Without the proper support of rafter ties or a structural ridge, a typical gable or sloped roof will sag downwards while pushing the building walls outwards towards a catastrophe such as that shown in our page top photo of a barn in Amenia, NY: the failure of rafter ties in this building combined with snow loading in Amenia, New York, pushed the ridge down and the walls outwards as the building slowly settles down to the earth. This article series describes and illustrates the different types of support that prevents roof sagging and wall bulging at buildings, including definitions of collar ties, rafter ties, and structural ridge beams. Roof structure definitions & support choices for sloped roofs, cantilevered roofs & cathedral ceiling roofs. Roof Framing: collar ties, rafter ties, tension beams & structural ridge beams: some of these can support the roof and prevent ridge sagging and wall spreading. POST a QUESTION or COMMENT about roof framing, support, construction specifications & practices.Thanks, for everyone's time.Common rafter, Valley rafter, Valley jack rafter,Ĭollar Ties, Rafter Ties, Structural Ridge Beams If they're installed somewhere along the rafter span, they should be considered in your analysis and the impact on the rafter tie considered. If you want collar ties to only help with uplift resistance at the ridge, place them as high as possible. So, to summarize, collar ties see compression loads. I've seen and fixed lots of roofs where the rafter ties are too high. Below are the moments in the various members under the same loading conditions noted above:

As the tie placement moves further up the rafter, the bending in the rafter increases significantly. Similar to collar ties, traditional rules-of-thumb would have them located anywhere in the bottom third of the rafter span ( ). Improperly located rafter ties can be even more dangerous. Yes, they'll go into tension in the event of uplift, but if they aren't designed to resist the appropriate compressive forces, the member or connection may be damaged and unreliable when it's needed in the uplift condition. The frames at the bottom are for the rafter tie discussion to follow.Īs you can see, the collar tie goes into compression under load. Frames on the left have full snow on the roof, frames on the right have full snow on the left side, 50% snow on the right. Collar and rafter ties (where shown) are at the third points. Below are the axial loads in the members.

The span is 24', c/c, 4:12 slope, 50 psf snow, and i can't recall the dead load but it's not significant relative to the snow. This puts the collar tie in compression and increases the tension force in the rafter tie (the moment arm between the compression and tension forces in the collar and tie, respectively, decreases). However, when you add collar ties, you are introducing a lower point where the rafters can react against each other, like they do at the ridge. Common rule-of-thumb practice is to locate the collar ties in the top third of the rafters. Their intended purpose is primarily to prevent roofs from splitting at the ridge in the event of large uplift forces as the connection to the ridge is traditionally tenuous (end and/or toe-nails). I'm not a layman, so let's get it on! :)Ĭollar ties are common in residential roof construction. The thread has since been removed (layman question i suspect). I posted an unpopular fact in a thread the other day that i was hoping we could discuss a little more.

Roof rafter ties