What Is a Ballasted Roofing System Covered With?

Stone Aggregate

• The most common ballast used to hold down a thin sheet of roofing membrane is stone aggregate. Basic gravel isn't sufficient for keeping the sheet on the roof under high winds. The stones need to be an average of 1.5 inches to 2.5 inches in diameter, with larger stones required in areas that get hurricane winds, according to the Buildings Journal. Naturally tumbled river stone is often used because it has no sharp edges to puncture or stretch the membrane.

Recycled Ballasts

• It may be possible to use crushed concrete if enough of this material is used to give at least 10 lbs of weight over each square foot of the membrane to prevent wind lifting, says International Diamond Systems. However, local building code may not allow for it. Recycled rubber pavers are heavy enough to keep the membrane flat, but don't have any risk of flying off of the roof like stone. This means the parapet built around the edge of the roof can be lower.

Concrete Pavers

• Slabs and interlocking pavers made from pre-cast concrete offer plenty of anchoring weight and durability beyond what other paving materials can provide. Waterproof Magazine states that many of these products feature interlocking edges to keep them from shifting and come in a range of color options. These slabs are heavier than stone in many cases, so you will need to check that the roof structure and deck can support them before installation is carried out.

Dirt and Plants

• Ballasted roofs make a great place for a green roof or garden. As long as a waterproof membrane completely covers the roof deck, lightweight growing medium and plants can be added to hold it down. Extra drainage in the form of pavers or gravel are added around beds of growing medium to ensure water doesn't stand and pool on the roof under the plants, explains the Center for Environmental Innovation in Roofing.