While a barn house is sometimes considered a farm-house-plan, there is a difference. I just have to please the search engines with various keywords so my site keeps coming up on top.
A really important thing about building a barn house is that the walls need to be sheathed with plywood or OSB before you start on the second floor. The weight of the second floor can put stress on the first floor framing.
Once the walls are sheathed, the structure will be much stronger. You will want to use at least ½” wafer-board. I prefer using 5/8” sheathing on the walls. The roof sheathing is the same provided the spacing of trusses is no more than 24” apart. On the walls, you will need to start at a corner and include the rim joist and sill plate as part of the wall being covered by sheathing.
When you get to the windows and doors, you can either piece together strips of sheathing or just cover up everything and use a router to cut them out. I like to keep the sheets big because it gives more strength. You can use a jigsaw or a saber saw to cut things out either.

If you are using plywood, you might consider running the sheets sideways so that the wood grains give more strength. Wafer-board doesn’t matter because the chips run in different directions each glued layer for extra strength. The roof sheathing is applied the same as the walls and the floor.

Now we can start on the upstairs because the first floor walls are strong with sheathing.

The framing part of a gambrel roof or a farm-house-plan uses heavy trusses. You will need to find a buddy to help you lift them. The wind can also be a factor, so you will want to wait for a calm day.

As illustrated above, a 30/60 slope isn’t difficult. It can just about be attained by guesswork alone. It is important to distribute the weight from the ridge to the knee. Also a collar beam will be necessary and it will help relieve pressure from the sides of the roof.
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