A high bill is always an unpleasant surprise. Additional cost can often come in the form of roof decking replacement, which is not included in the original replacement bid; without tearing off the roof, it is impossible to give an accurate estimate of the amount of water damage to the sheeting. But what caused it to rot? Sometimes the reason is obvious: storm-damaged shingles sprung a leak, or an improper installation caused premature deterioration that rotted the sheeting out, but either way, these types of problems would be easily fixed by the new roof installation. Other times, it is a little more complicated. The water is not leaking in from above, but instead condensating on the underside of the sheeting. Why?
Attic perimeters in older homes are oftentimes not properly insulated or vented. Improvements are difficult because the crawl space tends to be very shallow towards the gutter edges. During the heating season condensation inevitably occurs there, destroying the roof sheeting. On a cold day, the heat from indoors carries up moist air that, when it comes in contact with the cold underside of the roof deck, condenses into water droplets. Wherever there is moisture in an organic surface like wood, mold will inevitably grow, causing rot and structural damage. In the picture below, black spots on the plywood correlate to the areas with the greatest amount of condensation, and where the most heat was lost. You can also see the greater amount of black spots around the nails, because metal nails become cold faster than plywood, and therefore cause more condensation.
Of course, the delaminated sheathing needs to be removed and replaced, but that does not address the cause of the problem. To prevent condensation, the roof perimeter needs to be properly insulated and vented. In an ideal scenario, air is drawn into the attic through the intake vents in the eaves; as the heat naturally rises to the top, it carries all the moist air in the attic out the ridge vent, before it has a chance to condense inside (see diagram). This means soffit vents should be drilled in, as needed, to allow this intake; insulation should be installed over the perimeter edge of the ceiling and wrapped over the edge of the outer wall; and styrofoam baffles should be installed under the new plywood to keep the insulation from blocking air movement between the soffit vents and ridge vent.
Correcting this weak point makes the whole building more sound and long-lasting, minimizes ice dams, and decreases heating costs in the winter. It’s well worth the investment.
Please note, this solution only works in open attics that are easy to vent. For more complicated attics that have knee walls, skylights, or a vaulted/cathedral ceiling, please see the following article.