Prepping for PV

Installing and Flashing PV Roof Mounts

By:

Johan Alfsen

Issue Date:

August / September 2011

Last Updated:

12/20/11

Intermediate

Inside this Article

Quick Mount PV Racking

Intermediate

There are online instructional videos to teach just about everything—from playing your favorite guitar riff to installing dual-pane windows. But when you search for “how to mount PV modules on the roof,” you will most likely get an outdated video showing improper roof penetrations that actually void roof warranties and violate roofing codes and standards.

Only in the past few years have manufacturers designed mounting and waterproof flashing systems that are easy to install and meet the codes (see “Modern PV Roof Mounting” in HP137). Here is a step-by-step explanation for installing a code-compliant rack system on a composition/asphalt shingle roof.

Before You Start

Safety & roofing. Working on a roof can be dangerous and OSHA safety standards should always be followed (see www.osha.gov/dep/greenjobs/solar_falls.html). Roof warranties should also be considered. The roof’s condition will need to be evaluated to determine if a new roof will be needed. If not, contact the original roofing company to determine if there are any workmanship and product warranties that might be affected. If a new roof is going to be installed prior to installing the PV system, stand-off posts can be attached, which can then be flashed and waterproofed by the roofers as they install the new roof.

Layout & Attachment. Plan ahead so that you know how many roof penetrations need to be made—prior to stepping onto the roof. Online rack calculators or manufacturer specifications can help determine the proper number of roof attachments needed for your system and location (see Access).

Proper attachment to the roof structure is key to a strong rack system. If possible, it is best to attach additional wood blocking between rafters. When blocking is not an option, then you must pre-drill into the rafter and attach with a lag bolt or hanger bolt. Staggering the mounts on the roof to avoid attaching every rail to the same rafter helps distribute the load.

A common method of finding rafters is to transfer measurement points from inside the attic to the roof. If attic access is not an option, there are other methods and tricks that you can use. Looking at the gutters and bays from the ground, use a mallet to gently knock and determine rafter placement, or use a deep scan rafter finder. As a last resort, you can also use a small (typically 5/32-inch) bit to drill through the roof to find the rafters. If the bit misses the rafter, you can then use a wire coat hanger to fish through the hole and locate the rafter. Be sure to seal any holes you’ve made.

Once the rafter layout is determined, make chalk lines down the roof to mark the rafters and across the roof for your rail placement.

Choosing Your Product

There are a variety of code-compliant mounting and flashing products on the market. Before you buy, consider the following things.

Roofing code-compliance— The International Building Code (IBC) calls for flashing to be installed on all roof penetrations. A minimum of 4 inches of flashing should be on each side of the roof penetration to divert wind-driven rain.
Product certification (Example: ICC-ES)—There are product evaluation services such as ICC-ES that perform technical evaluations to determine that products comply with codes and standards.
Approval from roofing material manufacturers— Many roof manufacturers, such as GAF & Owens Corning, will test and evaluate products for warranty approval. It is important that your flashing/mount does not void the roof manufacturer’s warranty.
Mount engineering strength—The rail will govern how many mounts need to be installed based on the span charts. Mounts are tested for pull-out and shear strength. A well-engineered mount will allow further spans and fewer penetrations in the roof.
Ease of installation—After the rafters are located, a well-designed mount/flashing should only take a few minutes to install. Spending less time on the roof brings down labor costs and promotes a safer work environment.
Product longevity and life span of materials—The waterproof flashing and mount should have a life span that meets or exceeds the life of the roof and/or the system. Galvanized flashings only have about a 15-year life span, while aluminum has a 50-year life span. The Sheet Metal & Air Conditioning Contractors’ National Association (SMACNA) states that galvanized flashings should not be installed on roofs that will exceed 15 years.

Many manufacturers have this information on their websites. Obtaining a product sample for personal inspection can help verify the quality of the product being used, noting the workmanship, and weight, thickness, and size of the material.

No matter what product you choose, always follow the manufacturer’s installation instructions. Many manufacturers provide installation videos. While they may not teach you how to play your favorite guitar lick, they will certainly help you install code-compliant mounting and flashing solutions for your roof.