Spaced Decking Under Older Roofs

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Homes built in the first half of the twentieth century, and quite a few built afterward across the rural Midwest, were often roofed with spaced decking rather than the solid plywood or OSB sheathing familiar today. Spaced decking, sometimes called skip sheathing or board sheathing with gaps, consists of individual wood boards, typically one by four or one by six dimension lumber, nailed to the rafters with a visible gap between each board rather than a continuous surface. It was the standard method for supporting wood shakes and shingles, which needed airflow behind them to dry out after rain or snowmelt, and it remains common enough that appraisers working in this region encounter it regularly during roof inspections.

When a home with spaced decking is reroofed with modern asphalt shingles, the gaps present a practical problem, since shingle nails need continuous, solid backing to hold properly and manufacturers generally require solid sheathing for warranty compliance. Many reroofing projects address this by installing new plywood or OSB directly over the existing spaced boards, effectively creating a solid deck without removing the original structure. Others involve furring or shimming to close gaps, and some jurisdictions require the added sheathing to meet current code regardless of what existed before. The approach chosen affects roof assembly thickness, which in turn can affect flashing details, drip edge extension, and how ice and water shield membrane performs at eaves. Building departments in some Minnesota municipalities require this transition to be inspected before the new roofing material is installed, since it affects long-term deck integrity as much as it affects the shingles themselves.

Spaced decking creates specific considerations during a damage inspection. Nail penetration through overlaid sheathing into the original boards can miss solid wood if the boards have shrunk, cupped, or shifted over decades, leading to localized areas of reduced fastener holding strength that may not be visible from the exterior. Attic-side inspection, where accessible, often reveals more about deck condition and spacing than a surface walk of the shingles. Ventilation is another factor, since older homes with spaced decking sometimes lack the continuous soffit and ridge ventilation that current code favors, which can influence ice dam formation along Minnesota eaves during freeze-thaw cycles in January and February. Where accessible attic space allows a visual check, inspectors sometimes look for daylight visible between boards, staining patterns that suggest past moisture intrusion, or fastener corrosion that can develop over decades of humidity cycling.

None of this makes an older home with spaced decking deficient by modern standards; it reflects the building practices of its era, and many such roofs have performed for generations. What it does mean is that documentation during an inspection benefits from noting decking type, spacing, board condition, and whether solid sheathing has been added at some point in the home’s history, since these details inform repair feasibility and cost estimating regardless of who is examining the roof or on whose behalf.

Questions about whether existing conditions such as inadequate ventilation, deck spacing, or prior sheathing upgrades affect coverage under a specific policy are matters for the policy language and the parties involved, not for the person conducting a physical inspection. An appraiser’s function is to observe and value the property as it exists, working from a scope that others have determined.

This article is general education about how the appraisal process commonly works. It is not legal advice, and specific procedures can vary by state and policy.

Russ Lis works nationwide as a property insurance appraiser and umpire, based in Minnesota. Contact Appraisal Resolution.