Problem: Rotting of wood roofing; rusting of metal roofing.
Cause: Condensation on the underside of the roofing.
When water vapor migrates to the underside of wood shingles and shakes and then condenses, the moisture can cause decay.
Condensation can form on the underside of sheet metal when warm moist air leaks or diffuses from the interior and comes in contact with roofing at a temperature that is lower than the dew point. Condensation can also form on the underside of sheet metal as a result of rapid heat transfer by conduction and radiation. This condensation will cause the metal to corrode and rust.
Solutions: Reduce the amount of moisture entering the roof cavity by installing a good air and vapor barrier. Ventilation of the attic space will lower moisture levels as shown in Figure 6.
Figure 6 - Moisture Damage To Wood And Metal Roofing
Note 1: Condensation forms on framing members, sheathing and underside of roof covering.
Note 2: Roof covering absorbs moisture and deteriorates.
Note 3: Air leakage: Warm, humid indoor air leaks through holes.
- Install good air and vapor barriers in the house. This is especially important in ceilings, because a positive pressure near the ceiling will force warm, moist air into the attic.
- Ensure all penetrations through the air barrier are properly sealed.
- Ventilate the attic according to local building codes.
- Use pressure-treated shingles and shakes to achieve better service life.
- Prevent contact with the attic conditions by installing foil or felt underlay directly under the metal roofing. Avoid thermal bridges through fasteners, as they provide a good surface for condensation.
Additional information on roofs and roofing