8.3 Steep-Slope Reroofing & Overlay Rules

Reroofing is where steep-slope theory meets jobsite economics. Property owners ask for the cheapest second layer; manufacturers and codes ask whether the existing roof can legally and safely accept another covering. In Illinois, freeze–thaw, ice dams, and humid summers mean many “simple overlays” hide soft decking at eaves and valleys. Exam candidates must know when recover is allowed, when tear-off is mandatory, and what inspection steps follow once the old roof is gone.

Overlay (Recover) Versus Full Tear-Off

An overlay (also called a recover) installs a new steep-slope covering over an existing one without removing all prior layers to the deck. A full tear-off removes existing roof coverings down to the sheathing so the deck can be inspected, repaired, and detailed with new underlayment and flashings.

Overlays can reduce labor and disposal cost when conditions are ideal. They also add weight, can trap moisture, hide decay, and leave failed flashings in place. Tear-off costs more up front but reveals the truth about the deck and allows correct ice-and-water shield, drip edge, and ventilation corrections that Illinois roofs often need.

When Overlay May Be Allowed

Recover is a conditional permission, not a default right. Typical constraints—always verify the adopted code edition, local amendments, and the new shingle manufacturer’s written instructions—include:

• Existing roof is structurally sound and essentially smooth, without cupped, brittle, or missing shingles that prevent uniform nailing
• Deck remains sound—no soft spots, delamination, or widespread fastener withdrawal
• Layer limits are respected (commonly no more than two asphalt shingle layers total after the recover)
• Roof slope and product remain within the new covering’s approved applications
• Flashings can be made weathertight; often step flashing and counterflashing still need selective replacement even on recovers
• Ventilation and moisture conditions will not trap water between layers

Nail length on overlays must penetrate the new shingle, pass through the existing layer(s), and achieve the required embedment into the deck. Short nails that stop in the old shingles are a classic recover failure mode.

Layer Limits and Manufacturer Constraints

Many jurisdictions effectively cap asphalt shingle roofs at two layers. A third layer is typically prohibited because of dead load, uneven surfaces, and inability to inspect the deck. Even when a local code might seem permissive, manufacturer instructions control warranty and are often incorporated into code compliance. If the shingle wrapper says “apply only to a clean deck” or “maximum one overlay,” that instruction governs the job you are installing.

Other manufacturer and code constraints that block casual overlays:

• Prohibitions on recovering over saturated or curled roofs
• Limits on recovering over insulated roof assemblies that need drying
• Requirements to replace drip edge, valleys, and wall flashings rather than bury them
• Specific underlayment and ice-barrier rules for cold climates that assume deck-level installation
• Wind-warranty nailing patterns that cannot be achieved through irregular old surfaces

Illinois wind and winter events make “fasten through junk” a poor bet. When in doubt, tear off.

Mandatory Tear-Off Triggers

Plan a full tear-off when any of the following appear:

• Existing roof already at the maximum layer count
• Soft, spongy, delaminated, or mold-stained sheathing
• Widespread leaks, especially along eaves and valleys after ice dams
• Warped or uneven surfaces that prevent flat shingle courses
• Need to correct deck slope washouts, add ventilated baffles at eaves, or reset ice-and-water shield to cold-climate widths
• Building official or manufacturer requirement for deck inspection on that product/system

Tear-off is also the right ethical recommendation when an overlay would conceal known decay. Passing a short-term visual cover-up onto the next owner is not competent practice.

Deck Inspection After Tear-Off

Once the roof is stripped to sheathing, slow down. Illinois moisture often concentrates where snow sits longest—north slopes, valleys, behind chimneys, and at eaves inside the wall line. Inspect for:

• Soft spots under foot pressure; probe suspect areas
• Dark staining, fungal growth, and delaminated plywood/OSB edges
• Overdriven or withdrawn nails from prior roofs
• Inadequate thickness or spanning for current code loads
• Knot holes, gaps, and damaged panels at hips and ridges
• Evidence of chronic condensation (rusty nails, frost marks historically reported by owners)

Replace damaged panels; match thickness and panel rating; leave the deck clean, dry, and suitable for underlayment. Only then install ice-and-water shield at eaves/valleys, synthetic or felt underlayment as specified, new drip edge, and flashings. Reroofing without resetting cold-climate membrane details wastes the tear-off investment.

Using the Reroof to Fix Climate Systems

A steep-slope reroof in Illinois should be treated as a system reset:

• Add or restore soffit intake and ridge exhaust with proper NFA balance
• Install rafter baffles before insulating contractors refill eaves
• Air-seal attic bypasses when the project scope allows access
• Install ice-and-water shield to cold-climate eave dimensions, not token starter strips
• Replace compromised step flashing rather than caulking over old metal
• Confirm the chosen shingles are approved for the slope and for any recover scenario actually used

Code minimums, manufacturer instructions, and climate reality must agree. If an overlay prevents you from installing required ice barrier at the deck, the overlay is the wrong scope. Competent reroofing chooses tear-off whenever deck health, layer limits, or Illinois cold-climate detailing demand a clean start.