Tag: hipped roof

Metal Roof Pitch Part II: Panel Selection for Your Building End-Use

Posted on by Ken Buchinger

In part one of our series on metal panel roof pitch, we looked at the engineering and strength considerations that need to be taken into account when designing or specifying a building. In part two, we’ll review how to make the right metal panel selection for your building end-use by looking at the building type and roof pitch to find the optimal combination of form and function.

Types of roof panels:

Different types of metal roof panels are better suited for certain roof pitches and this mostly has to do with panel rib height and how they’re seamed together. For instance, the lower the pitch, the taller the rib of the roof panel needs to be in order to optimize water carrying capacity. Also, the lower the pitch, the slower water runs off the roof; combined with wind pressure, this can hold water at the eave. As an example:

  • On 2:12 and lower pitches for commercial/industrial jobs, weathertightness warranties will use a mechanically seamed panel because it creates an impenetrable lock that keep water from seeping between the panels. Seamed panels can be thought of as more hydrostatic (resistant to water), whereas non-seamed panels are hydrokinetic, i.e., water runs off of them, but they are not designed to be a water barrier.

Building Type and Use:

Whether residential, post-frame or commercial, structures follow general design criteria regarding metal roof slope, based on to the building type and use. For instance:

Gabled v. Hipped: gabled roofs provide more interior headroom while with hipped roofs, endwall sheet height can be reduced, making for easier ordering and installation as you don’t have to cut/miter the sheets.

  • Residential v. Post-Frame and Commercial Use: While it can generally be assumed that thicker-gauge panels are stronger, the type of building construction, location and end-use can affect the choice in panel thickness. Residential roofs generally have steeper slopes (often due to aesthetic sensibilities), while commercial and post-frame buildings will typically have a shallower pitch to increase the strength of the building. Buildings such as pole barns, post-frame buildings, warehouses and garages can require roof panels to span longer distances between supporting trusses and purlins and therefore require thicker panels to ensure the roof is structurally sound and can aid in reducing the chance of collapse under the weight of snow and water—particularly when the roof pitch is low. For smaller buildings—like residences—with supporting trusses and purlins that are typically closer together, thinner panels can be used in conjunction with thicker trim for an economic, structurally ideal residential metal roofing system. See our blog on metal panel gauges for more.
  • Economics: When a roof has a shallower pitch, less material is needed to cover the same roof span. This can be more economical since fewer square feet of material needs to be purchased, or at least help in offsetting the cost of the thicker panels commonly used on low-slope roof—a consideration often important to companies with multiple properties. Likewise, while more material may need to be purchased for a high-sloped roof, you may be able to buy thinner, less expensive panels since high-slope roofs don’t typically need to be able to support heavier loads like low-slope roofs do.
  • Aesthetics: Traditional residences typically feature a higher roof pitch while this may be less important to commercial building owners. However—from a functional standpoint—thicker panels can be more forgiving of installation errors such as over-tightened screws and aren’t as likely to transfer waves and bumps in sheathing.

Solar Panels and Roof Pitch:

The geographic location of a structure will determine the angle at which the solar panels should be installed. Generally. the closer to the equator you get, the lower the slope the solar panels can tolerate as roofs closer to the equator have more consistent exposure to sunlight as compared to those farther away. If you plan to add solar panels to your metal roof, be sure to check the Solar Reflective Index (SRI) values and minimum slopes set by the ASTM and local governing codes. These standards will advise adequate slope based on your location as well as help control things such as heat island effects, etc.  For more information on solar reflectance and how it is affected by the color of your metal roof panels, see our blog post on cool metal roofs.

Conclusion:

Roof slope is a factor that affects many decisions when it comes to any metal building roofing project regardless of the building’s end use. When embarking on any metal roofing project, be sure that the panels you’ve selected meet local building codes and are produced by a quality manufacturer with knowledge and experience designing for different applications to ensure you get exactly what you’re looking for. For more information on metal panel selection for your building end-use and to learn more about recommendations on roof slope for different building types, contact your local ABC representative today.

 Additional Resources:

For more information on slope requirements, visit NRCA’s website. For more information on steep-slope metal roofs and their benefits, visit the Metal Construction Association’s website.

Metal Roof Pitch Part I: Engineering and Strength Considerations

Posted on by David Lester

Whether you’re an owner who needs to consider roofing material options because of the nature or design of your building, or a distributor/contractor whose job it is to recognize whether a suggested design will or won’t work based on a range of circumstances, it is critical to understand the engineering behind why roof pitch and tensile strength are important on a metal building project. In part one of our series, we look at the engineering and strength considerations that need to be taken into account when designing or specifying a building, while part two will review the best type of metal panel to use based on the pitch and end-use of your building,

Engineering Considerations:

Ideally, your roof pitch will be the “sweet spot” between having a roof pitch that’s so shallow that the beared load is increased and having a roof pitch that’s so steep that wind uplift probability is increased—a factor which can also vary based on the building width. Specific engineering considerations with regard to roof slope include:

  • Snow Load Requirements: lower slopes create more load that the roof/structure must support. For example, when you have a flat roof and it snows, the snow will stay on the roof, adding more weight; on a sloped roof, a good amount of the snow will slide off of the roof, so the load will remain in an acceptable range the structure can support.
  • Wind Load: Preventing roof damage from high winds is largely dependent on roof geometry, slope and building material used. If outside air is allowed to penetrate a structure through cracks and other openings when wind flows over a building, the air pressure beneath the roof assembly increases. When this pressure becomes greater than the pressure directly above the roof (which happens as wind flows over a building), this can cause upward force on the roof system, leading to loss of structural integrity and significant damage. Wind uplift rating and minimum slope requirements are dictated by building codes in the market, so be sure to check with your local municipality for specific details.
  • Water Diversion: the steeper the pitch the faster it will channel and divert water, preventing it from pooling and causing other issue like mold, mildew and paint degradation. As with the snow load considerations mentioned above, the less additional weight burdening the roofing system, the better.
  • Minimizing Debris: steeper-sloped roofs also help minimize the chance that debris, mold and mildew have to build up on the roofing system. Unless there is a catastrophic weather event that drops large amounts of heavy debris—tree branches for example—there is a small chance that debris on a roof of any slope will exceed its ability to carry the load. However, because the more debris is able to slide down and off of a metal roof, the less that needs to be manually cleaned. Additionally, access to and cleaning the eaves of sloped roofs is much easier and less time-consuming that trying to perform the same maintenance on a roof with less slope.

The Strength of Metal Panels:

  • Tensile strength: This refers to the panel’s ability to resist breaking when it’s placed under pressure or tension—meaning the panel has a measured ability to bend under a certain amount of stress before it snaps. Galvanized steel panels can withstand between 50,000 and 80,000 pounds of pressure per square inch (50 KSI – 80 KSI)—significantly higher than the tensile strength of other roofing materials. Despite this, metal panels remain lighter and more impact resistant, keeping the likelihood and cost of repairs to a minimum. Check the tensile strength of your metal panels with the manufacturer and be sure to reference common testing standards related to roof slope, including ISO ratings and ASTM standards that test tensile strength.
  • Steel Hardness and Density: While it can generally be assumed that thicker-gauge panels are stronger, the type of building construction, location and end-use can affect the choice in panel thickness. Buildings such as pole barns, post-frame buildings, warehouses and garages can require roof panels to span longer distances and therefore require thicker panels as they are more structurally sound. These thicker panels will also aid in reducing the chance of collapse under the weight of snow and water—particularly when the roof pitch is low. For smaller buildings—like residences—with supporting trusses and purlins that are typically closer together, thinner panels can be used in conjunction with thicker trim for an economic, structurally ideal residential metal roofing system. See our blog on metal panel gauges for more.

Conclusion:

Following common standards, doing adequate research and conforming to manufacturer guidelines regarding slope will ensure you’re reaping the full benefit of your metal roof panel selection. For more information on metal roofs and to learn more about recommendations on roof slope, contact your local ABC representative today.

Additional Resources:

For more information on slope requirements, visit NRCA’s website. For more information on steep-slope metal roofs and their benefits, visit the Metal Construction Association’s website.