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Q. My husband and I bought our end-unit townhouse last year knowing it would need a new roof soon. At the time, the house inspector noted that the plywood underneath the shingles (decking?) was 1/2 inch thick and though common in 1965, when the townhouse was built, home builders now use 3/4 inch plywood. He also noted that there are some "spongy" spots as well as moss growing in some areas, likely from the extensive tree canopy we have. This year we are ready to invest in the project but were wondering what we need to know about getting a new roof. We would like to know if new plywood is worthwhile and what to look for to prevent staining from the tree debris and ice dams. Most importantly, we are very interested in how a roof can improve the efficiency of our "whole house system" in order to save on energy bills and also reduce our carbon footprint. Anything you could recommend would be greatly appreciated!
A. As part of your reroofing you should tear off the old shingles and tar paper right down to the wood deck. At that time the top of the plywood decking can be inspected and any repairs and replacement can be done as needed. Normally the places where the decking will get bad on an older roof are at edges and terminations, like near chimneys or roof penetrations for vents or skylights that have started to leak slightly causing wood rot. These locations may not be visible from inside the house so it’s only with the shingles off can they be detected. The going rate for plywood replacement is in the $100. per sheet range so it’s always wise to consider the roof replacement before the plywood is allowed to get too bad.
Townhouse roofs are typically framed with pre-engineered roof trusses set on two foot centers. Way back when we used 3/8ths inch plywood for sheathing with double aluminum clips between the trusses at the side joints of the sheets. When we walked the roof we taught ourselves to step from truss to truss and not to put our weight near seams. Anyone stepping between the trusses on the roof surface would certainly feel a deflection from the concentrated load of a foot on the center span of that deck material. The other condition that causes a spongy townhouse roof deck is degrading FRT (fire retardant treated) plywood but the bad years for that material was from 1981 to 1988. Today it's a nominal 1/2 inch sheathing that's used on roofs and really that's all you need unless you’re thinking about something much heavier than common asphalt roof shingles-- like slate. It is the building code that in this region roofers should roof against ice damming but they frequently don’t so you have to be sure when you are getting estimates that you develop a scope of work that describes the shingles you want and ice damming protection. I like a modified bitumen sheet material like Grace Ice and Water Shield® for ice dam protection. Since your desire is to go “green” as you can with this project part of your scope of work will be to specify a shingle that by design and color will reflect the sun’s heat better than the standard asphalt shingle that only reflects from five to fifteen percent. Take a look at Elk Cool Color Series that offers a line of shingles with reflective granules in various colors. As for the staining, that’s an algae that settles between the shingle’s tiny surface granules and grows. On darker shingles it’s less visible but since you will be selecting a lighter color for coolness see if you can specify the shingle to have copper and magnesium granules added to the surface to prevent the growth. Since you tell me this unit dates from 1965 if the attic insulation hasn’t been upgraded it surely needs to be. The target depth is about twelve inches of fiberglass (R38) and when your unit was built it was lucky to get half of that and insulation settles over time lowering its R value.