Common Issues in Rowhouse Flat Roofs

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Installation of a modern, single-ply, fully adhered thermoplastic polyolefin roof to a 115+ year-old-house. This type of roof system is both strong and energy efficient with high solar reflectivity.

Here on Capitol Hill the vast majority of roofs are flat. Flat roofs are common to commercial buildings, but residential flat roofs are rare except in historic urban areas such as Capitol Hill. There are several different types of flat roof systems, but across the board, a handful of issues make up the most common problems found on flat roofs.

Often the shared ownership of demising parapet walls between neighbors creates a challenge in the coordination required to properly build a new roof termination or coping. (A demising wall is one which divides one townhouse from another.) As a result, parapet coping here in DC is almost always handled very poorly. Sometimes it’s a matter of a homeowner or contractor cutting corners to save costs. In other cases, the absence of approval from a neighbor means the roof installer will just stop short of wrapping or terminating the roof membrane properly. Often, this incomplete detail allows water to enter and the roof to leak.

Also common are issues related to the replacement of historic pre-A/C, vented skylights, often built without curbs that meet modern code requirements. The International Residential Code requires curbs for skylights to be at least four inches above the roof surface. Most modern manufacturers require curbs to be at least eight inches above the roof surface, which is better practice, especially with snow loads in our region.

The International Residential Code allows for recover roofing, which is the industry and building code name for applying a new of layer roofing over an existing layer. However, the building code only allows for recover of a roof with one layer. In other words, the maximum number of roof-membrane-layer recovers is two, total. A very common issue is that many roofers, after tearing off an existing roof, install the new roof on top of the existing wood sheathing without replacing and therein lowering the drip edge. A drip edge is the metal transition that joins the lower roof edge to an adjacent, normally rear, gutter.

If the gutter is not lowered, the result is ponding with water buildup just above the drip edge. Ponding accelerates the deterioration of the roof membrane at that location. This problem is common throughout the homes of Capitol Hill. The best explanation is that most roofers do not go as far as lowering the existing gutter and therefore do not rework the drip edge. It would be best practice to remove the existing gutter, modify the downspout, and install the new area of roofing. These steps require extra cost, but going the extra mile leads to a longer-lasting roof.

Commonly in Capitol Hill, flat roof membranes are built where they join adjacent vertical walls. This is found where porch roofs meet front facades, or where one rowhouse happens to be lower than the next and the demising wall runs higher than the lower roof, and at rear lower additions. Frequently at these locations, counterflashing or a termination bar has not been used to connect the roof membrane to the adjacent wall. This condition is problematic.

Cliff Kornegay of Capitol Hill Home Inspection is a veteran home inspector with over 10 years of experience identifying roof issues and working with homes and problems just like the ones described. He agrees that the shortcuts and problems discussed, related to important details, are prolific on this type of residential flat roof. “It is rare to see properly installed termination bars or counterflashing in Capitol Hill roofs. Almost every manufacturer requires it, but it is a commonly skipped step. Most roofers will torch the flashing and think that is fine. It’s not enough.”

Improperly sealed penetrations after the new roof is installed, such as at air-conditioning lines, create a weak link in an otherwise good chain. We often find that good, expensive, and relatively new roof systems have had non-roof-related contractors come and make penetrations, which causes leaks. We see this with HVAC exhaust vents and other systems, low-volt communication cables for television and internet, roof deck installers, and solar panel installers. It is normally something very simple that is installed in a path-of-least-resistance type of manner but causes expensive damage.

Ice damming on flat roofs causes problems only in rare weather patterns, but applies a unique problem at rear additions to Capitol Hill houses. Where drip-edge or gravel-stop flashing is left unsealed at the bottom edge, ice damming with snow and ice buildup can cause flooding and considerable water damage to the home.

Kornegay explains that professional metal roof paint is expensive and can’t be bought at big box stores. “A good metal roof can last for decades. We often see somewhere along the timeline of the roof that a homeowner or low-quality contractor has gone over a metal roof and applied hot asphalt or mastic. They must think it is a good fix, but it actually traps moisture between the mastic and the metal roof, and the trapped moisture will create holes in the metal substrate. It ruins the roof.”

Gary Barnhart is a Capitol Hill contractor with over 20 years of experience in historic restoration. He has done hands-on construction and has been a certified building, plumbing, electrical, and mechanical inspector. His firm, GL Barnhart Construction, based and focused on Capitol Hill, is licensed and insured in both residential and commercial construction and is a certified residential and commercial roofer. You can find detailed resources related to the history, upkeep, and renovation of Capitol Hill homes at www.glbarnhart.com.