Built Right Detail 14: Vaulted & Flat Roofs

A builder’s net zero envelope course by Xerox Builders · October 2026

In a vaulted or flat roof there is no attic to help, so the roof deck has to stay warm and dry on its own. This Detail compares vented, above-deck and split insulation, explains where moisture problems start on the BC coast, and covers low-slope membranes and what the code expects.

No attic, no second chance

In a vaulted (cathedral) or flat roof the insulation sits right under the roof deck. There is no attic to buffer temperature swings, no space for the deck to dry into, and no way to crawl in and look. On the BC coast, long wet winters and cold, clear nights put the roof deck at risk, and problems often show up years later as rot or mould. The question to ask of every roof design is simple: will the roof deck stay warm and dry all winter?

Where moisture problems start

• Indoor air leakage. Warm, moist air leaks through the ceiling and condenses on the cold underside of the deck. Air leakage moves far more moisture than vapour diffusion.
• Night sky cooling. On clear nights the roof radiates heat to the sky and the sheathing can drop below the dew point of the air around it. RDH flags this as a real risk in Vancouver and other Pacific Northwest cities.
• Built-in water. Rain-soaked framing or sheathing closed in before it dries.
• Exhaust and ducts. Fans that end inside the roof or leak humid air into it.

RDH monitored test roofs at its Vancouver building from 2012 to 2014. Sheathing moisture content reached 20% to 30% through the rainy months, above the roughly 20% level where mould becomes a concern, and mould appeared on every test roof in the first year, first on north-facing slopes.

Three common build-ups

Spray foam under the deck is a fourth approach. It air seals and insulates in one step, but roofs with insulation under the deck and no vent space often need extra approval, so settle the build-up with your designer before framing. Keep in mind that joists bridge cavity insulation: RDH’s worked example of R-35 insulation between joists comes out at about R-28 effective once the framing is counted.

Vented vaulted roofs

A baffle holds the air space open from the soffit intake to the ridge exhaust in every rafter bay. Skylights, hips, valleys and dormers can dead-end bays, so check the vent path for each bay on the drawings. Under the insulation, the ceiling air barrier has to be very tight, because a vent space will not save a roof that is fed moist indoor air.

Low-slope membrane roofs

BC Housing’s airtightness guide notes that an exterior air barrier is usually the simplest choice on low-slope roofs: a membrane on the deck, before the insulation, has few intersections and penetrations. Above it, slope the roof to drains, often with tapered insulation, so water never ponds. Turn the membrane up every wall and curb and cover the top with a metal cap or counterflashing. Plan drains and overflows before framing, and keep other trades off the finished membrane.

What the code says

The BC Building Code (2018 text, Section 9.19) requires that where insulation is installed between a ceiling and the underside of the roof sheathing, a ventilated space be provided above it, unless it can be shown to be unnecessary. Vent area is at least 1/300 of the insulated ceiling area in most cases, and at least 1/150 for roofs sloped less than 1 in 6 or built with roof joists, with vents split between the top and bottom of the space. There must be at least 63 mm of space between the top of the insulation and the sheathing. A roof with all of its insulation above the deck has no insulated space under the sheathing, so it is not the case this rule is written for; confirm how your building official treats it. Split, spray-foam and other cavity-insulated roofs without a vent space therefore often go through an alternative solution, and some cities have restricted those: City of Vancouver Bulletin 2017-004-BU, effective September 1, 2017, stopped accepting alternative solutions for unvented roof and deck assemblies (Vancouver now has a 2025 building by-law, so check whether that position still applies). Confirm the current BC Building Code edition and your city’s position before you design.

Site checklist

• Roof build-up and vent path confirmed with the designer
• Approval in place for any unvented roof
• Framing and sheathing moisture-checked before closing in
• Baffles open from soffit to ridge in every vented bay
• Air and vapour membrane on the deck sealed at edges and penetrations
• Rigid insulation joints staggered, cover board installed
• Split roof built to the modelled ratio, no extra cavity insulation
• Slope to drains checked, no ponding spots
• Membrane turned up walls and curbs, cap or counterflashing installed
• Fans ducted to the outside, ceiling air barrier tested

Test yourself

Try to answer each question, then click it to check.

Next: Detail 15, Roof-to-Wall Transitions. See also our Roofing Systems & Ice Dam Prevention Field Guide.