Roof Structure From Below
~11 min read · Read sheathing sag, delamination and framing distress from inside the attic.
From inside the attic, the roof's skeleton confesses everything: sagging ridges, delaminating sheathing, cracked rafters, and the cardinal sin — cut truss members. The underside view is where roof-structure findings live.
Sheathing
Read the deck from below: delamination (plywood plies separating — chronic moisture), dark staining (leaks and condensation histories, active vs dried judged by moisture meter and correlation), sag between rafters (undersized/overspaced sheathing — the wavy roofline seen outside), and the old spaced skip-sheathing under original wood-shake roofs (fine for shakes, needs overlay for shingles). FRT plywood degradation in some 1980s-era buildings is a specialty note.
- Delamination and staining = moisture history written on wood
- Sag between supports: thickness/span mismatch
- Skip sheathing belongs to shake roofs — note conversions
Rafters and trusses
Rafter systems (stick-framed) fail by sagging (undersized, overspanned, overloaded by heavy coverings), cracking at knots, and thrust problems: rafters push walls outward unless tied — missing/cut collar ties and rafter ties, or ceiling joists disconnected, show as spreading walls and ridge sag. Trusses are engineered: every member matters, so cut, drilled, notched, or 'modified' truss members (the attic-storage chainsaw special) are reported for engineering evaluation, full stop. Gusset plates that rusted, popped, or were repaired with plywood-and-nails are the same referral.
- Ridge sag + spreading walls = tie failure
- Trusses: no field modifications, ever — refer any cut member
- Watch gusset plates for withdrawal and rust
Bearing and load history
Follow loads: purlins and struts must land on bearing walls, not ceiling drywall; added loads (solar arrays, second covering layers, HVAC platforms) demand structure that shows no distress; and sagging that FOLLOWS a re-roof to heavier material is a capacity finding. Document with sight-lines along the ridge and rafters, and photographs.
Worked example
In the attic of a truss-roofed 1998 house: the homeowner built a storage platform by cutting three diagonal webs 'that were in the way,' the ridge line shows a subtle dip over that area, and several gusset plates nearby have popped proud of the wood. Outside, the roof plane reads slightly wavy there. Report it.
This is the truss chapter's cardinal case. Trusses carry load through EVERY member — the three cut webs converted engineered triangles into a hinge, and the structure is already answering: ridge dip above the cuts, gusset plates backing out as members rack, the exterior waviness mapping the same zone. The report: cut truss members (condition) — structural capacity compromised, progressive deformation evident (implication) — evaluation and engineered repair by a structural engineer required; no field carpentry fix (action). Framing it as safety-significant is correct: snow load finds hinges. The storage platform, meanwhile, added load exactly where capacity was removed — note it.
Common exam pitfalls
Treating a cut truss web as minor because 'it still stands.'
Trusses have no sacrificial members — any cut/drilled/notched member is an engineering referral.
Reading every stain as an active leak.
Stains record history — correlate with moisture readings and roof condition to call active vs past.
Missing the tie system on stick frames.
Ridge sag with out-of-plumb walls points to missing/cut rafter ties — look for the thrust mechanism, not just the sag.
Sight the ridge, read the deck, follow the loads — and a cut truss is a call to an engineer, not a carpenter.
Recap
- Sheathing: delamination, stains, sag, skip-sheathing conversions
- Rafter systems: sag, thrust, tie integrity
- Trusses: zero tolerance for field modifications — refer
- Gusset plates: rust and withdrawal
- Added loads (solar, overlays) need distress checks
- Correlate attic findings with exterior roofline
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