What’s Under Your Shingles? Roof Layers
Shingles are only the top layer of your roof. Learn what’s underneath, what goes wrong, and how The Great House repairs roofs in Northern Virginia.
ROOFING
10/25/20263 min read
ASK MASON · ROOF
What’s Under Your Shingles? The Layers That Keep Your Roof Dry
Mason shows the layers under your shingles, the warning signs, and how we fix them.
Your shingles are only the top layer of your roof.
Watch Mason’s video, then read what’s underneath, what goes wrong when shingles fail, and how we fix it.
IN THIS GUIDE
01
What Mason Checks
The Layers Under Your Shingles
WOOD DECK
Plywood nailed to the rafters.
UNDERLAYMENT
A waterproof layer, with ice-and-water shield at edges and valleys.
SHINGLES
Overlapped so water runs down and off.
First comes the wood deck: plywood boards nailed to the rafters.
Then a waterproof underlayment, with ice-and-water shield at the edges and in the valleys.
Shingles go on top, overlapped so water always runs down and off.
Mason’s diagram: the wood deck, underlayment and ice-and-water shield
When the top layer goes, the layers underneath weren’t built to take the weather alone.
02
Warning Signs
Signs Your Shingles Are Failing
MISSING SHINGLES
Missing, curled or cracked shingles.
GRANULES
Gritty granules in your gutters.
SAGGING LINE
A roof line that dips or sags.
When shingles blow off, crack or wear thin, rain gets through the underlayment and reaches the wood deck.
The plywood soaks up water, swells and rots. The roof line starts to sag, and leaks show up inside.
Check this today
From the ground, scan the roof with binoculars for missing or curled shingles. Check the gutters and downspout outlets for piles of granules.
03
How We Fix It
A Repair Now, or a New Roof Later
We remove the damaged shingles and inspect the deck. We replace rotted plywood, install new underlayment and ice-and-water shield, and lay new shingles, nailed the way the manufacturer requires.
Caught early vs. caught late
Caught early, it’s a repair. Caught late, it’s a whole new roof.
In Northern Virginia
The DC area averages about 42 inches of rain a year (NWS, 1991–2020). In Fairfax County, replacing shingles doesn’t need a permit, but replacing 256 square feet or more of roof plywood does. We fixed this on our Landview Dr roof in Manassas.
Planning a bigger project?
Need a new roof? We inspect and replace damaged decking before the new shingles go on. See prices and real projects on our roofing page.
The fix: new shingles nailed right, with ice-and-water shield at the edge
ASK MASON · ROOF
Want to know another problem?
SPOTTED ONE OF THESE SIGNS?
We’ll check it before it turns into a big repair
Free in-home visit from a Class A licensed team. We check the problem, your roof and your attic, then give you a clear written plan.
ABOUT THE AUTHOR
The Great House team
Written by the team at The Great House LLC, a Class A licensed builder and remodeler in McLean, VA (License #2705197237). We have renovated 30+ Northern Virginia homes since 2022.
PLANNING A PROJECT?
Our roofing service
See prices, real photos and how we work: our roofing page. Real projects: our project gallery.
Serving McLean, Vienna, Great Falls, Fairfax, all of Northern Virginia.