A roof decking inspection is the part of a roof inspection that establishes whether the structural deck under the roof is sound. It cannot be done by looking at the deck, because the roof is on top of it, so it is done by assembling indirect evidence: what the roof does underfoot, what the underside shows from inside the building, and what moisture testing says about the layers in between.
It matters because the deck is the one layer whose condition changes the whole decision. A tired membrane over a sound deck is a reroof. The same membrane over a corroded deck is a structural job with a reroof attached, and the difference is large enough that nobody should discover it after the old roof has come off.
The three sources of evidence
| Where it comes from | What it shows | What it cannot show |
|---|---|---|
| Walking the roof | Deflection, soft areas, movement, fastener pull out | The cause of any of it |
| Inside the building | Corrosion, staining, deflection, light through seams | Anything where a ceiling blocks the view |
| Moisture testing | Where water is sitting above the deck, and how much | The deck’s remaining strength |
| Core samples | The actual condition of every layer at that point | Anything more than a few inches away |
The method is cumulative. A soft area underfoot, rust streaking below it and a wet reading on the scan is a finding. Any one of the three alone is a reason to investigate. What a moisture survey detects covers how the middle row is actually carried out.
What gets recorded from above
- Deflection between structural supports, and whether it is uniform or local
- Soft or spongy areas, marked and measured rather than described
- Movement or noise underfoot, which can indicate a loose or failing deck
- Fastener performance: plates pulling through, screws standing proud, backed out fixings
- Standing water, because the deck below it is the one most at risk
- Previous repairs, which mark where somebody already found something
What gets recorded from below
The interior walk is the most informative and the most often skipped part of a deck inspection. Wherever the soffit is exposed, it shows directly what has to be inferred from above.
Rust and staining trace where water has run, and the pattern usually points at the detail above that let it in. Deflection is easier to judge against a straight line of purlins than from standing on the roof. Fastener penetrations visible from underneath show both the attachment pattern and any that are leaking. In a steel deck, daylight through a side lap means an opening somewhere above it.
What happens during a roof inspection covers how the interior and exterior halves of the visit fit together, and the inspection preparation checklist covers the access arrangements that make the interior walk possible.
Deck types, and what each one does when it fails
Steel. Corrodes, typically at the flutes where water collects and at fastener penetrations. Loss of section reduces both strength and the ability to hold a fastener, which is why a corroded deck often shows up first as attachment failure rather than as a structural concern.
Wood. Rots, delaminates where it is a panel product, and stops holding nails or screws. Older boarded decks also shrink and open gaps.
Concrete. Rarely a structural problem, but holds moisture for a long time, which defeats adhesives and vapor retarders applied to it. Testing for moisture content before anything is adhered is standard.
Gypsum and cementitious decks. Vulnerable to any sustained wetting and found on older buildings. A wet area here is a serious finding rather than a maintenance item.
What the report should say
A deck finding has to be written so that somebody pricing the work can act on it: the location on a plan, the extent in area, the evidence behind the conclusion, and a clear statement of what is known against what is inferred. Roof condition assessment covers how that language works across the whole document.
Where to go next
If the roof is heading for replacement, the report review checklist covers how to read the deck section and the allowance that should follow from it. If the question is whether water has already reached the deck, an infrared roof inspection is usually the quickest way to find out across a large area.
Common questions
Can a roof deck be inspected without opening the roof up?
Largely, yes, by combining three sources of evidence: how the roof feels and behaves underfoot, what can be seen of the underside from inside the building, and what moisture testing says about the layers above. None is conclusive alone, and together they are usually enough.
- Deflection and soft areas underfoot indicate a problem below
- The underside is visible from inside wherever the deck is exposed
- Moisture testing shows where water has been sitting on the deck
- Fastener pull out and loose plates point at a failing substrate
- A core cut confirms it, and should be the last step not the first
A core sample settles the question definitively and is worth taking where the indirect evidence is ambiguous or the number at stake is large. On a purchase or a capital decision one or two cores are cheap insurance. On a routine condition walk they are usually unnecessary, because the question being answered is whether anything needs investigating rather than exactly how bad it is.
What does a soft or spongy area underfoot mean?
It means something below the membrane has lost strength, and it is one of the few findings on a roof that justifies immediate attention. The usual causes are saturated insulation, a deteriorated cover board, or a deck that has corroded or rotted under long standing water.
- Wet insulation compresses and no longer supports the membrane
- A failed cover board feels similar and is less serious
- Corroded steel deck or rotted wood is the serious version
- The extent is mapped by testing, not by walking
- Soft areas are a safety matter as well as a condition finding
A competent inspector marks the area, notes the extent, keeps off it and recommends testing rather than guessing at the cause. The three possibilities carry very different costs, and distinguishing between them is exactly what a moisture survey and a core cut are for. Anybody who names the cause confidently from a walk alone is telling you what they expect rather than what they found.
Which roof decks fail first, and why?
Steel decks corrode, usually from above where water has been sitting on them, and from below in buildings with high interior humidity. Wood decks rot and lose fastener grip. Concrete rarely fails structurally but holds moisture that defeats anything adhered to it.
- Steel corrodes at the flutes and at fastener penetrations
- Wood decks rot, delaminate and stop holding fasteners
- Gypsum and cementitious decks are vulnerable to any wetting
- Concrete retains moisture and needs testing before adhesion
- Every deck type fails faster under standing water
The common factor is water that stayed. A deck under a membrane that drains properly tends to outlast several roof coverings. A deck under a bay that ponds can be the reason the roof is replaced rather than recovered. That is why drainage findings and deck findings in a report should be read together, and why a report that records one without the other is incomplete.
What can be seen of the deck from inside the building?
More than most owners expect, wherever the underside is exposed. In a warehouse, a plant room or any space with no suspended ceiling, the deck soffit is visible along its whole length and tells you about corrosion, staining, deflection and fastener penetrations directly.
- Rust streaking and staining mark where water has been
- Fastener points visible from below show attachment and leakage
- Deflection between supports is easier to judge from underneath
- Light through a steel deck seam means a hole somewhere above
- Suspended ceilings hide all of this, so access has to be arranged
This is why the interior walk is part of a roof inspection rather than an add on, and why it is the part most often skipped. Arrange access to plant rooms, back of house areas and anywhere with an exposed soffit before the visit. Where ceilings cover everything, say so in advance so the inspector can plan the testing that has to substitute for looking.
When does decking get added to a reroof scope?
It belongs in the scope as a priced allowance on almost every reroof, because the deck can only be fully seen once the old roof is off. What varies is how large the allowance should be, and that is what the inspection is for.
- Replacement decking is priced per unit area as an allowance
- The inspection sets how big that allowance should be
- No allowance at all is a bid that will change later
- Agree in advance who decides what gets replaced
- Ask for the deck to be photographed once it is exposed
A bid with no deck allowance is not cheaper, it is simply incomplete, and the conversation about cost will happen after the roof is open and the building has no cover. Settle the unit rate and the authorization process before work starts, so that a finding becomes a priced variation rather than a negotiation conducted under time pressure with weather on the way.