Detailed answer
Factors That Change The Recommendation
Tipping roofers isn't standard like restaurant tipping - but it's more common than you'd think for jobs that go well. Crews work hard in hot weather, on steep roofs, hauling heavy material. A small gesture goes a long way.
If you do tip, $10-$40 per crew member is the typical range. A small crew (3-4 workers) on a one-day job might get $15-$25 each. A larger job that ran long might warrant $30-$40 each, especially if conditions were tough. Hand it to the crew foreman to distribute, or give individually if you've had direct interaction. Alternatives that crews appreciate just as much: bottled water, cold drinks on a hot day, lunch (especially pizza or burgers - easy to share, doesn't need utensils), and clean bathroom access. A positive online review with the foreman's name mentioned often matters more financially than cash tips. None of this is required - excellent service is what they're already paid for.
If You're In California Or New Jersey
California crews working through summer heat (Central Valley, Inland Empire) particularly appreciate cold drinks and lunch on long days. New Jersey crews working through unpredictable spring weather appreciate flexibility when weather pushes the schedule.