How to Ventilate a Low-Slope or Cathedral Roof
No attic, no stack effect, and a tighter space. Channels, the right ratio and, sometimes, an unvented assembly.
- Professional
- 1 to 3 days
- Updated
The short answer
A low-slope or cathedral roof has no open attic, so each rafter bay needs its own vent channel between the insulation and the roof deck, running from the eave to the ridge. The BC Building Code asks for vent area of at least 1 in 150 of the insulated ceiling area where slope is under 1 in 6 or the roof is built with joists. Where a channel cannot work, an unvented assembly needs engineered design.
A conventional attic ventilates itself with a large open space and the stack effect. A cathedral ceiling or a low-slope roof has neither. The insulation sits just below the roof deck, in a bay barely deeper than the rafter, so there is very little room for air, little height difference to drive flow and little margin for error. Moisture that gets into the bay has nowhere to go but the roof deck, which is why these roofs are more prone to condensation, frost and rot than open-attic roofs.
There are two ways to deal with it. A vented assembly keeps a clear air channel, commonly at least an inch or two deep, between the insulation and the roof deck in every bay, fed at the eave and exhausted at the ridge or high point. An unvented assembly has no air channel at all and relies on air-impermeable insulation and careful moisture design. The first is simple to understand but hard to get right, and the second needs engineered detail.
For the basics, read how attic ventilation works. For flat roofs, see also how to choose a flat roof membrane and how to install tapered insulation.
Tools
- Tape measure
- Staple gun
- Utility knife
- Drill and driver
- Hole saw for roof vents
- Level
- Caulking gun
Materials
- Rigid vent channels or baffles, sized to keep a clear airspace in each bay
- Insulation sized to leave the channel depth
- Continuous eave intake vents and a ridge or high exhaust vent
- Air and vapour control layer and sealing tape
- Low-slope rated vent products, where the roof has no ridge
Safety first
- Work tied off. WorkSafeBC requires fall protection at 3 metres (about 10 feet) and wherever a fall could cause serious injury. Low-slope roofs still have edges and openings.
- Do not cut through rafters or joists to fit vents. Structural changes need proper design and, often, an engineer.
- Wear a dust mask and eye protection when working with insulation and cutting sheathing.
Step by Step
Step 1: Identify the roof assembly
Look at how the roof is built. A cathedral ceiling has rafters that also form the ceiling. A flat or low-slope roof may have joists with insulation between them, or it may have insulation on top of the deck under the roofing, which is a different system. Check at an eave, at a vent or from a hole in the ceiling if you need to.
The assembly decides the method. Insulation on top of the deck with membrane over it needs no vent channel. Insulation between the rafters or joists needs ventilation or an engineered unvented design.
Step 2: Know the ratio the code asks for
The BC Building Code asks for vent area of not less than 1/150 of the insulated ceiling area where the roof slope is less than 1 in 6, or where the roof is built with roof joists. Confirm the wording in the current code edition and with your local building department.
Work out the net free area as shown in how to calculate attic ventilation, using 150 instead of 300. Split it between eave intake and high exhaust.
Step 3: Keep a clear channel above the insulation in each bay
Install rigid vent channels stapled to the underside of the deck to hold the space open, and fit insulation below them. The BC Building Code calls for at least 63 mm (about 2-1/2 inches) between the top of the insulation and the underside of the sheathing, and deeper is better for longer runs. The channel must not be blocked by bridging, blocking or insulation.
This leaves less room for insulation. Cathedral bays are often too shallow for the required R-value plus a channel, so extra insulation may be added below the rafters or as a layer under the ceiling finish, or an unvented design is used. Do not compress insulation to make room.
Watch out: Do not fill the channel with insulation to reach a target R-value. A blocked channel traps moisture against the roof deck.
Step 4: Provide continuous intake at the eave
Every bay needs its own open inlet. Continuous soffit vents are better than individual vents, since they feed every bay. See how to add soffit intake vents.
Where the rafter tails and soffits are closed, the intake may be at the fascia or a special eave vent, see how to install soffit panels.
Step 5: Provide exhaust at the high point, one type only
On a cathedral roof, a continuous ridge vent gives the best exhaust. Each bay must open into the ridge, and in shed or single-slope roofs, a high-side vent at the upper edge is used instead.
Low-slope roofs have little height difference and weak stack effect. Use vent products rated for low slopes, and do not mix exhaust types. See why you should not mix roof exhaust vents. Ventilation on very low slopes works poorly, so the air barrier below matters even more.
Step 6: Install a continuous air and vapour control layer on the warm side
In a cathedral roof, there is no attic to absorb leaks, so every gap at a recessed light, penetration or wall top sends moisture straight to the deck. Tape and seal all seams, penetrations and edges.
Avoid recessed lights that are not airtight and insulation-contact rated in these ceilings. Every hole is a moisture path.
Step 7: Consider an unvented assembly where a channel will not fit
An unvented roof has no air channel. One approach is air-impermeable insulation, such as closed-cell spray foam, applied against the underside of the deck. Another is continuous rigid insulation above the deck, under the roofing. Both depend on the right thickness and vapour control for the climate. This is an engineered detail, not a DIY substitution.
Check with the building department and with the insulation manufacturer. The requirements vary with the roofing type, the shingle warranty and the building code edition. Some shingle manufacturers limit warranty on unvented roofs, so ask before choosing.
Step 8: Check for moisture after the first winter
Because the space is tight and hard to see, problems show up as stains or drips on the ceiling. Check that eave vents and the ridge are clear of debris and snow. See how to fix attic mold caused by poor ventilation if growth appears.
When Ventilation Is Not Possible
Some roofs simply cannot carry a vent channel: complicated cut-up roofs, bays that are closed off, shallow rafters that leave no room for insulation and a channel, and flat roofs with no high point. In those cases, the choices are to change the assembly, by adding insulation above the deck, or to design an unvented assembly. Either option should be designed, not improvised.
Adding ventilation after the fact to a sealed, unvented roof can make things worse. Do not cut vents into a roof that was designed as unvented.
Snow and Fraser Valley Conditions
Wet snow and long damp winters make low-slope and cathedral roofs prone to condensation and ice dams. Snow covering the ridge or eave can block vents, and in the canyon and near Hope, deep snow matters. Keep vents clear and consider higher-set vents in heavy snow areas. See ice dams.
Common Mistakes
- Filling the channel with insulation. The deck stays cold and wet with no airflow.
- No air barrier at the ceiling. Warm moist air reaches the deck directly.
- Blocked bays. Bridging or blocking cuts the channel off from the ridge.
- Mixing exhaust vent types. Air short circuits.
- Adding vents to an unvented assembly. It breaks the design.
- Using the 1 in 300 ratio. Low-slope and joist roofs need 1 in 150 under the BC Building Code.
When to Call a Professional
These roofs are hard to get right and expensive to fix when wrong. Have a roofer and, for unvented designs or structural changes, a building envelope professional or engineer involved. Check permit requirements with your municipality.
If there is staining on the ceiling, a musty smell or sagging, get it assessed quickly, since rot can develop in a closed bay without being seen.
Fraser Roofing does free roof inspections across the Fraser Valley, from Hope to Abbotsford. See our Roof Replacement service or the towns we cover.
How to Ventilate a Low-Slope or Cathedral Roof: Questions
How do you ventilate a cathedral ceiling?
Keep a clear air channel, at least 63 mm (about 2-1/2 inches) deep under the BC Building Code, between the insulation and the roof deck in every rafter bay, running from continuous soffit intake at the eave to a ridge vent at the top. Use rigid vent channels to hold the space, and seal the ceiling side with an air and vapour barrier.
How much ventilation does a low-slope roof need?
The BC Building Code asks for unobstructed vent area of not less than 1 in 150 of the insulated ceiling area where the roof slope is less than 1 in 6 or the roof is built with roof joists. That is double the 1 in 300 used for pitched roofs. Split it between intake low and exhaust high.
Can a flat roof be ventilated?
Sometimes, with continuous channels between joists and vents at opposite edges, but flat roofs have little stack effect so ventilation is weak. Many flat roofs are built with the insulation above the deck, so no ventilation is needed. Adding vents to a roof designed as unvented can cause damage.
What is an unvented roof assembly?
It is a roof with no air channel, where air-impermeable insulation, such as spray foam against the deck, or continuous rigid insulation above the deck, controls condensation. It needs careful design for the climate and usually building department approval. It is not something to add to a roof built with vents.
Why do cathedral ceilings have moisture problems?
There is no attic to buffer the roof, the bays are shallow and the insulation sits right under the deck, so any warm moist air that leaks up condenses on the cold sheathing with nowhere to go. Air leaks, blocked channels and too little insulation depth are common causes, and problems often stay hidden until stains appear.
Related Guides
All Ventilation and Attics guides|The full How-To Library
This guide is general education. Always follow the product manufacturer's installation instructions, the BC Building Code and WorkSafeBC requirements, which take priority over anything written here.
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