Temporary Measures
A tarp is a bridge to a repair, not a repair. Done properly it holds for weeks; done badly it holds water against the roof.
Short answer
A proper tarp is run up over the ridge where possible, fastened along battens rather than through the field of the shingle, and tensioned so water runs off rather than ponding. In a Toronto winter a tarp is a short-term measure, because wind, ice and snow load all work against it.
What makes a tarp work
Enough coverage, extending well past the damaged area in every direction and up over the ridge where the geometry allows.
Fastening along wood battens rather than nailing through the middle of the tarp, which is what tears.
Tension, so water runs off rather than pooling in a sag that then freezes.
Securing the upslope edge under a shingle course or over the ridge, so water cannot run under the top edge.
Keeping the fasteners out of the undamaged field of the roof where possible, because every penetration is a future leak.
And a heavy enough tarp, because a light one shreds in the first Toronto windstorm.
How long it lasts
Weeks rather than months, and less in winter. Ultraviolet degrades the material, wind works the fasteners, and snow load pulls it. A tarp still on a roof in spring has usually been leaking for a while. Treat it as a scheduled item rather than a solution.
The other temporary measures
A patch on an accessible flashing, which on a chimney or sidewall leak can hold through the winter.
A new vent boot, which is a quick permanent repair rather than a temporary one.
Peel-and-stick membrane over a damaged area, which is more durable than a tarp and is a legitimate interim measure.
Clearing an ice dam and the eave, which addresses the cause rather than covering the symptom.
Clearing a blocked flat roof drain, same.
And interior containment, which is frequently the right answer overnight and costs nothing.
What to agree with the contractor
What the temporary measure is and roughly how long it is expected to hold.
What the permanent repair will be, and when.
Whether the temporary work is credited against the permanent work.
That the emergency invoice is separate from the repair invoice, for the insurance claim.
That the tarp will be removed and the fasteners properly dealt with when the permanent repair happens.
And a date for the permanent work in writing, because a tarp with no scheduled repair behind it is how a temporary measure becomes a two-year one.
Common questions
How long does a roof tarp last?
Weeks rather than months, and less in a Toronto winter.
Is a tarp a repair?
No. It is a bridge. Get the permanent repair date in writing.
Can a flashing be patched for the winter?
Frequently yes, and on a chimney or sidewall leak it can hold.