A cracked lintel can be severe if left to expand. Diagnosing the cause of the cracking should be the first step; this is done to prevent the same problem from reoccurring. Modern mortars and masonry paints trap moisture in porous materials and cause them to erode. This trapped moisture also causes thermal expansion leading to cracks forming within stone lintels. If the damage hasn't fractured the lintel, it can be repaired with a surface mortar. Any non-porous materials should be removed from around it and replaced with a lime-based product. Concrete lintels crack because, over time, the steel inside them expands in damp conditions causing the concrete to fracture. Concrete lintels are best suited for internal use in modern homes.