Installing solar roof attachments requires drilling dozens of holes through roofing material, making any roof vulnerable to water intrusion. Given this reality, it’s important to understand how water intrusion (and the resulting building damage) occurs and ways installers can prevent it from happening.
Improperly waterproofing a rooftop solar system is expensive. The labor costs to repair smaller leaks often range between $500 and $1,000. If the problem is bigger, flashed mounts or the whole roof may need replaced. A PV system “remove and reinstall” (often referred to as “R&R”) costs from 50¢/watt up to $1.50/watt depending on roof type and array configuration. This equates to a whopping 20 to 40% of the price of a new PV system.
Before we talk about waterproofing, we need to address roof inspection. Shingle life typically ranges from 15 to 30 years, and the same is true for tile underlayment. Cracked or curling shingles or underlayment is a strong indicator that the roof is beyond middle age. If the roof is more than five to 10 years old, solar installers should recommend roof replacement to customers, as it is a sensible investment. Installing on older roofs may be inadvisable in some cases, but another option is reroofing only on the part of the roof that will be getting solar.
Rooftop water management basics
Two basic strategies exist for eliminating the risk of leaks on roofs: “water-proofing” and “water-shedding.”
Roofs with a pitch below 2:12 (low-slope roofs) will use waterproofing membranes. The two common types are asphaltic “built-up-roofing” or “single ply membranes” made from plastic or rubber sheets. Waterproof membranes work by sealing every seam, crack and crevice. This roofing method results in a watertight membrane that protects in the heaviest rains, even when ponding occurs, for 20 to 30 years. These roofing systems require a qualified roofer to seal every penetration, taking great care for long-term reliability. Biannual inspection and service by a qualified roofer are required to maintain most membrane roof warranties.
Unlike waterproofing membranes, water-shedding systems like tile and asphalt shingles rely on the steepness of the roof for effective waterproofing. Shingles are most effective when the roof has a steeper pitch and they are less effective on lower slope roofs. When the roof pitch goes below 4:12, special waterproofing measures are required, including fully adhered underlayment.
How water gets in
Contrary to popular belief, rainwater does not always flow downhill. Wind blows water up between shingles or tile, and capillary action draws water sideways and even uphill. When water finds even the smallest pinhole, it seeps under the roofing. Over time it soaks and rots the sheathing and rafters, leading to structural damage, weakened attachments and possible mold formation. Larger holes will result in damage occurring more quickly. This is the reason that building codes and roofing manufacturers mandate the use of underlayments below water shedding roofs and require roofing manufacturer-approved flashing for all penetrations.
Choosing proper flashing methods
Single ply membranes like TPO, PVC and EPDM use thermally or chemically welded flashings while torch-down flashings are the norm on built-up asphalt roofs. Most roofing manufacturers require that a qualified roofer perform flashing installation to preserve the warranty. This typically means a licensed roofer would install low slope flashings. Some membranes like granulated rolled roofing may be installed by solar contractors with newer flashing systems, such as sealant pitch pockets or fabric-reinforced polymeric membrane flashings.
Choosing the proper metal for flashings on shingle or tile roofs is critical to the long life of the flashing. A corrosion-resistant material like aluminum is preferred because it can last more than 30 years. Galvanized steel may only be reliable for 10 to 15 years, depending on the climate. In wet climates or corrosive coastal conditions, solar installers often choose thicker flashing metal to combat the effects of frequent wind-driven rain or salt spray.
Elevating the waterproofing seal
The water sealing surface is the critical barrier against rainwater getting under the flashing and into a penetration. Elevating this critical barrier is one of the key elements to reliable, long-term waterproofing. On low-slope roofs, this is accomplished by sealing the base of the flashing to the membrane and elevating the sealing area above the flood line. On steep-slope roofs, elevating the water seal has major advantages. First, it slows down the deterioration of the rubber seal as it reduces water exposure, and the seal dries out faster, preventing deterioration from biological growth. Additionally, ice formation in freezing climates can wear out the seal through successive freeze-thaw cycles. An elevated seal reduces or eliminates ice contacting the seal.
The sheathing of this roof is badly stained from leaks near the ridge. The sheathing and rafters will weaken over time and may need to be replaced.
Sealant by itself has poor long-term waterproofing reliability, but when used in combination with manufacturer-approved flashing methods it can provide decades of reliable performance. Sealant is typically applied into all pilot holes and under the flashing around the lag bolt hole.
There are several common sealants used in rooftop solar installations, and it is crucial to verify sealants are compatible with the roofing, flashing seal and any other materials they contact.
Asphaltic roofing cement (mastic) with reinforcing mesh is a code approved method for sealing underlayment flashings below tile, and it is also used as sealant on shingles, but most solar installers rely on roofing sealants like those from Chemlink and Geocel. These synthetic sealants can provide decades of performance when properly applied. Each sealant is unique in its performance characteristics. Some sealants can cure underwater while others prefer dry application. Some can handle temperatures over 200°F while others should never be used in areas with full sun exposure. Research is important to make sure the sealant can last the life of the roof and array.
Proper waterproofing is not always quick and easy. Solar installers are encouraged to get proper education on roofing systems and waterproofing methods. It can take time to master the subtleties of flashing installation but those that make the effort will be rewarded with reliable waterproofing on penetrations for the life of the roof and the solar system.
This article was contributed by Jeff Spies, senior director of policy for Quick Mount PV.