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What Causes Mold Growth on Siding?

Mold growth on siding doesn’t seem to follow the rules.  One house has persistent mold growth throughout the siding, while the neighbor’s home is spotless.  Why such discrepancies?

Paint Quality

Latex paint is a wonderful food source for mold.  In fact, if the manufacturers didn’t add mildicide to the paint, you’d find a layer of mold growth inside the can.   However, these mildicides are designed to prevent mold growth during storage.  They are not designed to prevent mold growth once the paint has been applied. To prevent future mold growth, secondary additives are mixed into the formula.  Sometimes these work great.  Sometimes they do not…

siding mold growth

Mold growth on siding

Poor quality materials used for sheathing

Sheetrock is great when it’s used appropriately (inside the home, in areas with little chance of water intrusion).  The exterior of a home is not one of these locations.

moldy siding

Mold growth found on sheetrock used as sheathing beneath the siding.


wet siding and mold growth

Watch where you point your sprinklers.


mold damage beneath siding

Mold and rot damage on sheathing due to failed masonry.


Project Report > Inspection for Mold due to Exterior Siding Failure

Mold Growth from Siding Failure

Reason for inspection:  Major remodel at the property.

  • Sheetrock along the exterior wall was removed and mold growth was identified by the contractor on the gypsum sheathing along the western wall of floors 2 and 3.

Summary of concerns:

  • Mold growth is likely secondary to previous leakage or failure of the stucco siding along the western wall.
  • Active leaking from around the elevator shaft above floor 3 was reported, water damage was noted in these areas but no active moisture content was present.

Water Damage from Siding Failure

Level 2:  Observations

  • Obvious signs of water damage and mold growth on gypsum sheathing along southern

 Level 3 Electrical room:

  • Obvious signs of mold growth and minor water damage on gypsum sheathing at bottom 3’ near the electrical panel.

Level 3 Shaft:

  • This area was inaccessible at the time of inspection, but a small inspection hole was cut by the contractors.
  • Inspection through the hole revealed dark staining consistent with mold growth on the bottom 4” of all visible sheetrock.

Level 3 Stairwell:

  • Contractor reports intermittent leakage from the ceiling in this area.
  • The leakage may be stemming from the adjacent elevator shaft.
  • Obvious deformation to the paint and sheetrock ceiling was noted in this area.
  • Hire an appropriate roofing contractor to address area of leakage.

 Level 3 Northern portion of West Wall:

  • Visible mold growth noted on gypsum sheathing along the northern end of the western wall.

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