I’ve got a detached single car garage.  The interior is unfinished, exposed studs and rafters, cement slab floor.  Exterior is vinyl siding over old wood shingles.  It has a gable roof, fairly low pitch, maybe 4/12, 2 layers of tar shingles than need replacing.  There used to be gable vents but they were blocked off when the vinyl siding was put on ~20 years ago.  The garage has electricity but no heat.

I want to set up a gym in the garage but using it in the winter will require running a heater, at least enough that my hands don’t freeze to the weights.  I haven’t had any problems with leaking even with the old shingles falling apart but if I raise the temperature in there above melting for 30-60 minutes a day in the winter with snow on the roof, I’m afraid I’ll get ice dams and leaks.  If the shingles are getting replaced, what’s the best way to do the roof to prevent this?  Ice and water membrane all the way to the ridge?  Do I need venting to keep the framing from rotting?  Or should I just put regular underlayment and shingles on there and rake the roof regularly?