Tar & Gravel Roofing (Built-Up Roofing) Systems
Perhaps you have been driving through a mid-century neighborhood and asked yourself, "Why are there rocks on that roof?" Tar and gravel roofing, also known as built-up roofing, is a very common roofing application utilized in construction of low-slope or flat roofs.
On top of the roof sheathing (also called "decking" and commonly plywood), consecutive layers of felt paper are laid, and then the surface is "mopped" with hot asphalt - which contains tar - to seal that layer. Sometimes multiple layers of felt and asphalt are applied before the flood coat, a mixture of asphalt and gravel, is applied for durability. Lastly, a layer of gravel or "ballast" is laid to protect the roof as a whole.
Tar and gravel roofs, with proper maintenance, can last 30+ years. That being said, given that this is an uncommon residential roofing application in modern times, many roofers are hesitant to repair or modify them. Some insurance companies will not insure homes with these roofing types. There are modern replacement roofing systems, which include motified bitumen that comes in a roll and acts much like an asphalt shingle, EPDM, or TPO. EPDM and TPO are considered membrane systems. Modified bitumen is probably the most cost effective, while EPDM and TPO are more expensive.
The roof is a difficult component on a mid-century modern home to "preserve", but one of the most important pieces of the home. I recently had a conversation with a potential buyer who was concerned that replacing the tar and gravel roof would diminish the style, to which I agreed. I find this style to be unique and beautiful. A white TPO roof may disrupt the cadence of elevations from the street, but there might be options to maintain that orginal look with a creative roofer, though such ventures are uncommon. If you need assistance with your roof, let me know. I'm happy to share resources!