What boards go under a flat roof?

Usually polyiso insulation boards, often tapered for drainage, with a thin, dense cover board on top that protects the insulation and gives the membrane a firm surface.

What is a cover board on a flat roof?

A thin, dense board installed over the insulation and under the membrane. It protects the insulation from foot traffic and hail and gives the membrane an even substrate.

What insulation is used on flat roofs?

Most commonly polyisocyanurate (polyiso), because it gives the most R-value per inch. Tapered polyiso also creates the slope that moves water to the drains.

Where do flat roofs usually leak?

At the edges, curbs, penetrations, drains and transitions, rather than in the open field of the membrane.

What Are the Layers of a Flat Roof?

Short answer: From the bottom up: the structural deck, a vapour retarder where the building needs one, insulation boards, a cover board, and the membrane, tied together by edge metal, flashings and drains. The “roofing boards” on a flat roof are usually two things: polyiso insulation boards, often tapered to give the roof slope to its drains, and a thin, dense cover board on top that protects the insulation from foot traffic and hail and gives the membrane a firm, even surface.

The layers, bottom to top

Structural deck. Steel deck on most modern commercial buildings; structural concrete, lightweight insulating concrete, gypsum or wood plank on others. The deck decides how everything above it can be fastened.

Vapour retarder. Only where the building’s interior conditions call for one, so moist indoor air does not condense inside the insulation.

Insulation. Usually polyisocyanurate (polyiso) boards, the dominant material because it gives the most R-value per inch. The energy code sets the minimum on a re-roof. Tapered polyiso is how a genuinely flat structure gets slope to its drains.

Cover board. A thin, dense board over the insulation. It protects the insulation from foot traffic and hail, spreads loads, and gives the membrane a flat, firm substrate.

Membrane. TPO, EPDM, PVC, modified bitumen or built-up roofing: the waterproof layer.

Edges and details. Edge metal, coping, curbs, flashings and drains. This is where most leaks start, not in the open field of the membrane.

How the layers are held down

Each layer is either mechanically fastened through to the deck or adhered to the layer below, and some older roofs are loose-laid under stone ballast. Corners and perimeters see much higher wind uplift than the middle of the roof, so attachment has to be denser there. The deck sets the options: steel takes fasteners, structural concrete generally needs adhered systems.

Why it matters when you replace one

Wet insulation under a membrane keeps rotting the deck and rusting fasteners, so a contractor should take core cuts and run a moisture survey before choosing between a recover and a full tear-off. A proposal that prices only the membrane by the square foot leaves out the layers that decide how the roof performs. See whether a new flat roof can go over the old one.

How we handle commercial roofs

We core, survey and photograph before we recommend anything, then specify the full assembly, deck to edge metal, for TPO, EPDM, PVC and modified bitumen roofs across Maryland and Virginia.

MHIC #111971 in Maryland, Virginia Class A #2705183185, family-owned since 2005.

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We inspect across Maryland — Montgomery, Prince George's, Howard, Anne Arundel, Frederick, Carroll and Baltimore counties. Free inspection, photo report within 24 hours, and the report is yours to keep whether or not you hire us.