You have a few different options when building a log cabin roof. You can try the end gable system where you continue building the walls on the ends of the cabin up to a point. You can use a chalk line to get an even slope on each side.
Once you have the chalk line measurement, you can make a diagonal cut with a chainsaw to make a perfect slope. Be careful, chainsaws are nasty beasts!
The logs can be run from the gable ends lengthwise of the cabin as beam supports or they can be run as trusses. Either way you will need to make sure that the roof gets adequate insulation.
Many log cabins have open or vaulted ceilings with tongue-in-groove boards making it difficult to get adequate insulation. It looks nice, but doesn’t hold heat very well.
Many cabin owners design false roofs that have an extra foot thick of insulation in the ceiling.
Most log cabins get metal roofs. They look a little tacky with asphalt shingles. Log cabin roofs support very heavy snow loads due to the gable ends.
If you live in an area that has excessive snowfall, it would be wise to use a larger diameter log size for the roof. I have seen smaller log roofs fall in under heavy snow loads.
Large diameter logs are really expensive, but you only need a few to build a log home with a very strong roof. This is especially important if the roof members have a long span. It’s always best to overbuild when in doubt.
The purlins will be the areas where the most weight will be. These are where a larger diameter log needs to be used in cases of long spans and in areas of unusually heavy snow loads.
Purlins can be one long log piece or they can be broken up by log trusses to ease the weight.