How to Inspect a Roof Deck for Rot Before the New Roof Goes On

Step by Step

1. 1Step 1: Clean the deck and look at it in good light

   Broom off grit, granules and stray nails. Look along the deck toward the sun or use a flashlight held low to the surface. Dark patches, water rings, raised grain and panel edges that have swollen above their neighbours all stand out in raking light.

   Walk the whole roof, not only the areas you expect. Damage often shows up in the middle of a slope where a nail has been leaking for years.

2. 2Step 2: Probe every stained or suspect area

   Press the tip into the surface with moderate force, not a hammer blow. Sound plywood or OSB resists and leaves only a small dent. Rotten plywood lets the tool sink in, crumbles, or comes up as fibre. Check the edges of panels, which decay first, and the areas around nails and vents.

   Mark each suspect area with crayon and probe around it to find how far it goes. Decay spreads along the grain and under the surface, and the visible stain is often smaller than the damage.

   Tip: Test the lower eave edge carefully. Plywood at the eave often looks fine on top and is delaminated underneath.

3. 3Step 3: Check for delamination and edge swelling

   Plywood that has been wet repeatedly separates in layers. You will see the top ply lifting, checked or peeling. OSB that has absorbed water swells at the edges, turns fuzzy and flakes. A swollen edge is a sign that water has been getting in at a joint or at the eave.

   A panel that is delaminated will not hold a nail well, and it will deflect under load. It needs replacing even when the rest of the deck is fine.

4. 4Step 4: Check from the attic side

   If you can get into the attic, walk the framing with a flashlight. Dark water stains on the underside point to leaks above. Rust on nail tips shows condensation on the nails, a sign that warm moist air is reaching the cold deck. Black or green patches on the wood mean mold from poor ventilation.

   A moisture meter gives a number. Wood around 12 to 15 percent is normal, and readings above about 19 to 20 percent point to a moisture problem. Take readings in several places, and compare against the dry panels. If the moisture comes from ventilation problems, see how to fix attic mold caused by poor ventilation.

5. 5Step 5: Check deflection between rafters

   A solid deck on rafters at 16 or 24 inches on centre should feel stiff underfoot. A springy panel means it is too thin for the span, unsupported at the edges, rotten, or loose from its framing. Lay a long level across the rafters and look for gaps.

   If the sag runs across several rafters, it is a framing issue, not only a sheathing one. See how to fix a sagging roof deck.

6. 6Step 6: Check fasteners and panel joints

   Nails that stand proud of the deck or rattle are loose and should be driven or replaced. Rusted nails have reduced holding power. Panels should have a gap of about 1/8 inch at edges, and on unsupported edges of thinner sheathing H-clips or blocking should be present. Missing clips are a cause of deflection and wavy shingles.

   Check around penetrations and at the ridge, where sheathing can be cut short or badly fastened.

7. 7Step 7: Decide what to replace

   The rule is that anything that will not hold a nail or carry a worker's weight has to go. Small stains on the surface of solid wood can stay. Dark, soft or crumbling areas go, and so does a panel with swollen edges and a weak core. Cut back to sound wood so there is a full rafter to land the new edge on.

   If more than about a third of the deck is bad, it is often faster to re-sheet whole slopes. See how to replace rotten roof sheathing. If the deck is spaced boards, see how to resheet over plank decking.

8. 8Step 8: Find and fix the cause of the damage

   Rot from a leak at a chimney needs the flashing rebuilt. Rot along the eave points to ice damming or gutters. Rot on the underside points to ventilation or bathroom fans. Replacing the wood without fixing the cause only sets the next failure in motion.

   Record where damage was found and what it was caused by, so the homeowner sees the reasoning.

What a Good Deck Looks Like

Fraser Valley Notes

Common Mistakes

• Judging the deck by looking only. Wood that looks fine can be delaminated or soft. Probe it.
• Covering a bad deck with new underlayment. Nails do not hold in rotten wood and the roof will move and leak.
• Skipping the attic check. The underside shows stains, mold and rusted nails that the top side hides.
• Replacing panels without finding the cause. The new wood rots again if the leak, the gutter or the ventilation problem stays.
• Cutting out rot but leaving rotten edges at the rafters. New panels then have nothing solid to land on.
• Ignoring deflection. A springy deck cracks shingles and loosens fasteners even if the wood is sound.