If you keep the door closed, the vent fan won't move much air unless you cut some soffit vents or box vents down low. With the soffit vents you don't have to deal with shingles.

I'd imagine it depends on how the garage is oriented. You're right that only one vent fan won't work as it needs to pull air from somewhere. Typically a house with gable vents would have one at each end of the house. I'd look to go that route if the garage is situated like that.

I'm not sure what you mean about dealing with shingles, though. Gable vents are in the peak of the end wall, not on the roof.

Box vents are on the surface of the roof.

Thank you both. Going to drill out some soffit vents (wood trim) and put in the gable vent. Probably looking at a new roof in the next two years anyway, so just trying to get things going in the right direction. No pun intended.

Put two in the attic in my old house for HVAC, it set the one at 120ish and seemed to run nonstop from June-October.

right, but OP said gable vent, not box vent.

And I said gable vent too. A gable vent requires a supply whether it is an open door, box vent, soffit vent, ridge vent, or gable vent on the other end. Have I covered everything?

I’m good! Thank you.

Something I did was to put a 30”x30” powerful belt driven gable vent with a nice decorative front screened grill (bugs) up and inside the garage (and attic..I did 3 total) I attached a hygrometer AND thermometer (all one unit) and a snap covered on off switch for when belts need changing and I don’t want to lose fingers. The hygrometer is set for 37 percent humidity (in Mass you would be safe going with 50% to keep it from running all the time) and the temp was set to 140/80F. In the summer the peak of the garage hits 140F by 7am and either the humidity setting or the temp sets it off. If you are actually trying to use this to cool the garage you can set it for lower temps but you are going to use a LOT of electricity since the heat will constantly be high up near the fan. I would recommend putting window or wall mount fans at work level and have switch to flip them on when you are going to be in there. They are cheap at $120 each for professional units and will NEVER wear out (until your home burns down and they melt…or maybe that’s just me…and the fire was caused by something installed long before I bought the house). So that’s a thing I did to resolve both moisture and heat issues. The two in the attic of the house, Mid point and North end, were quiet and smooth operating and I never had a repeat of mildew and never overheated or overworked the air conditioning in the Summer. In 2006 this was $480 done professionally but that’s pretty meaningless now. What you are talking about with my recommendations will run about 550 DIY$ these days for the single fan and a couple window fans at level.

That's good to know. If I can help you, I will. Thank you and have e a nice weekend.

Venting roofs doesn’t really make them cooler. The shingles and sheathing are going to be roughly the same temperature vented or not, and that heat is going to radiate into the space. You might experience a sensation of coolness from the air movement on your sweaty skin, which will bring some relief. A radiant barrier below your rafters would likely do more than venting will to make it feel cooler. If you have a window, open that along with the garage door to get a cross breeze when you’re in there.

Cable vents on both of the gables rely on each other for a cross breeze. Gable vents are not compatible with soft advent or ridge vent. If you have soffit vent and ridge vent, the gable vents must be closed off. I've been a roofer for 18 years

I put in a radiant barrier in my attic and I verified attic temp reduction of 20F at the insulation. Best $100 I ever spent.

True that it only reduces upstairs living space about 5 deg but that's misleading. It basically eliminates evening heat soak. Before house actually got hotter from about 5pm onward. The other advantage is that before if I turned on AC at 3pm, it would basically never make any headway. Now I turn it on at 3pm and 30 mins its almost at the thermostat setting because my evaporator and ducting is in attic.

When temps are above 85f my neighbors turn on AC at 1pm and they run pretty much constant whereas I almost never use AC anymore unless there is a string of 100 deg days which is rare.

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I considered attic fan (I have a big gable vent) and came to same conclusion that they don't really work. Ridge vent is better but also has issues.

I am a shed, garage, and patio builder in Australia.

Vents will help but insulation is far better all year round. I assume this is a timber stud wall garage with cladding on the outside.

If you can induce a natural flow with solar passive design principles it will likely be far more efficient and comfortable than trying to force air with a fan. You want to try to have an inlet vent on the cool side down lower and an outlet vent on the hot side up high which should be the south west for the US. This will natural suck cool air through your garage.

Adding a thermal insulation layer will be far more effective against the heat. I use a product call aircell a lot. It's kinda like bubble wrap, two strong and durable foiled layers with bubbles sandwiched between. It has a very high insulation rating, and it's very easy to work with, cuts easily with a sharp knife and will hold with most fasteners or glue. But it's not cheap.

But do you have a flat soffit or a sloped soffit?? Sloped makes venting more complex and it’s completely untrue that you need soffit and roof vents for venting to work well.

Also it’s way way more complex to size all of this. Look in the IRC there is an entire code section on how to calc and what’s required Chapter 8