While both flat and sloped roofs can handle rooftop supports, the two require different calculations and, therefore, result in different specifications, especially when it comes to things like drainage, weight distribution, and wind resistance. Wind resistance is an especially critical factor: A sloped roof disrupts the wind’s airflow much more than a flat roof does, and that changes wind pressure distribution and magnitude. That’s why any change in a design’s slope also means recalculating your design for the roof supports.
Supports have to work with the roof’s engineered load capacity. This affects not only how many pipes, pieces of equipment, and ducts you can fit on a roof, but how closely their supports should be spaced. Don’t forget to include things like walkways and stairs as well as all potential live load stresses. At higher loads, you may need to place your pipe supports on top of joists for added support.
It’s likely that your client will, at some point, plan additional uses for the rooftop: like storage, parking, or green spaces. But there’s only so much available space on any given roof. Your design needs to consider any plans the building owner has for possible rooftop expansion.
Pipes move during operation due to things like temperature, load, and vibration. Depending on the pipe and its contents, that movement must be either supported or restricted. Your design should account for maximum movement in all possible directions at any point during an operational cycle, not just the beginning and ending points.
All rooftop supports have to be weather-resistant, but your design should also incorporate the likelihood of natural disasters like hurricanes, tornadoes, and earthquakes. In high-risk areas, your pipe support design may need to comply with strict building codes. After Hurricane Andrew, for example, the state of Florida took a close look at existing building codes. They found that, not only were the codes weak -- especially the wind-load provisions -- they frequently weren’t being enforced. One of the state’s first moves was to adopt the wind-load standards supported by the American Society of Engineers.
Pipes should be wide enough that, even under maximum velocity, friction doesn’t result in erosion, noise, or water hammer. At minimum velocity, the width should prevent both surging and buildup.
The best pipe wall thickness also varies depending on what the pipe is going to hold and where it will be used. Some of the factors that should be a part of any pipe thickness calculations include maximum and working pressures, maximum and working temperatures, chemical properties and velocity of the fluid or gas moving through the pipe, the material the pipe is made of, and any applicable industry or local code requirements.
It’s also important to consider the way in which the support bases will be attached to the roof. If you plan to use bolts, you’ll have to make sure that there’s enough room to accommodate the bolt and that workers can actually access the attachment point from the inside. If you plan to use adhesives, you’ll need to choose one that’s compatible with the roof surface material.
Rooftop equipment needs maintenance. Your specifications need to ensure that employees can access it easily and safely both for inspections and for repairs. Pipes and rooftop supports that need the most maintenance should be designed with the easiest access.
The process of designing a rooftop system is far too complex to completely cover in one blog post, but this is a good overview of some of the factors it’s important to consider. What other factors go into your designs?