Structural Considerations for PV Installations on Older Row Houses
Inside this Article
While Washington, DC, has a vibrant PV market, streamlining the design and permitting process has been a persistent challenge. As a result, many installation companies migrate toward standard designs in terms of system topology and mounting method. This strategy helps companies control costs, minimize unforeseen challenges and achieve predictable engineering and permitting results. It is challenging, however, for companies to develop standardized design approaches suitable for the more than 70,000 row houses that comprise more than 25% of the housing stock in Washington, DC. These row houses are typically two to three stories above grade and 14–20 feet wide. Similar housing stock is found in many other US cities.
As a structural plan reviewer for solar projects, I have evaluated hundreds of engineering plans for retrofitting residential PV systems. Here I present some key considerations for installations on older row houses. This is intended as a high-level overview; some issues are entirely unaddressed and others could stand greater scrutiny. However, PV installation contractors, structural engineers and plan reviewers can apply some of these lessons learned to the older structures prevalent where they work.
Unique Design Challenges
Standard approaches to retrofitting PV systems on residential structures are not well suited to the peculiarities of older row homes. Largely built in the period between 1900 and 1940, row homes in Washington, DC, present a unique set of design challenges. The roof framing is rarely up to code, which raises concerns about connecting rail-mounted PV systems to the rafters. Further, the nearly flat roof slope, typically around 5°, is not ideal for making dozens of roof penetrations. However, the light framing typically also rules out the use of a ballasted mounting system. The alternative mounting solution that many designers commonly propose is to suspend the PV system above the roof by spanning between parapets.
Party walls and parapets. In Washington, DC, row-house roofs are separated by 12-inch-wide party walls that extend above the roof about 6–8 inches as parapets. These are multi-wythe brick walls—meaning that continuous vertical sections of brick are laid next to one another to increase the wall thickness. Roofing material generally extends up and over the parapet. While torch-down modified bitumen is a common modern roofing material, many of these structures originally had standing-seam metal roofs with coping (a metal cap flashing) atop the parapet; painted and patched variations of this original roofing are still found in the field. Often the parapet has wood cap board, of uncertain and variable age, on top of the brick and under the roofing or coping. The condition of these components and the wall in general can vary considerably, due in part both to the effects of aging and to a wide variation in the original materials and build quality.
Because brick walls have very high compressive strength—1,000 pounds per square inch is a reasonable minimum value—parapets in older row houses are attractive to solar contractors and structural engineers as a means of supporting the PV array. The general idea behind this approach is that the array can be mounted on a system of beams that span from parapet to parapet, as shown in Figure 1. As long as the bearing surface of each beam end is at least several square inches, compressive loading is unlikely to limit the structural design of the mounting system. The viability of parapet-to-parapet mounting, therefore, depends on whether the design addresses uplift loads in addition to the compressive loads due to dead weight and snow.
Resistance to uplift. I have reviewed many row house projects where the engineer had not properly accounted for wind-related uplift loads. The most commonly proposed method of handling uplift in parapet-to-parapet mounting is to use some arrangement of bolts or threaded rods installed in the top of the parapets. Proposed embedment depths are typically in the 4- to 18-inch range, although some are specified as deep as 50 inches. In nearly every case, the bolts or threaded rods are to be embedded in injectable mortar or hydraulic masonry cement, such as Rockite anchoring cement or Hilti HIT-HY 70. The problem with these designs is that they do not take into account the fact that row homes have unreinforced masonry walls. Section 2.2.4 of the Building Code Requirements and Specifications for Masonry Structures, published by the American Society of Civil Engineers (ASCE) as Standard 5-11, effectively states that anchors embedded in the top of a multi-wythe brick wall cannot be considered to provide resistance to uplift: “The tensile strength of [unreinforced] masonry shall be neglected in design when the masonry is subject to axial tension forces.”