In asking this question, the interviewer wants to determine if you have the qualities they are looking for in a Roofer. The strength you describe should relate to the position. Review the job announcement, look at the qualities that the company is looking for, and use one of those keywords in your answer. If the company is specific about the traits they want in a Roofer and you use one of those as your greatest strength, the interviewer will be more likely to look at you as a strong candidate. For example, suppose the company advertises they are looking for a dependable, safety-conscious Roofer with excellent communication skills and attention to detail. You should then choose one of those keywords as your greatest strength.