Illinois Roofing License IDFPR #19HC00108700 · GAF Certified Contractor
Roof decking is the sheet material – plywood or OSB – that’s nailed to the roof rafters. The shingles, underlayment, ice barrier, and flashing all attach to it. When decking goes soft or rotted, the roof system above it has nothing solid to hold onto, and shingles pull free even in moderate wind. This guide explains the differences between OSB and plywood decking, when decking needs replacement, and what to expect during a Trill Roofing tear-off when soft spots are found.
OSB vs plywood – what’s actually different
Both materials are wood-based panels glued and pressed into 4×8 sheets. The differences:
Plywood:
- Made from thin layers (“plies”) of wood veneer cross-laminated for strength
- Common in residential construction from the 1950s through about 1990
- More expensive than OSB
- Generally heavier
- When exposed to moisture and then dried, plywood delaminates but retains most structural strength
- Holds nails very securely
OSB (Oriented Strand Board):
- Made from wood strands oriented in layers and bonded with adhesive
- Standard since the early 1990s – most Illinois homes built since then have OSB decking
- Less expensive than plywood
- Lighter
- When exposed to prolonged moisture, OSB swells and loses structural strength permanently
- Holds nails well but slightly less than plywood
For roof decking purposes, both materials are acceptable when properly installed and protected from moisture. The bigger variable is age and exposure history.
How decking fails
Roof decking has one job: stay dry. When it doesn’t, several failure modes appear:
Soft spots from chronic leaks. A failed pipe boot, flashing, or roof valley can leak slowly for years without showing up inside the house – the water gets absorbed by the decking. The wood gets soft, then rotted. Eventually the soft section can’t hold a nail or support foot traffic.
Edge rot at the eaves. When drip edge is missing or the gutter has been backing up for years, water runs down behind the gutter and into the lowest course of decking. The fascia board rots first, then the deck edge.
Ice dam damage. Repeated ice dam events at the eaves push water under the lowest shingles. Decking absorbs the water, freezes, expands, separates from the rafter beneath.
Condensation rot. Inadequate attic ventilation lets warm humid air rise into the attic and condense on the underside of cold roof decking. Over winters this can cause a steady drip pattern that rots a large area of decking from the underside.
Original construction issues. Some Illinois homes from the 1950s and earlier were built with 1×8 boards (“plank decking”) instead of plywood. Plank decking is fine structurally but doesn’t always create a flat surface for modern shingle installation. Plank gaps can also fail at fastener holds.
Inspecting decking – only possible during tear-off
This is one of the strongest arguments for tear-off over overlay: you can’t reliably inspect decking from on top of the shingles. The shingle covers the wood. Soft spots feel slightly different under foot but the assessment is much more accurate once the shingles come off.
During a Trill Roofing tear-off we walk the entire roof deck and probe with screwdrivers in suspect areas. Indicators of decking that needs replacement:
- Visible water staining (dark spots on the wood)
- Spongy feel when stepped on (compared to firm plywood)
- A screwdriver can be pushed into the surface without much force
- Separation between the deck panel and the rafter below
- Delamination of plywood layers visible at the edges
- Mold/mildew growth on the wood surface
Soft or rotted sections get marked with paint and replaced before the new underlayment goes down.
What decking replacement costs
Trill Roofing charges our cost on replaced decking – no markup on materials. Typical pricing:
- 4×8 sheet of 1/2″ OSB or plywood: approximately $40-$60 material (varies with current lumber prices)
- Installation labor: built into the replacement quote
- Total cost per sheet installed (replacement decking only): typically $60-$90
For a typical 2,000 sq ft roof with maybe 3-5 sheets of damaged decking found during tear-off, that’s a $200-$500 add to the project. For a roof with widespread decking failure (10+ sheets), it can be $1,000+. The estimate is finalized after tear-off when we can actually see what needs replacement.
The alternative – leaving soft decking and installing new shingles over it – guarantees future failure. New shingles nailed into soft wood don’t hold. Within a few years you’ll be paying to replace decking and re-shingle that area, on top of the cost you just paid.
Older Illinois homes – plank decking considerations
If your home was built before about 1960, you might have plank decking (1×8 or similar lumber boards) instead of plywood. Plank decking is structurally fine but has two roof-related quirks:
- Gaps between planks – sometimes wide enough that asphalt shingles can sag slightly into the gap. Not a structural problem but can create a slight “wavy” appearance.
- Fastener purchase – older shingle nail patterns assumed solid plywood. Plank decking sometimes has gaps that miss the nail.
For homes with plank decking, we usually recommend overlay-with-OSB during a roof replacement: leave the plank decking in place and add a 1/2″ OSB layer on top. This creates a flat, modern surface for the new shingles while preserving the original framing. Adds about $1.50-$2.50 per square foot to the project.
What to ask on a quote
If you’re getting roofing quotes, ask each contractor:
- How is decking replacement handled? At what cost per sheet?
- Is the cost “at cost” or marked up?
- Is the decking replacement allowance specified in the written estimate?
- Will you replace decking before installing shingles, or just install over whatever’s there?
If the answer is “we’ll let you know if any needs replacement” without any pricing structure, that’s not a real quote – it’s an opening to upsell during the project. Get the pricing structure in writing.
Schedule a free Trill Roofing inspection at /free-inspection/ for an honest assessment.
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