Answer by
LeheckaG (1826)
Roof-trusses support your: roof's & rain/snow weight, walls from collapsing & wind-forces.
Architects/engineers take your roof-truss spacing, span-length, & roof-pitch;
& then calculate downward (weight/rain/snow/wind), horizontal (walls/wind), & upward (hurricane/tornado) forces.
Modern roof-trusses are designed/engineered to exceed such-forces & typically-pre-manufactured from 2"x4" lumber.
Local/state building-zoning codes can specify roof-truss-sizes.
