We need to redo the roof on our house, and we're planning on doing it ourselves and with some friends ( one of whom is a construction worker, though not a roofer).
My question is how steep is too steep for a DIY roof to be attempted safely?

The roof is approx 8.5/12, which is less then the roof of the house I grew up in which I used to play on as a kid, but still pretty steep. I don't want to get any of my friends hurt, so I'm just looking for some opinions or experience with similar roofs.

The roof is a normal 3 tab shingle roof. Currently a couple layers on it, we're tearing it all off.

Thanks