To repair sagging upholstery on a car roof, buy upholstery glue from a local hardware store, and then use an aerosol can to spray the glue to areas in which the fabric is sagging from the roof. If there are leaks in the roof that are causing the fabric to sag, use a silicone roof sealant to stop the leaks.
It can be very expensive to have a professional repair the roof of a car. Often times, there can be a leak in the roof that causes the upholstery to warp and sag. A local hardware store should carry both silicone roof sealant and upholstery glue, which will allow the user to stop the leak and reupholster the roof fabric to make it look new.
Once a leak has been detected, fill in the leak with the silicone sealant. After the sealant has dried, locate the areas in which the upholstery is sagging and cut a small hole in the fabric near the sagging areas. Use the aerosol upholstery glue to adhere the fabric back to the roof of the vehicle, and have an assistant hold the fabric in place until it dries. Repeat this process to stop the fabric from sagging, and make sure to tend to all possible leaks in the roof to prevent future sagging.