How Thick Can Roof Coverboards be?

28 December 2017

In low-slope roofing, coverboards have become ubiquitous as concerns about gaps and design requirements looking for high-performance roof systems demand protective layers used on top of thermal insulation. Coverboards are typically thought of as 1" thick or less, dense boards that provide sound and fire resistance to the assembly.

However, you don't have to limit yourself to 1" coverboards. Thicker coverboards, such as 2" or 3", could be used to replace 1/2" or 5/8" thick boards and provide insulating value that can reduce the thickness of the thermal insulation to allow only one layer of thermal insulation to be used.

In a common roof design, 2 layers of polyisocyanurate insulation would be used to meet an R30 roof design totaling 5.5" thick of insulation as well as the coverboard (1/2" or 5/8"). With 2" thick of mineral wool as a coverboard (which provides R3.8/in), an R30 roof system can be met with just 4" of polyisocyanurate insulation. This system does not increase the height of the total assembly but will perform better in cold temperatures based on third-party research into effective R-value performance.

Roof systems are increasing in height as codes require more and more thermal resistance. Taking advantage of coverboard materials that provide added thermal resistance in addition to meeting the demands of high-performance assemblies can be beneficial to all involved.

ROCKWOOL TOPROCK® DD and TOPROCK® DD Plus products are offered in 2 - 6" thicknesses to meet today's energy codes, although the benefits of the coverboard can be realized with a minimum of 2" thick TOPROCK DD on top of the polyiso insulation.

Check out How to Install: ROCKWOOL TOPROCK® DD Plus with a Spray-Applied Adhesive.