Home cable trusses require tensioning in areas according to building report. Is this a thing?

[Rob001]

I've lived in my house 2-3 years now and looking over the building inspection report I received when I was purchasing it, it has 'Cable trusses require tensioning in areas, this should be performed every 5 years or so as cables stretch over time - roughly $200'

Is this a thing? I can't find any info online about roof cable trusses needing tensioning. The home is 42 years old but recently renovated and an extension added 20 years ago.

If it is a thing could I do it myself or who would I hire to do it? Google isn't providing any info.

[MitchellMc]

Welcome to the Bunnings Workshop community @Rob001. It's fantastic to have you join us, and many thanks for your question about roof cable trusses.

In a non-truss roof system, the underpurlins can be strengthened by steel cables and a compression post. The cables are typically tightened, and the tension in the cables can be adjusted if necessary to get the alignment of the roof right. Over time there might be a need to have the cables either tightened or loosened. If the inspector has determined that the roof is sagging or there are other issues present, then re-adjustment of the cable's tension could be required.

This process is not as simple as just tightening the fixture. I'd recommend you enlist the services of a qualified carpenter who is a roof and wall frame specialist to do this adjustment for you. 

Please let me know if you have further questions.

Mitchell

[MikeTNZ]

@MitchellMc,

Is this actually a thing?

I can understand trussed roofing members being braced with wire rope and turn-buckles, after repair, but what sort of roof structure would this be?

Cheers,

Mike T.

[MitchellMc]

Definitely a thing, @MikeTNZ. However, typically only seen in older dwellings. 

A trussed roof is self-supporting. The rigidity and weight-bearing capacity comes from the triangular frames. The frames transfer the weight of the roof onto the outer load-bearing walls.

In a non-trussed roof, the individual rafters extend from the ridge board down to the top plate and load-bearing walls. There are no frames as such, and underpurlins support the rafters. The underpurlins run from one end of the roof to the other. To increase the load-bearing capacity of these underpurlins, they can be supported by struts or, if there is no location for a strut to be placed, a cable tie-bolt truss. The cable connects at either end of the underpurlin and an adjustable threaded support is fixed at the mid-point. When the support is extended via a nut and threaded rod, it pushes up on the middle of the underpurlin. In short, a cable truss is a way to increase the loadbearing and span characteristics of a piece of timber. It's the same concept used in bridge design. This underpurlin cable truss is not to be mistaken for a roof frame truss.

Trussed roof frames are typically constructed off-site and transported to it. A non-trussed roof is constructed on-site. It's quicker and more cost-effective to build a trussed roof, and you'd only really see a non-trussed roof these days if there is an open attic space planned for the build.

I've included some images below, which should help explain better. I couldn't find a good example of the cable truss, so I have created a rendering of one. If you are interested in the topic, I'd encourage you to check out the many different types of roof trusses. 

Please let me know if you have further questions.

Mitchell