In part it depends on how well ventilated your attic is. If like mine it has two attic fans that will run if the temperature reaches a certain point and lots of vents you will not build up a much heat but if your attic has little to no ventilation, like how older houses were built, it could likely be a problem.
If you can run your lights during the day without your grow room temperature getting to high for security it would be best to do so because during the day your roof will naturally get hot and what you will add will make little difference, unless you live in an area where it never gets very warm. Especially if you have dark shingles, assuming that is that you do have a shingled roof, your roof will get rather hot and then it will retain the heat for some time after the sun goes down where lighter shingles will reflect more light rays that produce heat and will not get as hot and will cool down faster after the sun goes down. If you can match the natural heating and cooling of your roof as much as you can it reduces a suspicious roof heat signature.
Something to consider is if your attic has vents or attic fans or both is running your lights exhaust line all the way to a vent or fan so it will blow out or be sucked out faster and be less likely to build up. If your attic does not have vents and or fans what I have heard some people will do is to run long exhaust lines back and forth across their attic and then block the end and make a hole every so often, precisely how far apart I am not sure but as even of a spacing as you can make is best, and that spreads the heat more evenly over the entire attic space and when it radiates up it does so more evenly and would appear to be more normal of a heat pattern than if all the heat blew out in one area and then rose and heated one portion of the roof more than the rest.
And yet one more thing to consider is if you have a bathroom fan with an exhaust line running through the roof you can tie into it, use or make what would be like a ‘V’ connection and run your light’s exhaust line into the bathroom vent line and you will not build up any heat at all that way, well not if you used an insulated exhaust ducting and if not a slight amount of heat would radiate through a thin uninsulated duct but not enough to wave any red flags.
