Thread: collar ties in roofs

I always assumed they were in tension. They would prevent the rafters from spreading - purlins will help stop sagging but as the weight comes down, they would tend to spread the rafters. If opposite rafters are connected by a ceiling joist (like a truss), you might argue that you don't need them.

Hi

Originally Posted by beaconsbob

can any one tell me what use collar ties in roof serve. If a rafter is supported by purlins, which is struted to a wall below and the ceiling joist is bolted to the rafters how can the roof move i.e why have collar ties. Most homes of the 40s didnt have them and most of those roofs are fine. Are the collar ties in compression or tension, my theory is neither.

We have too many trees and they figured out that they can use the timber for collar ties to make it more difficult to move about in the roof space

Collar ties are under tension as they help stop the spread of the rafters.

Even though a roof is supported "from underneath" in many cases the supports are NOT directly in line with the downward pressure.

WIthout collar ties it would still be possible for the rafters to spread outwards toward the eaves.

Collar ties are generally used when the rafters are continuous over more than one span. In other words, when the roof construction involves one or more lines of under-purlins and struts to support the rafters in mid span.

You are quite right in that the ceiling joists will form the tension member of the roof triangle just as the bottom chord of a roof truss does, and prevents the rafters spreading apart at the base. If you added a collar tie to this single rafter span geometry it would indeed tend to act as a compression member, which is not the design purpose of a collar tie.

However, as soon as you add a purlin run to the rafter span the upward force of the purlins/struts introduces a new "lateral spreading" force at the centre of the rafters, even though they are securely fixed at the bottom by the ceiling joist. A collar tie then becomes effective in negating this force, and they are most definitely in tension. The same problem does not present in a trussed roof because all of the upward support is located at the outsides of the triangle. The upward support requirement at mid rafter of a truss is effectively transferred to the outside points via the strutting/web members, bottom chord and gang nails.

It's makes more sense when you see an actual Engineer's drawing of the compression and tension forces at work in roof construction. Quite fascinating, actually.

Wayne

On larger roofs the rafters could be in more than one piece. Joined over an under purlin, a collar tie at the joint stops the rafters spreading.

If under purlins are used the tendency also was for less depth in the rafters, so again collar ties were used to stiffen things up.

A lot of design effort goes into resisting uplift forces. The few stitch nails etc that used to be used for fixing under purlins and struts were useless for this, but collar ties did help in this respect.

Cheers
Bill

Since a collar tie is under tension, i expect you'd be able to use a steel cable or rod instead of a wooden beam. Something like a 3/8" steel rope bolted to the rafter at each end?

I think they might have figured it out after 10 years...

All it takes is for me to walk around the local neighbourhood and look at the old, sway-backed tile rooves around here to know why these ties're there.

It's often a simple matter to spot the ones without... they also have a bit of a bloated belly look, as well as the sway back!