In selecting shim material, an important consideration is Fire Rating. For Concrete Contractors, shims often are used in the forms prior to pouring concrete. Since they will not be a permanent part of the structure any material that provides the necessary spacing should be acceptable. For steel Fabricators and Erectors, the shims are more often permanent and required to last the life of the structure. This requires proper material selection not only for Compressive Strength and Durability but also for Fire Rating.