The Anatomy of a Roof: What's Actually Up There

Most people think of a roof as the part they can see — the tile or the shingles. But that's just the outer shell. The roof is really a system of layers working together, and when you understand the layers, you understand where your money is going and where a contractor can cut corners.

Start at the bottom. The decking is the wood surface nailed to your trusses — usually plywood or OSB. It's the structural base everything else attaches to, and on an older roof it's often where you find rot once the old roof comes off.

On top of the decking goes the underlayment. This is the layer that actually waterproofs your house. The tile or shingle takes the abuse from the sun and the wind, but the underlayment is what stops water. People skip right over it because they never see it, and that's a mistake.

Then comes the flashing — the metal that seals all the spots where the roof meets something else: walls, chimneys, vents, valleys. More on that in its own article, because flashing is where most roofs actually leak.

After that, the roof covering itself — the tile, shingle, or metal — and the finishing pieces: the ridge at the peak, the eaves and drip edge at the bottom edges, and the vents that let your attic breathe.

Here's why this matters when you're shopping. When a contractor hands you a price, you're not buying "a roof." You're buying decisions about every one of these layers — what underlayment, what flashing, whether they replace bad decking or roof right over it. The cheap bid and the good bid can look identical from the street and be completely different roofs underneath. Knowing the layers is how you ask the right questions.