Horizontal Thrust Roof

This horizontal load or thrust can be considerable especially on a low pitched roof.

Horizontal thrust roof.

This horizontal load or thrust can be considerable especially on a low pitched roof. Rafter and collar tie tension equation. 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. 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.

This horizontal movement translates into a vertical deflection which is most visible at the ridgeline. As the roof load pushes downward vertically the rafters tend to spread outward at their lower ends pushing against the walls of the house. This horizontal outward thrust can be considerable. 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. To resist thrust the irc calls for a structural ridge required for any roof with a pitch less than 3 12 or for each pair of rafters to be securely connected to each other by a continuous ceiling joist r802 3 2006 irc. The pitch is the number of vertical inches that a roof descends over the span of 12 horizontal inches. The use of projections onto the horizontal plane seems to be the most commonly used approach.

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. To resist thrust the irc calls for a structural ridge required for any roof with a roof pitch less than 3 12 or for each pair of rafters to be securely connected to each other at the lower end of the rafter by a continuous joist. 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.