Layer Cake With Frosting? Photo of the Month

Before you start to get hungry, this is NOT a picture of a layered dessert. It’s a commercial roof core sample taken from one of the buildings at the Centers for Disease Control and Prevention (CDC) in Atlanta.

The black cylinders layers that look like chocolate cake are a combination of modified bitumen, insulation, felt and plies. The “frosting” is a layer of Polyiso, a rigid foam board insulation — the sample is upside down in the photo. The thinner bottom layer, or the roof’s top layer, is the cap sheet. 

Taking core samples (we took eight from this particular roof) is the best way to determine if there’s moisture present in the roofing system. An accurate sample must include the entire system down to the concrete, metal or wood deck (substrate). Thermography is a non-invasive alternative to core sampling but isn’t appropriate for every situation.

The thickness of a particular core sample depends on the type of roof assembly and the amount of tapered insulation that was needed on that part of the roof to ensure proper drainage. It’s not uncommon for roofing systems to have core samples as thick as eight inches. In this sample, you can see the asphalt in the modified bitumen has embedded itself into the fiberboard insulation and turned it black.

We found moisture across the entire 40,000 square foot roofing system and are in the process of replacing the roof. When a core sample is borderline, we send it to a lab to determine the exact moisture content but that wasn’t necessary for this roofing system. The new roof will be a modified bitumen roof assembly, similar to the current roof.

Congratulations to Peter Saucedo for our delicious photo of the month!