How to Build a Deck over an Insulated, Heated Space – GreenBuildingAdvisor

By Chuck Bickford | August 11, 2025

Ah, flat roofs. Maybe a design made without a call to 1-800-FUNCTION? Roofs were made to shed water, after all, and most so-called flat roofs actually have a slight pitch to assist with drainage. But that slight pitch must still rely on the surface material, not gravity, to keep water out.  Yes, even back in the bad old days of tar paper, asphalt, and gravel, or soldered metal sheets, flat roofs held up. Until they didn’t. These days, of course, we have materials such as EPDM (ethylene propylene diene terpolymer), or TPO (thermoplastic polyolefin) that are better suited for the job. But there are still issues, like when you have to pierce the membrane with posts that support a deck railing. And then there’s the roof structure itself. Flat roofs are not an uncommon question for GBAs Q&A Spotlight; here is one example from a few years ago. Adding a deck to the flat roof is a new twist.

A member named “KBurnham” recently posted that he’s designing an open deck and railing on top of a new roof for the local fire hall in Cariboo, British Columbia, Canada. The region is climate zone 5, on the snowy side of things. He wants answers to questions such as “Would the roof assembly be vented?” and  “How might the roof be insulated?”

The topic for this Spotlight: What’s this builder’s best path to a durable, insulated, watertight flat roof that will support a deck?

Vents and this flat roof don’t go together

Despite his terse post, there are lots of questions lurking, something member “jhwehrli” was quick to point out. What’s the structure below? How is the roof framed?  Other questions include “What about vents?” A vented roof needs convection to work. In…