Inspect Existing Roof
1-2 hoursAssess decking condition, identify rot or soft spots, check ventilation, document existing layers, and determine if tear-off or overlay is appropriate.
Field context
This workflow is part of 3 niche fields
End-to-end roofing project guide covering material estimates, pitch calculations, underlayment, flashing, and safety planning for shingle or metal roofs.
Assess decking condition, identify rot or soft spots, check ventilation, document existing layers, and determine if tear-off or overlay is appropriate.
Measure each roof plane length and width, record pitch, note valleys, hips, ridges, and penetrations (vents, chimneys, skylights).
Determine shingle bundles, underlayment rolls, drip edge, ridge cap, nails, ice-and-water shield, and gutter capacity based on measured area and pitch.
Remove existing shingles and underlayment, inspect and replace damaged decking, install drip edge at eaves and rakes, and dispose of waste properly.
Lay ice-and-water shield in valleys and at eaves, roll synthetic or felt underlayment, install step flashing at walls, and set pipe boot flashings.
Install starter strip, field shingles from eave upward with correct exposure and nail pattern, cap ridges and hips, and seal exposed nail heads.
Install counter-flashing at chimneys, seal all penetrations, hang and pitch gutters, install downspouts, and perform a final walk-through inspection.
Determine roof pitch to assess walkability and select appropriate materials. · Convert rise/run measurements to pitch factor for area calculations. · Confirm nail pattern requirements based on roof slope.
Calculate true roof surface area using pitch multipliers for each plane. · Cross-check total squares (100 sq ft units) for material ordering. · Estimate tear-off waste volume for dumpster sizing. · Calculate underlayment roll count from net roof area plus overlaps. · Verify total catchment area for gutter capacity validation.
Verify rafter length and spacing when replacing damaged structural members.
Calculate shingle bundles needed based on roof area and exposure rating. · Track bundle consumption during installation against calculated order.
Size gutters and downspouts based on roof catchment area and rainfall intensity. · Size and position downspouts based on roof drainage area.
Apply waste factor (10–15%) for complex roof geometries.
Multipliers to convert flat footprint area to actual roof surface area.
| Pitch (rise/run) | Angle (°) | Pitch Factor | Walkability |
|---|---|---|---|
| 4/12 | 18.4° | 1.054 | Easy |
| 6/12 | 26.6° | 1.118 | Moderate |
| 8/12 | 33.7° | 1.202 | Difficult — use roof brackets |
| 10/12 | 39.8° | 1.302 | Steep — safety harness required |
| 12/12 | 45.0° | 1.414 | Very steep — professional only |
Never tear off more than you can dry-in in one day. Keep tarps ready and monitor hourly forecasts — a surprise rainstorm on exposed decking causes thousands in damage.
Work from the bottom up so each course overlaps the one below. Starter strip overhangs the drip edge by 6–10 mm (1/4–3/8") — more causes cracking, less allows water wicking.
Open metal valleys last longer than woven shingles in heavy snow regions. Closed-cut valleys look cleaner but require precise cuts. Pick one method and use it consistently across the entire roof.