Valleys
A valley collects the water from two roof planes and concentrates it into one channel, which is why it wears out before the rest of the roof.
Short answer
Valleys fail because they carry the highest volume, because ice forms in them and sits, and because the way they were detailed decides everything. An open metal valley over ice and water shield is the robust detail here; a woven or closed-cut valley with no membrane under it is the one that leaks.
Why they fail first
Volume. Two planes drain into one line, so the water velocity and quantity are far higher than anywhere else on the roof.
Granule loss accelerates in the valley because of that flow, which exposes the asphalt.
Ice forms and sits in the valley, and in a Toronto winter it stays there for weeks and works at every lap.
Debris collects — leaves, needles, granules — which dams the flow and holds water.
The detail is frequently wrong, with no underlayment membrane beneath it.
And valleys are where somebody has usually smeared roofing cement at some point, which cracks and then holds water.
Ice and water shield underneath
A self-adhering membrane the full length of the valley, under whatever the finished detail is, is the thing that makes a Toronto valley hold. It is cheap at install time and it cannot be retrofitted without opening the valley. A reroof quote that does not mention it in the valleys is a quote to question.
The three details
Open metal valley: a metal channel with the shingles cut back either side. Most robust in a snow and ice climate, easiest to clear, and the detail to prefer here.
Closed-cut valley: shingles from one plane run through and the other plane is cut to a line. Common, acceptable with proper membrane beneath, and the cut edge is the vulnerable point.
Woven valley: courses from both planes interlaced. Looks neat, holds water at the weave in freezing conditions, and is the least forgiving detail in a Toronto winter.
In all three, the membrane beneath matters more than the detail above it.
Metal valleys need the right profile and a hemmed edge rather than a flat sheet.
And fasteners must stay out of the water channel, which is a common installation error.
Repair and maintenance
Keep them clear. A valley full of leaves is a dam, and clearing it is a twenty-minute fall job that prevents a repair.
Repair means opening the valley back to sound material and redoing the detail with membrane beneath, not cementing over it.
Replacing a valley on an otherwise sound roof is a legitimate repair and is frequently what is needed.
Check the valley at every roof inspection, because it tells you the remaining life of the roof better than the field does.
Where a valley has leaked for a while, expect sheathing work beneath it.
And on a reroof, specify open metal valleys with full-length membrane, which is a small cost and a meaningful improvement.
Common questions
Why do valleys leak first?
They carry the water from two planes, and ice sits in them all winter.
Which valley detail is best in Toronto?
Open metal over full-length ice and water shield. It is the most robust and the easiest to clear.
Can a valley be repaired on its own?
Yes, and it frequently is the right repair on an otherwise sound roof.