The Cap
A cap is the cheapest item on any chimney quote and it prevents more problems than anything else on the list.
Short answer
A cap keeps rain and snow out of the flue, keeps animals out, stops debris falling in, and with a spark arrestor screen reduces the risk of embers landing on the roof. On a Toronto chimney the water exclusion is the main value, because water in the flue is what destroys the liner and the masonry from inside.
What it does
Keeps rain and snow out of the flue, which is the main value. Water in a flue attacks the liner, the smoke chamber, the damper and the masonry from inside.
Keeps animals out, which saves a wildlife removal and a flue full of nesting material.
Keeps leaves and debris out, which otherwise block the flue.
Acts as a spark arrestor where it has a mesh screen, reducing the chance of an ember landing on the roof.
Helps with downdraft in some configurations, where the cap design shelters the flue from wind.
And protects the top of the flue tile from the freeze-thaw cycling that cracks it.
Water in the flue
An uncapped flue takes the rain for the whole footprint of the opening, straight down onto the smoke shelf and the damper. That is why uncapped chimneys have rusted dampers, deteriorated smoke chambers and water staining on the chimney breast inside the house. A cap costs very little against that.
What to specify
Stainless steel rather than galvanized, because galvanized rusts out in a decade and stainless does not.
A lifetime or long-term warranty, which good caps carry.
Mesh sized to exclude animals while allowing adequate draft — too fine a mesh blocks with creosote and ice.
Adequate height above the flue, so the cap does not restrict the flow.
Proper fixing to the flue tile or the crown, because a cap that blows off in a Toronto windstorm has achieved nothing.
And a multi-flue cap where there are several flues, which covers the whole crown and protects it as well.
The related items
The crown, which should be sound before a cap is fitted, because the cap does not protect the masonry from a cracked crown.
The top-sealing damper option, which replaces the throat damper with a sealing cap at the top — a good solution on an unused or drafty fireplace.
The flue tile condition at the top, which is visible when the cap is fitted and is worth checking then.
The flashing at the roof line, which is a separate water path.
Clearing the cap periodically, because a mesh cap on a wood-burning flue blocks with creosote.
And on a gas appliance, confirming the cap is appropriate for that flue, because the requirements differ.
Common questions
Does a chimney need a cap?
In Toronto, yes. Water in the flue is what destroys the liner, damper and masonry from inside.
Stainless or galvanized?
Stainless. Galvanized rusts out in about a decade.
Can the mesh block?
Yes, with creosote on a wood flue or with ice. Check it periodically.