Just How Dense Do You Need to Be?

By: Tamara Middleton, Sales Representative – Architectural Products

ASTM C578 uses 10% deformation as it base for compression standards in EPS, including EPS roof insulation. 10% deformations is the weight at which the foam compresses to 90% of its original thickness, and still maintain all its ASTM C578 properties.

ASTM C578 standard for EPS type I is 10psi or 1,440 pounds per square ft (1 square foot is equal to 144 square inches). One square inch of Type I EPS can support 10 pounds; however, set a one square foot board on the foam and it can support 1,440 pounds.

How much would a roof really need? Park City, Utah at an elevation of 9,000 feet has snow loads of 240 pounds per square foot. Add one 200 pound person standing on one foot, approximately 8” x 3” or 24 square inches. The Park City snow load in that 24 square inches is 40lbs (240/144 = 1.67lbs. per square inch, 1.67* 24). Snow load and person equal 240 lbs on that 8” x 3” spot. Type I EPS in 24 square inch supports 240 lbs. Type VIII EPS in 24 square inches supports 312 lbs. Type II EPS in 24 square inches supports 600 lbs.

With DensDeck or similar product the weight is distributed over 32 square feet, EPS Type I can then support 46,080 lbs (32*144*10). The Park City snow load for that sheet of DensDeck would be 7,680 lbs (240*32) add four 200 pound people and the weight of ½” DensDeck (2lbs per square ft = 64 lbs) and the total would be 8,544 lbs far below the max weight for ASTM standard Type I EPS.

A car weighs approximately 4,000lbs that’s 1,000 lbs per tire, one tire sits on about 6” X 8” or 48 square inches Type I EPS foam in 48 sq inches can support 480 lbs, however drive the car on to 2 sheets of 4 x 8 OSB, 2 tires on each, now 32 square feet of foam is supporting the ½ the car or 2000lbs, 32 X 144 = 4,608 square inches or 46,080 lbs.

To stay at or below 1% deformation – 1lb foam supports 3.5 pounds per square inch. 10ft x 10ft slab of concrete weights 50lbs per square ft X 100 Square feet = 5,000 lbs. 100 square feet of Type I foam can support 50,400 pounds (100*144*3.5 lbs per sqin) at 1% deformation. One full size king cab 4 wheel drive pickup truck can weigh 12600 lbs. Parked on 10ft x 10 ft slab the total weight is 17,600 lbs well below the 50,400 lbs 100 square feet of 1 lb. EPS can support with 1% deformation or less.

On most roof decks 10% deformation is sufficient as it is not part of the structural support. In most cases there is no need to calculate the local snow load plus the weight of several individuals and Type I EPS with DensDeck or similar. Type II EPS would be enough without the additional board. Even on a rooftop patio the Type I with a board or the Type II without would support the additional weight of the pavers and people.

A one square foot paver weights approximately 25 lbs, EPS Type II can support 2,160 pounds at 10% deformation and 936 pounds at 1% deformation. Add the Park City snow load on that paver and the total weight on the foam is 265 pounds. That leaves more than 600 pounds before the foam compresses to 99% of its original thickness and 1,895 pounds before is compresses to 90%. Pedestal pavers need to be calculated based on the size of each pedestal and may need a Type IX EPS if a DensDeck or similar is not used.

Learn more about EPS roof insulation.

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