Case Study: Ice Dam Attic Mold — Sandy Craftsman, Salt Lake Valley

Event date: March 2023 | Property: 2001-era hip-roof craftsman with wide overhanging eaves, Sandy, UT | Event type: Ice dam meltwater intrusion into attic assembly; Cladosporium colonization of OSB roof sheathing underside | Duration: Multiple winter seasons of undetected meltwater intrusion | Insurance carrier: Allstate HO-5 | Approved amount: $8,440 | Deductible: $1,000

What Happened

In March 2023, a Sandy homeowner noticed a ceiling stain in the master bedroom — an 18-inch-diameter dark ring approximately 3 feet from the exterior wall, visible through the painted ceiling surface. A roofer had inspected the roof in December and found no damage. The homeowner had assumed the stain was from a roof nail pop and had been watching it since late February to see if it grew. It had grown slightly each week for three weeks when she called us.

The stain itself was not the event. The stain was the most recent evidence of an event that had been occurring every winter for at least two seasons. A hip-roof craftsman with wide overhanging eaves creates a large cold eave zone — the section of the roof that extends beyond the exterior wall plane and receives no heat from the building interior below it. When the R-22 attic insulation above the master bedroom — the 2001 installation, significantly below the current IECC climate zone recommendation of R-49 — allows sufficient heat loss through the ceiling plane, it warms the roof deck above the master bedroom. Snow on that section of the deck melts from below. The meltwater runs down toward the cold eave, reaches the unheated section above the overhang, and refreezes. The accumulated ice dam forces subsequent meltwater to pool on the roof surface above it until it finds a pathway under the roofing membrane — at a shingle seam, a nail penetration, or a section of weathered underlayment.

The roofer who had inspected the roof in December had found no damage — because in December, the ice dam had not yet formed. The roofer was looking at the roof surface. He was not looking at the attic assembly below it.

What We Found

When we accessed the attic above the master bedroom, we found Cladosporium colonization on the underside of the oriented strand board roof sheathing covering approximately 34 square feet — a fan pattern radiating from the eave entry point, spreading laterally across the sheathing underside from the location where meltwater had entered the attic assembly at the cold eave zone. The colonization pattern was consistent with at least two to three seasons of recurring meltwater intrusion, not a single event. Each winter’s ice dam had introduced meltwater to the same sheathing section; each cycle had wetted the OSB before it fully dried from the previous cycle; the accumulated sustained moisture had enabled Cladosporium colonization to establish across the full 34-square-foot zone.

Calibrated moisture meter readings at the sheathing surface: 21% to 28% at nine monitoring points across the colonized zone, 16% to 19% at the thermal boundary — above dry standard throughout. FLIR thermal imaging of the master bedroom ceiling showed the cold zone extending 28 inches from the exterior wall, consistent with meltwater migration from the attic assembly through the insulation to the ceiling surface below.

What We Did

Mold remediation in the attic proceeded under HEPA negative air pressure containment: wire brushing of all colonized OSB sheathing surfaces to remove loose spore-bearing material, two passes of EPA-registered antimicrobial treatment at required concentration and dwell time, film-forming encapsulant applied to the full remediated surface and a 6-inch conservative extension beyond the thermal boundary. Ceiling drywall at the stain location was replaced. Post-remediation clearance testing by an independent certified industrial hygienist returned spore counts within normal background range.

Insulation above the affected master bedroom area was upgraded from R-22 to R-49 — the current International Energy Conservation Code recommendation for this climate zone — before the ceiling was closed. The insulation upgrade addresses the root cause of ice dam formation in this section: by reducing heat loss through the ceiling plane to the point where the roof deck above the master bedroom no longer warms sufficiently to melt the overlying snowpack, the ice dam formation cycle is broken. We do not guarantee that ice dams will never form again — the Wasatch Front’s January and February freeze-thaw cycling can produce ice formation at any roofline under the right conditions — but upgrading from R-22 to R-49 substantially reduces the probability of recurrence at this specific location.

Insurance Outcome and the Three Prior Seasons

Allstate HO-5 covered the event as weather-related roof leak damage. Total approved: $8,440. Deductible: $1,000. The homeowner asked at the walkthrough whether the previous two winters’ ice dam events — which she had not known were ice dam events, attributing the brief ceiling stains to condensation — should have been claimed separately. They could not have been: the prior stains had not been professionally assessed, the moisture extent had not been documented, and the prior events had resolved visibly without intervention. What was claimable was the current event, which we documented with the full thermal imaging record, moisture meter baseline, mold colonization findings, and remediation scope. The prior seasons’ history was relevant background. The current event was the covered claim.

Frequently Asked Questions

Why does ice dam damage appear in the ceiling weeks after the ice forms?

Ice dam meltwater enters the attic at the cold eave zone and migrates through the insulation layer by capillary action and gravity before appearing at the ceiling surface below. This migration takes days to weeks. The visible ceiling stain appears long after the ice dam event — which is why homeowners see a stain in late February from an event that occurred in early January.

Does upgrading attic insulation prevent ice dams?

Upgrading from R-22 to R-49 (the current IECC recommendation for this climate zone) substantially reduces heat loss through the ceiling plane that warms the roof deck and initiates the snow-melt cycle. It does not guarantee no ice will ever form — eave overhangs, penetrations, and other uninsulated areas can still become cold enough. But it breaks the recurring cycle at the insulated section where the ice dam forms most predictably.

Is ice dam damage covered by homeowners insurance?

Most HO-3 and HO-5 policies cover ice dam damage as weather-related water damage — meltwater entering through the roof structure as a result of the ice dam. Documentation of the entry point, the meltwater migration pathway through the attic, and the resulting moisture and mold conditions is the basis for the claim. An HO-5 open-perils policy generally provides broader ice dam coverage than an HO-3.

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True Day Water Damage Restoration | 11268 S 2865 W, South Jordan, UT 84095 | (385) 247-9359 | Utah Contractor License: #960332-3505 | IICRC Firm ID: #927354-5258