Ridge Caps and Vented Ridge Assemblies on a Metal Roof
The ridge is the highest point on the roof and the place warm air leaves the attic. It has to shed water, exhaust air, resist blowing snow, and let the panels move.
The ridge does two jobs that pull in opposite directions. It has to close the top of the roof against rain and wind-driven snow, and on most houses it also has to be the exhaust point for the attic ventilation. Building an opening that lets air out and keeps weather out is the whole problem, and in a climate with blowing snow it is harder than it looks.
What Happens at the Top of the Panel
Before any cap goes on, the panel itself has to be terminated properly. On a standing seam roof, the top of each panel gets an upturned leg — a bend at the panel end, made with a hand seamer or a hemming tool, typically an inch or two high. That leg is what stops water running up the panel under wind pressure and over the top edge into the assembly. A panel simply cut off square at the ridge, with the cap laid over it, has no such stop, and wind-driven rain will walk right up the pan.
Behind that leg sits a Z-closure or offset cleat, fastened to the deck, running the length of the ridge. The ridge cap engages that closure. This is what allows the cap to be secured without driving screws through the face of the panel — which would pin a panel that is supposed to be free to move.
The Cap Itself
The ridge cap is formed sheet, folded to the roof angle, wide enough to cover the closure and the panel ends on both sides with a generous margin. Individual lengths lap — commonly around six inches — with butyl tape in the lap, and the lap runs so that the prevailing weather does not drive into it. The ends of the ridge, where the cap runs out at the gable, get a formed end cap or a folded closure; an open cap end is an invitation to wind-driven rain, snow, wasps and birds.
- ✓Match the cap material, gauge and finish to the panels. A lighter-gauge cap in a slightly different colour is visible from the ground for the life of the roof.
- ✓Lap uphill over downhill on hips; on a horizontal ridge, lap with the prevailing wind at your back.
- ✓Butyl in every lap, compressed by the stitch fasteners — not a bead of caulk smeared over the outside afterwards.
- ✓Fasten to the closure and to the panel high ribs where the system calls for it, never through the flat pan of a floating panel.
- ✓On exposed-fastener panels, the cap sits on profile-matched foam closures with butyl on both faces of the closure.
Vented Ridge Assemblies
To vent at the ridge, a slot is cut through the sheathing on each side of the ridge board, leaving the structural ridge intact and stopping short of the ends of the roof. A profiled vent material — a purpose-made mesh or baffled plastic product sized for the panel — sits over the slot, and the ridge cap goes over that. The vent holds the cap up off the roof, keeps the opening clear, and provides the resistance that stops wind blowing straight through.
The important specification is net free area: the actual open area the vent provides per running foot, after accounting for the mesh or baffle. It is published for every vent product and it is the number that gets balanced against the intake. A vent that looks generous can have modest net free area once the filter material is accounted for.
A ridge vent without adequate soffit intake is worse than no ridge vent. With no low intake, the ridge cannot draw fresh air up the underside of the deck. Instead it pulls conditioned air out of the house through every ceiling penetration — potlights, bath fans, the attic hatch — which increases heating costs and carries moisture into the attic, where it condenses on the cold deck. Intake first, exhaust second.
Balancing Intake and Exhaust
The rule of thumb the trade works to is a total ventilation area of roughly one square foot of net free area for every 300 square feet of insulated ceiling area, split approximately evenly between low intake and high exhaust, with a more generous ratio used where a vapour barrier is absent or the split is unbalanced. Your local building department sets the actual requirement, and the specifics of an assembly can change the answer. What holds regardless is the split: exhaust at the ridge only works if there is at least as much intake at the eaves.
In practice, the intake side is where most Ontario houses fall down. Soffit vents get painted over, blocked by blown-in insulation that has drifted into the eave, or were never adequate to begin with. Installing baffles at the eave to hold the insulation back and keep a clear air path is part of doing a ridge vent properly, and it happens during the re-roof when the eave is accessible.
Blowing Snow
This is the Ontario-specific failure. In a driving snowstorm, fine snow behaves like a fluid and is carried through any opening the airstream can reach. A poorly baffled ridge vent will pass snow into the attic, where it melts on the next mild day and shows up as a ceiling stain that everyone assumes is a roof leak. The mitigations are a vent with an external wind baffle that deflects the airstream up and over the opening, an internal filter or mesh, and the vent stopping well short of the gable ends where wind pressure is highest.
Do Not Mix Exhaust Types
A house with a continuous ridge vent should not also have gable louvres or a powered attic fan on the same attic space. Multiple exhaust points at different heights short-circuit the intended airflow: the ridge vent nearest a gable louvre will draw air in from the louvre rather than up from the soffits, so the deck over the far side of the attic gets no flow at all. During a metal re-roof is the natural time to decide on one strategy and close the others off.
Frequently Asked Questions
Does a metal roof need a ridge vent?
If the assembly is a vented attic or a vented cathedral ceiling, yes — the roof needs a high exhaust point, and the ridge is the best one. If it is a designed unvented assembly with insulation in contact with the deck and the vapour control handled correctly, it does not, and cutting a ridge slot in that assembly would be a mistake.
Will snow blow in through a metal ridge vent?
It can, if the vent is not designed for it. Use a vent product with an external wind baffle and a filter, terminate it short of the gable ends, and make sure it is the right profile for the panel so there are no unintended gaps under the cap.
Can I use a shingle-style rolled ridge vent under a metal cap?
Only if the product is rated for use under metal and matches the panel profile. Rolled vents made to sit under cap shingles are not shaped to seal against a ribbed or seamed panel, and the resulting gaps are exactly where snow gets in.
What if the house has no soffits to vent from?
Houses with no overhang, or with cathedral ceilings and no clear air path, need a different strategy — edge intake vents at the fascia, a vented over-roof assembly, or an unvented design with the insulation and vapour control detailed for it. Putting a ridge vent on a roof with no possible intake creates problems rather than solving them.
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