What’s Actually Under Your Roof? Decking by St. Louis Home Era

The most common source of an unexpected bill during a roof replacement is decking, the structural sheathing your shingles are nailed to. Nobody can see it until the old roof comes off. What is under there depends heavily on when your home was built, and St. Louis has housing stock spanning more than a century. Knowing what to expect for your home’s era lets you budget realistically and tells you whether a contractor’s estimate is being honest with you before the tear-off starts.

What Decking Is Under Homes of Each Era?

Plank decking is common in St. Louis city neighborhoods and older county municipalities. Individual boards were laid with small gaps for expansion. The wood is often excellent old-growth material and can be in fine condition after a century. The complication is the gaps. Modern shingles need consistent nailing surface, and where gaps are too wide or boards have cupped, an overlay sheet of plywood may be required across the whole roof rather than a few board swaps.

Plywood became standard mid-century and holds up well. Failures are usually localized, concentrated around plumbing penetrations, chimneys, valleys, and eaves where water has been working at a specific point for years.

OSB is the current standard and performs fine dry. Its weakness is water. Once saturated, OSB swells at the edges and loses structural integrity faster than plywood does, so a long-undetected leak on an OSB deck often means more replacement than the same leak on plywood.

How Much Does Decking Replacement Cost?

Decking is typically priced per sheet or per board as a separate line item, because no contractor can quote it accurately in advance. A reputable estimate will state the per-unit price up front so you know the rate before work begins.

Ask three questions of any estimate: what is the per-sheet or per-board price, how many units are included in the base quote, and will you be shown the damaged material before it is removed. The last one matters. Decking replacement is the easiest place for a dishonest contractor to inflate a bill, because the evidence goes in the dumpster.

Why Do Some Estimates Not Mention Decking At All?

That is a warning sign rather than a convenience. A contractor who does not address decking is either planning to surprise you mid-job with an unpriced change order, or planning to shingle over compromised material and leave. Both outcomes are worse than a slightly higher quote that names the rate honestly.

This is also one of the practical arguments against a layover, where new shingles are installed over the existing layer without a tear-off. A layover leaves the decking unseen and unaddressed, which is fine until it is not. St. Louis County code limits roofs to two layers, and the second layer hides exactly the information you need.

What Can I Learn Before the Tear-Off?

More than you might expect, from inside the attic with a flashlight. Look at the underside of the deck. Plank decking is visibly individual boards with gaps. Plywood and OSB appear as sheets, and OSB has a distinctive flaked appearance. Look for dark staining, which indicates past or present water intrusion. Look for daylight, which means active openings. Press gently on suspect areas for softness, and check around every plumbing vent and chimney, since those are where problems concentrate.

Newer decking replaced during a previous roof job is usually obvious, appearing lighter in color than the surrounding original material.

Family First Exteriors states decking pricing as a line item on every estimate and shows homeowners the damaged material before it is removed. Request an estimate.

Frequently Asked Questions