Buildings with a Low Pitch Roof

These are typically on homes in the 1000 to 1200 square foot foundation size.  The slope of the roof ranges between 3” and 6” drop in 12 inches.

The attic typically has ventilation on either end of the peak and the eave vents.

Cooler outside air enters the attic through the eave vents and exits at the roof peak vents as warm air.

The warm air in the residence rises and ultimately enters the attic.  When the outside air temperature is at least 20 degrees cooler than the residence, any moisture in the air then condenses on cool surfaces in the attic. The coolest surfaces are the nails holding the shingles. The moisture collects around the nail and moistens the wood. Over time the moist wood attracts mold spores from the air and a dark stain forms.

In most cases, the section of the roof facing predominantly NORTH is more involved because the winter sun does not warm the North roof and evaporate the moisture.  The South facing roof is exposed to sun which heats the roofing material and evaporates the moisture.

The mold on roof sheeting is not a health issue because attic air is not shared with the indoor air stream.  Rather, the moisture damages the plywood sheeting that holds the shingles. If the plywood becomes wet enough to weaken the glue inside the sheeting, the plywood begins to delaminate or break apart. The only solution to delaminated sheeting is the replacement of the plywood sheeting.

The best solution for limiting moisture condensation in the attic is the following:

  1. Duct all exhaust fans to the outside, NOT INTO THE ATTIC.
  2. Duct the stove exhaust hood outside the residence to exhaust the cooking moisture.
  3. Verify that the eaves are open and not blocked by attic insulation or the application of vinyl or aluminum siding.
  4. Put additional vents in the attic near the top of the roof.
  5. If the roof is being replaced, have ridge vents installed.


Treating the roof sheeting with Microban or IAQ2000 will kill the mold but will not eliminate the stain.