We only worry about two things: 1) Live loads, and 2) Dead loads.
1) Live Load (loads that come and go): In your case, there isn't a second floor, so you're just concerned about "snow load". (If there was a second floor, then you'd add "people Load". In the U.S., its 40 lbs. per square foot.) You'll need to determine what your building code requires. You can call your local building department and they'll know. (It'll be something like 40 lbs. per square foot, or higher if you live in a snow zone.)
2) Dead Load (loads that are there all the time): You can add up the weight of all the roof components, or use 15 lbs. per square foot, unless you have concrete tile roofing.
Now, the Load on your header is based on those two factors added together and multiplied by one-half the span on your header. So, let's say you have roof trusses that span 28'. Just add 40 psf to 15 psf to equal 55 psf TOTAL Load. Then, multiply 55 psf by 28' and divide by 2 (because half the load goes the other direction away from the header.) which equals 770 lbs. per foot on the header.
Now you can check the tables for the span of your header (width of your window). If your window is 6' wide your header should be 2 - 2 x 10's. (Table 2.)