Re: Tell me how to build a triangle gable vent
I built a few of these for an old house (1820-ish) many years ago. First size and build the triangular "box" which must match the roof pitch exactly, and in a better quality example, will have the bottom sill pitched at a typical 6* angle to shed water. It should be sitting on a larger subsill that overhangs the siding so water drips clear. You can one-piece the box and make a wider, overhanging sill, but it does complicate the calculations and two-pieceing it lets you just think about the triangular box at this time.
Louver spacing is usually twice the thickness of the slat; 3/4" slat, 1 1/2" spacing. Between 30* to 45* slope to the slats is fine, lower slope , higher likelihood of rain blowing inside. Rip the slat angle on the edges of each louver so the outside presents a flush vertical face where it meets the box.
The compound angles (unless you're a trig wizard) are trial and error. I ended up buying a compound saw for that particular job, because I needed a saw that went to 62* miter angle for my design. I bought a bosch sliding compound saw, still have it.
Square louvers you can dado the slats in, but on a triangular one, you're going to be nailing and screwing the slats in place. If you dado these slats in you're building furniture, not millwork. You can play with the spacing so it's basically even-looking top to bottom without having a too-small non-functioning louver-ette at the top.
Generally it's better to plan on the casing overlap the slats by 1/2" to keep the ends out of weather.
Remove not the ancient landmark, which your fathers have set.