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About Purlins & Girts Now
Purlins need to be bolted to the primary frame by cleats
welded to the rafters or columns by a qualified steel
fabricator. The cleats and the associated hole geometry
have been standardised in the AISC Standardised Structural
The bolts are usually M12 Grade 4.6 requiring snug tightening
to make an effective connection.
To allow for minor variations in frame alignment, purlins
made from GALVASPAN® steel generally have quite
large clearance holes-18mm diameter holed for 12mm diameter
bolts. Some manufacturers produce 18mm x 22mm slots
for greater adjustment. These generous clearances make
for easier assembly, but do not affect structural performance.
When Z purlins are lapped, additional holes are provided
to ensure structural continuity. Purlins are engineered
for M12 bolts and in these cases, M16 should not be
used. With lapped Z Section Purlins in some of the thicker
gauges, the holes cover one another sufficiently to
prevent assembly with M16 bolts unless they are forced.
To minimise the tendency of the sections to rotate between
supports or bridgings, purlins need to be installed
with the top flange facing up the slope from the cleat:
- C sections should be fitted on the high side
of the cleat, open face facing up the slope.
- Z sections should be fitted with the web on
the low side of the cleat, with the top flange above
While purlin fixing to cladding is quite straightforward,
the sections are very flexible until they become part
of the total sheeted system. The aim should be to maintain
a stable framework by fitting the bridging as the purlins
Bundles of roof sheeting should not be placed on unsheeted
purlins, as this can cause overloading and result in
permanent deformation of the sections.