A new roof changes the day to day in ways you stop noticing after week one. The upstairs room stops getting that weird afternoon heat. Rain on the shingles goes quiet instead of loud. A full tear off and replacement on a typical single family home takes one to three days of actual work, with another day or two for cleanup and inspection, so you mostly just need to be out of the driveway while the crew is up there.
Late summer is a smart window to book the work. Schedules start loosening as fall approaches, and roofers will often sharpen their pencils to fill the calendar before storm season is fully underway. Get three written quotes at minimum, and make sure each one spells out the same things: tear off vs overlay, underlayment type, flashing, ventilation, and what happens if rotten decking shows up. Apples to apples is the only way to compare.
The 0% intro APR is a deadline, not a discount. If the balance is still on the card after 15 months, you start paying the regular purchase APR on whatever is left, and that rate is meaningfully higher than what a roofer would charge you to finance through them. Treat month 15 as the drop dead payoff date and you keep the savings real.