Solid fuel Fireplaces are used for burning coal or wood. A conventional hearth (Unslabbed hearth) is manufactured without an expansion joint and is not suitable for use with solid fuel. The intense heat generated may crack the hearth, as there is extensive expansion and contraction. Solid Fuel Hearths Slabbing is a method of filling the underneath of the heath with specialised cement (often called fondu cement). This cement is light in weight and is more heat resistant. By filling the hearth with cement & making expansion joints, the hollow hearth is reinforced and this helps spread the heat evenly reducing the risk of heat stresses which may have otherwise caused the hearth to crack.