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Frequently Asked Questions
What does a purlin anchor connect?
A purlin to the rafter it crosses.
Purlins run horizontally across the roof plane carrying the covering.
The joint is small, repetitive and structurally significant.
It is where uplift on the roof surface is first resisted.
Wind suction pulls on the covering and into the purlins.
The purlins then have to pass that force into the rafters.
Why do roofs fail along a purlin line?
Because the purlin lifts and takes the covering with it.
An inadequate purlin-to-rafter connection is the weak point.
The covering leaves in strips rather than scattered pieces.
That pattern is characteristic of this failure.
The fixings into the purlin may be perfectly sound.
It is the connection beneath them that gave way.
Why not just skew-nail the joint?
Because the geometry makes it awkward and unreliable.
A purlin crosses a rafter at right angles and often at a pitch as well.
Skew-nailing produces a joint whose capacity depends on how it was done.
An anchor puts fasteners into side grain where they work properly.
It also gives the joint a published capacity.
That is what an engineer designs to and an inspector checks.
How does roof pitch affect the choice?
The connector has to accommodate the angle between purlin and rafter.
Anchors are made to suit a range of pitches or a specific one.
Forcing a flat connector into a pitched joint leaves it bearing on an edge.
That concentrates load where the steel is weakest.
Pitch has to be established before ordering.
Measuring the built condition rather than scaling the drawing is more reliable.
Does one missing anchor matter?
Yes - it creates the weak point the roof fails at.
The joint is repetitive, so small errors multiply.
A line of sound connections with one gap fails at the gap.
Uplift concentrates on the remaining connections.
Eaves, verges and corners see the highest uplift of all.
Those locations frequently require closer spacing or heavier anchors.
How is the requirement determined?
From the wind loading for the specific site.
Exposed, coastal and cyclone-prone regions have far heavier requirements.
Roof shape and building height both affect the loading.
Local codes set the design method and the figures.
A detail adequate in one region can be badly inadequate elsewhere.
The manufacturer's tables for that market apply alongside the code.
Where else does the load path go?
Onward from the rafter, down through the structure to the foundation.
A purlin anchor alone does not resist uplift.
The rafter must be tied to the wall plate.
The wall must be tied to its base.
The chain has to run unbroken to the foundation.
A well-anchored purlin on an untied roof structure achieves very little.