The decision between installing a steep-slope or a low-slope roof is an important one as it can shape a building’s entire persona and purpose from design to construction and continue throughout the maintenance and upkeep required for each style of roofing. The “slope” can impact how a building is laid out, its curb appeal, and how it integrates with its surroundings.
In more current building Codes, technically, all roofs do have some degree of slope. Despite the name, even flat roofs are not completely horizontal. They have a very slight slope, some as minimal as 1/4:12, so for every 12 inches horizontally, the roof rises about 1/4 inch. This minimal slope helps drain surface water, though not as efficiently as more steeply pitched roofs. This is why sloped roofs can be preferred in areas with heavy rainfall or snow, as they are naturally able to shed water and debris.
There are two fundamental types of roof designs: steep-slope and low-slope. As a roofing contractor, understanding the differences between these two is paramount to help you choose the right roofing materials, and installation method, and determine the type and frequency of maintenance required.
What are Steep-Slope Roofs?
The word “steep” refers to the angle or pitch of the roof as represented here. It’s more pronounced than that of a low-slope roof. Also known as a pitched or high-slope roof, it has a pitch (the angle of the roof) that is at least 2:12, i.e., the roof rises more than 2 inches for every 12 inches it extends horizontally. Such a slope is visually noticeable and gives the roof a prominent, angular appearance.
The slope allows a more rapid shedding of water, snow, and debris to prevent any water from sitting on the roof. This natural drainage design reduces the risk of leaks due to standing water, water damage should any leak occur, or ice build-up which is common in colder regions. This design type seamlessly directs water and debris (fallen leaves, small branches, dirt, etc.) from the roof surface.
Although steep roofs are commonly associated with traditional residential designs – gabled, A-frame, Tudor, Victorian, and Cape Cod style homes – they are also featured in more modern and eclectic architectural styles. They allow for additional living or attic space beneath the roof and may include features like dormers, skylights, lofts, vaulted ceilings, and roof terraces.
Since there is an appeal of steeper pitched roofs in their ability to rapidly channel rainwater away from the roof surface to minimize the likelihood of water stagnation and seepage, the roof underlayment and roof covering material should be chosen wisely. Not all materials handle water runoff equally well. For steeper roofs, the materials for such applications include:
- Asphalt shingles
- Clay tiles
- Concrete tiles
- Slate
- Wood shingles and shakes
- Metal roofs
- Synthetic roof coverings
- Architectural shingles
- Composite shingles
- Solar shingles
Though most of the above can be successfully used for a wide variety of roof designs, complex roof designs, and climatic regions must be a consideration when choosing a roof underlayment and roof covering. These are readily visible from the ground and will last decades so it is best to choose carefully.
How is a Steep-Slope Roof Installed?
- Roof decking that is typical and commonly utilized for steep-roof designs (plywood or oriented strand board – OSB) is nailed to the structural rafters to provide a solid sheathing base for the roofing materials.
- Then an underlayment (organic felt, synthetic underlayment, or self-adhesive leak barrier) membrane is laid over the entire roof decking. Starting at the lowest point of the roof, the underlayment is rolled out, each row should overlap the one below it to ensure water sheds from the roof surface.
- Depending on the roof covering type, most utilize a metal flashing or “drip edge” installed along the roof’s edges to direct water away from the fascia and into the gutters. It’s nailed down along the eaves first, then along the rake edges (the sloped side edge of the roof).
- Assorted metal flashings are used around chimneys, skylights, vents, and other roof features to prevent leaks. Again, depending on the roof covering type, some are installed as a part of the underlayment, some as part of the roof covering and some have metal flashings at both. It is important to understand and follow the manufacturer’s instructions for the proper installation of flashings.
- Start installing the material (asphalt shingles, tiles, metal, etc. as discussed previously). The installation process should start at the drainage edge of the roof and go up from there. Steep-Slope roof coverings shed water so they are installed in overlapping rows to the proceeding row with mechanical fasteners placed into each piece at a precision location.
- Some roof coverings utilize special shingles known as “ridge caps” which should be installed to cover the ridge (the peak where two roof slopes meet). These should go over the ridge vent, if there’s one, and seal the top of the roof.
Pros and Cons of Steep-Slope Roofs
Pros:
- The steeper pitch or angle shed water and typically does not allow any water pooling, thereby extending the lifespan of the roofing system. Special attention to through-roof penetrations needs to be given to avoid these issues at larger openings such as chimneys and curbs.
- The steeper pitch and multiple slopes/sections add a unique curb appeal or look to the building, adding a dramatic, quaint, timeless, or storybook-like appearance, depending on the design elements.
- The ridge vents at the top and soffit vents at the bottom mean better natural airflow. Cool air enters, while heat and moisture go out from the attic effortlessly.
- Snow management can be considered to allow it to slide off quickly without the need for manual snow removal if the conditions as suitable.
- Steep-slope roofs can offer increased resistance to high-wind conditions because wind flows smoothly over the surface with less uplift pressure.
- The natural wind and gravity conditions can work in favor of removing leaves, twigs, and other debris from the roof surface.
Cons:
- The roof design and installation process can be more labor-intensive per square foot, especially on very steep slopes which is more time-consuming and adding expense.
- There is less usable roof surface that could be considered for solar panels, HVAC units, or rooftop gardens.
- The roofs are more difficult to access with additional safety issues for routine inspections, repairs, and maintenance.
What are Low-Slope Roofs?
The terms “flat roofs” and “low-slope roofs” are sometimes used interchangeably, but there’s a subtle difference. Flat roofs are almost level with most having some degree of slope but were specifically intended to retain water on the building instead of readily draining water from the roof.
Low-slope or low-pitched roofs should have a noticeable but continuous slope, more than 0/12 but less than 2/12 such as a steep-slope roof. The angle/slope of the roof is relatively shallow as it rises less than 2 inches for every 12 horizontal inches. This corresponds to a pitch of less than 14.5 degrees.
With proper design and construction of a low-sloped roof deck, will allow for surface water to drain from the roof, however, large roofs are at risk of effectively moving water to outlet (roof drains or scuppers) so it can exit the roof. This type of design often provides more functional use of space on the roof compared to steeper roofs. The slope is so modest that the roof may even appear almost flat when viewed from a distance.
Warehouses, shopping centers, factories, or any large, single-story, or multi-story building – where the roof covers a vast area – are ideal for a low-slope roof. Unlike higher-slope roofs, the low-pitched design is far more suitable for installing HVAC units, solar panels, recreational spaces, and other features without taking up valuable ground space.
If you decide to install commercial or residential low-slope roofing systems, make sure to choose materials that are designed for prolonged exposure to standing water, such as:
- Modified bitumen roofs
- Built-up roofing (BUR)
- Green roofs
- Spray polyurethane foam (SPF)
- EPDM (ethylene propylene diene monomer) rubber roofing
- TPO roofs (thermoplastic olefin)
- PVC roofs (polyvinyl chloride)
Like with any steep-slope roofing system, low-slope roofing assemblies have various considerations that are necessary to think about in the design and installation of these roof types. Choosing the incorrect roof type for the building conditions or occupancy use can lead to excessive coats and cost reworking of the roof.
Self-adhering Low-Slope Membrane Roofing
Self-adhering membrane roofing is modified asphaltic roof membranes (APP or SBS Polymers) with an advanced solution designed to deliver high performance and durability. These membranes are engineered to perform in any climatic region and most roofing conditions. They offer unique and often advantageous characteristics in their installation since they do not require the use of asphalt kettles and open-flame torching. Their unique design can address the unique challenges posed by traditional hazards associated with roof installation at a competitive price.
Fluid-Applied Low-Sloped Roofing
The expansive use of liquid or fluid roof coating products has added a new dynamic to the low-sloped roof options for certain building and roof conditions. Protective roof coating has been in the marketplace for decades historically more for roof repair, roof maintenance, and as a sacrificial surface protection for other roof types (Modified Bitumen, EPDM, TPO, PVC ) from UV exposure and environmental conditions.
Coatings materials are offered in a variety of chemistries from water-based acrylics, silicone, and rubber such as urethanes. These elastomeric coatings can be applied over various types of existing roofs, however, in recent decades the use of these has been expanded to include reinforcing fabrics and coating materials in multiple layers to form a fluid-applied system.
How is a Low-Slope Roof Installed?
- The main reason to design and install any roof is simple, keep the water leakage out of the building. As simple as that may sound it is truly more complicated than that and a variety of considerations must be made with every roof design. Moisture leakage is problematic to not just the occupants of a building but leakage internally into the components of a roof assembly is also very important.
- Proper roof design begins with the roof deck. It’s important to determine if the deck is appropriate for the building’s design and intended use. Additionally, the design may require the use of a vapor retarder to prevent condensation issues. After installing the vapor retarder, insulation boards should be placed to help maintain a comfortable indoor temperature, keeping the building warm in the winter and cool in the summer.
- On top of the insulation, cover boards may be added to create a smooth surface for the roofing membrane.
- Next, roofing system is installed. Depending on the roof type and design, various fields of the roof procedures must be followed prior to any detailed work.
- Flashings and/or details are next to be added around roof edges, vents, chimneys, and other protrusions to keep the water/moisture from seeping in at these vulnerable points. Drains, gutters, and downspouts must be installed after.
- In some circumstances specialized conditions must be considered such as the need for expansion joints or area divides in strategic locations to control expansion and contraction.
- For additional protection and to help direct water to drainage points, crickets are essential pieces to the roof system. In addition, parapets walls may be a best practice to control wind effects and limit the sight line of roof-top equipment.
Pros and Cons of Low-Slope Roofs
Pros:
- Low-pitched roofs can be more cost-effective to construct and maintain than large steep-slope roofs because they require fewer materials and are easily accessible (and safer) to workers.
- They provide more square footage if you want to install HVAC units, photovoltaic panels, cooling towers, chimneys, walkways, roof decks, illuminated signs, or rainwater harvesting systems on the roof.
- You can lower the building’s energy costs by up to 50% and cooling demand by up to 15% by use of additional thermal insulation and/or use of highly reflective roof membranes and/or coatings.
- The clean lines and minimalistic design are perfect for the utilitarian aesthetic, including industrial buildings, ranch-style homes, and urban-chic city environments without overwhelming the skyline.
Cons:
- The lack of incline makes the roof more prone to water pooling, leaks when damaged, and damage to the structure if not properly managed.
- Regular roof maintenance such as cleaning of the drains and scuppers is necessary to prevent blockage. It should be done at least twice a year (in the spring and fall), but if the roof is in an area with a lot of trees, or after a big storm is recommended.
- A strong supporting framework must be installed with the roof to handle the added load of snow and rainwater.
- UV-resistant surfaces such as granules, films or even coatings on materials are necessary to protect the roof because the horizontal surface area is more exposed to degradation.
Polyglass provides a comprehensive selection of dependable self-adhered modified bitumen products for low-slope roofs, as well as top-tier underlayments, elastomeric coatings, and additional solutions. Ensuring that sheets are properly secured to the roof deck is essential for achieving stability and durability in roofing installations.
Understanding your roofing needs goes beyond the low-slope versus steep-slope debate. You need to find the perfect balance of functionality, aesthetics, and longevity if you want to future-proof your project. This is exactly what we at Polyglass are here to help you with. Contact us to learn about our complete range of high-performance roofing solutions for every home and business.