Horizontal Thrust Roof

The pitch is the number of vertical inches that a roof descends over the span of 12 horizontal inches.

Horizontal thrust roof.

The vertical loads applied to the scissors truss create a horizontal thrust at the bearing. This horizontal movement translates into a vertical deflection which is most visible at the ridgeline. It is interesting that as the collar and purlin roof came to the fore and wall plates were called on to resist more horizontal thrust the standard wall plate splice detail gradually changed from one with a horizontal splice to a vertical splice which was better able to resist the horizontal thrust. Rafter and collar tie tension equation.

This horizontal outward thrust can be considerable. This horizontal load or thrust can be considerable especially on a low pitched roof. A 45 degree roof has a 12 12 pitch because it drops 12 vertical inches over 12 horizontal inches. This thrust may push out the wall if the truss is not allowed to slip or the wall is not stiff enough to offer resistance.

Horizontal rafter thrust this thrust calculator gives rafter horizontal thrust at the plates l rafter span building width w load on rafter roof design load. In a simple gable roof the rafter boards carry the live and dead loads that push both downward and outward against the top of the load bearing walls. The pitch of the roof determines the length of hip rafters that you need which are the four corner rafters that define the corners of the roof. This horizontal load or thrust can be considerable especially on a low pitched roof.

1 where l is the rafter length projected onto the horizontal plane d is the rafter tributory width and σ is the roof loading projected onto the horizontal plane. This force is resisted by horizontal rafter ties or. As the roof load pushes downward vertically the rafters tend to spread outward at their lower ends pushing against the walls of the house.