the speed and ease of installation, coupled with its relatively low cost. Torch Down roofing has become an industry standard for many years now. Like BUR, which we will cover next, it can also be layered to add redundancy and is fairly resilient as well as dependable. The rolls are heated with a large propane torch as they are rolled out which creates an instant bond and seals them. This bond continues to grow stronger as the sun heats the roof surface over the following days and weeks.
For all of the advantages that MBR has, there are some areas where it falls short. MBR doesn’t have the lifespan of other roofing products. It also doesn’t hold up to foot traffic as well as BUR or PVC. That said, most commercial roofing solutions are not made with this in mind. Overall the cost over time is comparable when you look at the cost of installation and number of years between replacement.
BUR doesn’t really have many disadvantages, but there are a few things to consider with this roofing material.
It’s more expensive due to additional materials and labor required, but generally lasts longer than MBR. Like any roofing materials, it needs to be inspected and maintained to ensure cracks in the tar aren’t ignored. BUR is what you might call an ‘old school’ approach to commercial roofing and you will see it used on older buildings that were built to support its weight. Newer structures might need additional structural support if they were not designed with BUR in mind.
BUR - Built Up Roofing
Before MBR roofing material was developed in the 1960’s most flat roofs were covered with BUR (Built Up Roofing). As the name suggests, this style of roofing consists of layers of bitumen (or roofing tar) and sheets of other materials such as roofing felt, or fiberglass. The sheets are soaked in the hot liquid tar as they’re rolled out creating a strong roof with redundant layers of waterproofing. The top layer is coated with small gravel or mineral granules which adds weather resistance and protects from UV rays. Built Up Roofing is naturally fire resistant and holds up to extreme weather conditions (with proper maintenance)
Single Ply Membrane Roofing:
TPO - EPDM - PVC
WIth all of these single ply roofing materials, it should go without saying they should be inspected regularly. Because there is no inherent redundancy with a single ply roofing material, any cracks or tears can quickly lead to a water leak and structural damage. While each of these synthetic materials differs in the installation somewhat, they are all fairly complex with a process that needs to be strictly followed by a qualified professional roofer to ensure they last as expected.
Thermoplastic Olefin (TPO)
TPO roofing is a single ply roofing material made from rubber and a lot of long scientific words that mean nothing to the average person ( polymerized thermoplastic, which is a combination of polypropylene and ethylene-propylene rubber). The formula for this man made roofing material has been improved over the years. Some earlier versions were known to age prematurely and form cracks, but reputable manufacturers have addressed many of those concerns.
Because TPO comes in lighter colors, it reflects a lot of the sun’s rays. This helps reduce the overall temperature of the structure and subsequent cooling costs. While its flexibility helps to eliminate cracks from building movements, TPO is fairly complex to install as it can be glued down or mechanically fastened. The seams are then welded together in place.
Ethylene Propylene Diene Monomer (EPDM)
EPDM roofing is a synthetic rubber-like roofing material. This option for commercial roofing is similar to TPO but generally comes in darker colors which absorb more heat. That said, it is still resistant to UV rays and remains flexible even when it’s cold. This gives EPDM a fairly long lifespan which makes it cost effective over time.
Polyvinyl Chloride (PVC)
PVC Roofing is similar to TPO and EPDM in the sense that it’s another single ply roof covering. Like TPO it’s available in light colors to reduce the thermal transfer to the commercial spaces below. While PVC roofing is generally more expensive than TPO and EPDM, it has some advantages when it comes to resistance to chemicals and durability. Because PVC is the same material used to make many inflatable boats, you may already be familiar with how well it deals with abrasions and impacts. This makes PVC roofing an excellent choice for rooftop decks or anywhere that will have foot traffic. Its resistance to chemicals also makes it ideal for commercial properties situated in industrial areas.
Drainage and Ventilation
Of Flat Roof Systems
As we pointed out earlier, no roof is truly flat because without drainage it would become a swimming pool. Water will always follow the path of least resistance, so by giving the water an easy route off of the roof, it is less likely to find its way into the building
To prevent pooling water one of several methods should be employed to create an adequate slope.
The Path Of Least Resistance
Framing and Tapered Insulation Methods
One such method is to create a consistent slope within buildings framing. This is common among classic retail buildings with a long rectangular shape. The minimum slope required is typically ¼ “ drop for every linear foot. However, most builders and roofing contractors prefer to create more of a slope to ensure water doesn’t stagnate and pool up in one place.
Another common practice for larger roofs is to foam insulation board that tapers towards drains or scuppers. While this method can be very effective, it requires a knowledgeable roofing contractor to plan out complex slope patterns on larger roofs. One advantage of using tapered foam board to create a drainage slope is it can be used on larger roofs or those with a complex shape by creating strategic high and low points.
Corners and Curbs
For buildings with parapet walls that extend above the roof decking, corners can be a common place for water to become trapped. To prevent this, crickets are added to the corners to direct water away from them. Additionally, a tapered build up along the curb or deck to wall joint to keep water from clinging to the surfaces. The roofing material is then run continuously up to the top of the curb / wall where it’s covered by a cap of some type.
Drainage Troughs
In conjunction with either of these methods to create a slope, drainage can be further increased by creating a trough to direct water from one or more sloped areas towards the drains or scuppers. Below is an example of a large commercial roof that had an area that collected standing water. By creating a trough, we were able to redirect that water towards the scuppers at the building perimeter.
Roof Drains and Scuppers
Since we just mentioned Drains and Scuppers, let’s talk about those next. Each of them have some advantages and disadvantages, but most of the time this has already been designed by the original architect and engineers of the building. While these systems need to be maintained and upgraded over time, most buildings will keep the system they have unless it’s no longer working sufficiently.
Flush Mounted Roof Drains
Drains have the advantage of being placed almost anywhere on the roof and they channel water away through a series of internal pipes. This makes them ideal for commercial roofs with large surface areas because they can be placed where two slopes meet or in rain troughs. The two common problems with roof drains is they can become clogged with debris (especially granules from BUR), and they have the potential to fail and become an entry point for water to cause damage inside the building.
Surface mounted drains will typically have some type of cage or grill around them to keep large debris out, but that doesn’t stop small items that can build up over time.The obvious way to keep them from becoming clogged is to do regular inspections and flush them out periodically. If a roof drain fails it may not be noticed right away if the pipes run in an internal bay that is covered below, but on larger commercial buildings they are often exposed in plain sight and can be easily noticed and serviced.
Occasionally a gutter and downspout system is used on smaller commercial flat roofs, but this is less common and more likely to become clogged with debris. Therefore, we typically don’t recommend this unless it’s for a small section of roof or the situation seems to call for it.
Scuppers and Downspouts
Scuppers have existed for centuries and can even be found on medieval castles and large ships as a way for water to leave the deck. They are essentially a hole in the parapet wall at the level of the roof decking. Their big advantage is the reduced likelihood of them becoming clogged due to their size. The scupper usually connects to a downspout and directs water away from the building’s foundation.
Some scuppers are designed with an extra lip or protrusion and simply allow the water to free fall to a paved surface below that also slopes away from the building. The advantage is water free falls so there is no downspout to get clogged. At the same time, this isn’t ideal for anything over a single story or anywhere people will walk. Also some thought has to be given to where that water will run downhill from there.
Flat Roofing Ventilation
Ventilation can be another problem with flat roofing. As we mentioned earlier, if a flat roof is enclosed extra effort should be taken to create air flow between the bays of the support structure. While it’s sometimes possible to create small vents at the ends of the bays or soffits, this isn’t very common in commercial settings. One creative solution is to frame boxes above the roofs decking to act as heat sinks which run perpendicular or diagonally to the rafters.
These boxes don’t have to be excessively high to work efficiently. The raised spaces are then fitted with passive vents to allow heat and moisture to escape which prevents mold and rot. This will extend the lifespan of the roof by allowing it to breathe. This is why many commercial buildings don’t have a finished surface on the interior ceiling, but rather a drop ceiling. The space between those two is usually several feet and allows the roof to breathe while also hiding much of the HVAC and wiring for the lighting system.
Sawtooth Roof Design
For Commercial Buildings
Another of the common commercial roofing types is the Sawtooth roof. The iconic look of sawtooth roofs dates back to the industrial revolution. The unique design solved several challenges for large commercial buildings and factories. By creating a series of asymmetrical pitched roofing sections, rain water could be easily managed regardless of the building’s size. Because each wall of the sawtooth contains windows, ample light fills the factory floor which reduced the need for artificial lighting at the time they were being designed. Of course now many of those previous factories have been converted to mixed use buildings containing retail spaces, artist lofts, or other industrial businesses.
The steep pitch of many sawtooth roofing systems allow for a wider variety of commercial roofing materials to be used compared to flat roofing types. While metal sheet roofing is a very common choice for sawtooth roofs, membrane systems are popular for their flexibility and resistance to chemicals and UV rays. In some cases common asphalt shingles are also used because of their durability to cost ratio.
How do Sawtooth roofs drain?
Looking at a sawtooth roof from the side, it might seem obvious that water is going to pool at the bottom of every slope. What you usually can’t see from the ground is how the water is directed to each end of the building. Similar to how flat roofs drain, sawtooth roofs use a variety of methods to shed water and direct it away from the building.
Internal gutters are used in some designs at the bottom of each angled pitch. Depending on the width of the building this gutter will either have one high point and direct all the water to the lower end where it exits through a scupper or drain. For wider buildings, the internal gutter system may have a peak in the middle and direct rain water to either side of the building. In many cases this gutter is really just a wide trench that is lined with a solid continuous sheet of waterproof material. Care to design and install the gutter must be taken to create a redundant and sufficient level of waterproofing that will last over time to ensure it doesn’t fail before its expected lifespan.
Natural Light and Ventilation
Besides natural light, another advantage of sawtooth roofing designs for commercial buildings is the ease of natural ventilation. Large warehouses and factories can act like a giant oven in warmer months and climates. In addition to that, running a bunch of machines can add to the heat and, back in the days before OSHA, could even produce fumes that were hazardous to the workers inside.
With this in mind louvered windows were often installed in each vertical wall segment which allowed air to vent out and prevented normal rain showers from entering. The louvers could be closed by a pulley system which has been replaced over time by modern mechanical. Many of these windows have now been retrofitted with electric shades as well to block direct sunlight during some hours of the day. Ventilation can be further increased by adding large ejection fans on the gable ends and passive vents on the roof peaks.
What is Green Roofing?
Living Roof Designs for Commercial Buildings
The popularity of Living Roofs, sometimes called Green Roofing, is actually a very old concept dating back thousands of years to the hanging gardens of Babylon. In modern times it’s become increasingly popular in dense urban building design because of the environmental benefits, aesthetic appeal, and noise reduction. We should probably point out the obvious, which is that Green Roofing is a complex system that must be addressed in the design of a building’s structure because it adds a significant weight load on the building. There’s also a big difference between an intentional Living Roof and one that’s been neglected for so long that it has become a defacto living roof.
How do Living Roofs Work?
It’s All About The Layers…
Green roof systems consist of multiple layers to effectively separate the building’s structural components from the vegetation layer and its root system. Starting from the bottom up, you can usually expect to find the following layers in a typical green roof on top of a commercial or residential building:
Vapor Barrier: Often made of MBR or a suitable equivalent this is considered the last line of defense to keep the building dry.
Waterproofing layer: Several options exist for this layer, but common choices are rubber membranes or PVC sheeting as we discussed earlier.
Root Barrier layer: This layer is crucial to prevent your beautiful plants from digging their roots into the waterproofing and causing leaks. Options for the root barrier include High-Density Polyethylene sheets, rubberized asphalt membranes, and geotextile fabrics. Some living roof designers go so far as to use a combination of two different types of root barrier for redundancy because it’s just that critical.
Drainage Layer: Another important aspect of any green roof system is adequate drainage to prevent the soil from becoming a waterlogged sponge. Depending on the designer, they may use synthetic materials like high-density polyethylene drainage mats or specially designed geocomposite drainage panels. However natural materials like pumice or gravel can also be used if the weight allowance will tolerate it.
Growing Medium: At this point you might think it’s acceptable to use topsoil, but most living roofs actually use a special blend of light weight soil, compost, and other organic materials to reduce the overall weight of the green roof. These growing mediums are carefully blended to keep them from shedding particles and nutrients which can cause drainage issues and increase the garden’s maintenance.t
Vegetation Layer:
The choice of plants you use for your living roof is more of a topic for your local landscape designer. After all, you wouldn’t ask your gardener about your roofing solutions, right? That said, we can tell you it’s generally thought that you should choose low maintenance plants that are drought resistant and common for your geolocation. Typical choices are grasses, herbs, sedums, and native wildflowers.
Irrigation system: In Seattle and the PNW in general, we are known for our long wet season. But our summer months can be brutal on plants, especially when they don’t get the natural shade from trees. So while an irrigation system is optional, it should be a consideration to keep your rooftop garden looking lush year round.
Hydroponics:
A Green Roofing Hybrid Approach
At Anytime Roofing, we love the idea of green living roofs. But the reality is that the amount of preparation and maintenance makes this an nonviable option for most commercial building owners. However, if you have your heart set on growing vegetation on your rooftop, there are other options. You could consider installing a semi-permanent Hydroponic system.
Hydroponic Rooftop Garden:
Hydroponic systems that use PVC pipes set in racks above the roofing material are another alternative to a green roofing. These rows of pipes can be configured to grow anything from decorative plants to edible gardens.
Believe it or not, in cities around the world fresh produce is being grown in urban environments with rooftop farming. Additionally, because they are self contained and separate from the roofing system, this presents a way for commercial and residential building owners to incorporate a living green roof without a significant retrofit and significant financial investment.
In some cases 3rd party growers may even pay for the investment and for the use of the roof space as well as utilities. We recommend that you have a professional inspection done to ensure your roof will support the added load, but in most cases a building’s rooftop is made to support much more weight than this.
Regardless of which type of green roofing works with your commercial building’s roofing system, it’s clear that this increasingly popular design aspect can increase building efficiency and enhance both the enjoyment of the building’s residents as well as supporting the local ecosystem.
Conclusion:
We hope this article has shed some light on the differences between Flat Roofs, Sawtooth Roofs, and how it’s possible to incorporate a Living Green Roof to your commercial building. We realize that most commercial building owners are not looking to change much about their roofing system, but the biggest decision to make when addressing a roofing issue is usually what materials to use going forward.
No matter what roofing materials you use or type of drainage system your commercial roof is using, we can’t stress the importance of regular inspections and maintenance enough. Making sure that water is not pooling up on your flat roof is very important if you want to keep the roof from failing and forming leaks.
At Anytime Roofing, we work with our commercial clients to set a regular inspection schedule so maintenance isn’t overlooked and any problems will be noticed sooner so they can be brought to your attention. We would be happy to schedule a meeting with you if you’re in the greater Seattle / Tacoma area, but if you’re somewhere outside of our service area, we recommend working with a reputable and trusted professional roofing contractor for the best results.
If you have any questions or think we missed something in this article, we would love to hear from you. You can Reach Us Anytime!
