At Wyssling Consulting, one of our core values is Engineering Excellence Through Best Practice and Industry Experience. We pride ourselves on holding this value paramount, in order the provide the servicethat is expected from the leader in the industry, and to safeguard ourselves, our clients, and the public. One of the ways we deliver this value is throughour experience in structural engineering.
The structural aspect of certifying solar can be tricky sometimes, especially when it comes to determining if a roof can support a new PV system. Around 95 - 99% of the time, a structure is already built and existing. Very rarely will we be involved in a new build that has solar integrated into the original design. In order to certify that the roof of a structure can adequately support a proposed PV system, an analysis of the existing roof systemis required. This means obtaining accurate information of the existing systemis paramount to providing an accurate analysis and delivering our service tothe level that is expected.
However, this is easier said than done in most cases. There are many obstacles in trying to get this information. The structures we review for solar installations are generally broken up between residential and commercial. Below I will go over these two types, the information typically required for each, the challenges with obtaining this information, and the ways to obtain the most accurate information possible.
Residential
Residential projects can be fun to me because you never know what you’re going to get or see. It could be a home built this year, or a home built before the 1900s, which have very different types of roof framing. Sometimes a home can be remodeled so it’s hard to tell the actual age from aglance. This is why we rely so heavily on the photos and measurements obtained on-site.
The typical information we need from these photos is:
• Measurements of the depth and thickness of the rafter/truss.
• On center spacing of the rafters/truss.
• Overall view of the attic, showing the framing as a whole, the condition, and if there are any additional supports.
• Exterior photos of the roofs showing general conditionand finish.
• If there are existing knee wall or purlin supports, we would need confirmation that a load bearing wall exists below the attic at their location.
All of these items are important to analyzing the roof to determine its capacity to support solar. While most of these items should be relatively straight-forward to obtain there can be some challenges. The #1 reason that this information can’t be obtained is vaulted/finished ceilings,where the roof is flat with no access to an attic or the ceiling finish follows the slope of the roof framing providing no access.
The following are ways to obtain roof information with vaulted/finished ceilings:
• Measure depth of skylights provides a good estimate on framing size.
• Measure the framing vis exterior overhang if visible or you’re able to access it through a soffit.
• Measure the depth of the framing by going througha ceiling light fixture with a rod.
• Measure the top of a window to the ceiling from the interior and the top of the window to the roof finish from the exterior, subtracting the two.
• Use a stud finder on ceiling or tap roof with a hammer to determine framing spacing.
A type of structure we encounter frequently on the residential side are pergolas and carports. These seem to be a popular option for installing solar on to avoid roof penetrations in the main home, or if there’s not enough roof area on the main home for the system size desired. There is additional information required to structurally certify these types of structures.
The following is information we require to certify pergolas and carports:
• Measure rafter size and spacing.
• Measure beam sizes.
• Measure length between rafter supports.
• Measure post size, spacing, and height.
• Provide photos of rafter connections to beams, beam connections to post, and post connection to slab/footing.
• Try to get an estimate on footing size, either from homeowner, contractor, or if there’s photos from the original installation.
Commercial
Solar projects on commercial buildings are becoming increasingly prevalent but come with their own challenges. Now we’re looking at structural steel, bar joists, etc. and not just wood rafters. The best thing isto obtain the original building plans. This will typically provide all the information required to analyze the building. However, sometimes these are not available,and we have to rely on site information and photos. There’s a lot more information to get (and a lot more information to miss). Most commercial projects are installed using ballast systems, which can be quite heavy, soobtaining the most accurate information is crucial. The site surveyor needs a different knowledge base to obtain the correct information than what is required on the residential side.
Let’s start by reviewing the different types of roof framing we usually see on commercial projects and the information that we require for each:
• Steel Bar Joists
o The Steel Bar Joist Institute (SJI) has a form to determine the model of existing steel bar joists. This will require the measurements and thickness of the top and bottom angles, depth, length (span),bar size and spacing, age of building, etc.
• Steel Wide Flange Beams (I-Beams)
o Measurements of depth, width, and flange thickness shall provide an estimate of the correct size.
• C/Z Purlins – Pre-engineered Metal Building (PEMB)
o Measurements of the depth, width, and thickness. It’s also good to provide a photo of it crossing over the supporting beams to show how far they ‘lap’ or splice each other.
While this is great for determining these induvial elements, additional information is required for the roof as a whole.
The following is what information is required for commercia lprojects:
• Size (depth and width), thickness, spacing of the roof framing.
• Span of roof framing or spacing between supports.
• Size of those supports if possible, or a roughidea.
• Makeup of the roof if possible (metal, wood, thickness of insulation over-top, especially if positive attachment to the roof is needed).
• Photos of all these elements.
One challenge in obtaining this information can be the height of the framing, especially in a warehouse or similar building when the ceiling height can be 15-20 feet. Most times a lift will be required to obtain accurate measurements. Sometimes you can measure the framing from a roof access ladder as well. You can measure depths and spacings from the ground using a laser measuring tool, but this will not provide accurate information on the thickness of materials and some other critical measurements outlined previously.
