Gravel vs. a Cap Sheet Roof

Weight

• Using gravel as your aggregate for a flat roof system creates far more weight, and therefore strain on the roof structure, than a cap sheet. A gravel and tar system weighs about 450 lbs. per 100 square feet, according to the Journal of Light Construction. In contrast, the cap sheet installation weighs only 220 lbs. per 100 square feet on average. If putting additional stress on the frame or deck of the roof is a concern, cap sheets are a better choice.

Installation

• While labor costs vary among roofing installation companies, applying a layer of gravel to finish a flat roof generally takes more time and is more hazardous for the installers. The gravel must be embedded in a layer of hot asphalt before it hardens, according to Inspectapedia. This requires the use of a hot asphalt roller or sprayer. Cap sheets are attached with torch welding, or melting spots of the sheet onto the layer below. This exposes the worker to less hazardous fumes and takes less time.

Wear and Tear

• Gravel finish tends to last longer because the stone pieces resist the damages of wind, rain and constant sunlight better than the mineral topping of a cap sheet, according to the "The Architect's Guide to Preventing Water Infiltration." Standing water and weathering causes them to weaken and become soft over time. This leads to the need for regular replacement, while a properly installed gravel bed will only need replacement if something penetrates the top layer of asphalt holding it on.

Finding Mistakes

• Sometimes a portion of the flat roofing materials don't adhere to the deck correctly. This causes a leak that is often spotted from the inside of the building. Locating that leak when it is covered with gravel is a lot harder than looking for it through the thin layer of mineral coating on a cap sheet according to "The Architect's Guide to Preventing Water Infiltration." The small pieces of mineral aggregate show holes or adhesion problems much easier.