Fixing Roof Decking Cut a piece of 1/2 inch plywood that will cover the area. Because driving nails up into the sheathing material will make things worse, glue a piece of 1-inch rigid Styrofoam to your piece of plywood. Next, place your piece of plywood and Styrofoam foam-side-up against the damaged roof decking.Click to see full answer. Also to know is, how much does it cost to fix a sagging roof?Most home owners spend between $300 and $1,100 for a roof repair, or an average of $650 to fix a roof related issue. Going beyond $3,000 is possible, but at that point a replacement roof could be your best bet.One may also ask, why is there a dip in my roof? That dip in the roof with a soft spot indicates that you should have your roof repaired ASAP. These soft spots usually form when the layer underneath gets soaked due to gaps in the shingles. Most roofs have plywood sheeting underneath the roofing tiles or shingles. Just so, is a sagging roof normal? Sagging or low spots in a flat roof may be due to undersized or weakened roof joists or problems with the buildup layers beneath the exposed roofing. As mentioned, roofs sag because there’s too much weight, or load, for the existing structure. Common loads include: snow and ice.How often should you replace your roof?For instance, a typical asphalt shingle roof lasts 20 to 25 years, while a roof installed over an existing layer of shingles should be replaced after 20 years.