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The size and spacing required of purlins (the
supporting bearers) frequently defeat new entrants to the world of
roofing and cladding in that they underestimate the size required
and overestimate the spacing distance between them, particularly in
relation to profiled metal sheeting where there is no standard
spacing distance as there are so many variables to be considered,
i.e. location, winds speeds and thus load, amount of snow load, Height above sea
level, thickness of metal, depth of profile, angle of roof pitch etc.
The professional Architect or Surveyor can achieve
the maximum spacing and loading in a given area by referring to and
interpreting wind/snow loading and deflection tables as found
elsewhere in this website. Without industry qualifications and
specialist knowledge these charts are meaningless to the lay man.
The DIY enthusiast is in need of a cover all
circumstances guide that is on the conservative side
regards spacing distance and load bearing capacity. It is this that
this page attempts to address.
One of the best means for the DIYer is to look and
copy what the qualified professionals do and if in the slightest
doubt consult a professional.
PURLIN SIZES
Remember it is not the actual weight of the sheets
on the purlins that is the most important But the snow and wind load
which is the same whether or not the roof is constructed of light or heavy weight
materials.
TIMBER PURLINS. Preferably Tanalised.
Up to a maximum of 16 ft long use not less than
7"deep x 3" wide. From 16ft to 20ft long use. 9" deep x 3" wide.
CLICK HERE
to visit the timber page.
GALVANISED COLD ROLLED Z PURLINS
Up to a maximum of 4.88m ( 16 ft) long use not less than 141mm
deep x 1.6mm thick . From 4.88m to 6.10m (16ft to 20ft) long use 178mm deep x 1.6mm
thick. Eaves Beams use at least 2mm thick CLICK
HERE to visit the Z Purlin page.
PURLIN SPACINGS.
For Metal Profiled Sheets with a reasonable steep
pitch and 0.7mm thick stick to a maximum of 1.230m (4ft) spacings. Thinner
sheets and low pitches bring the spacing down to a maximum of 1.067
(3ft 6")and even .914mm (3ft) if sheets are made of a very thin material.
FIBRE-CEMENT.
The manufacturers quote a maximum purlin distance
for PROFILE SIX 1.372 Metres (4ft-6"). For PROFILE THREE. 914mm
(3ft)
PVC and FREELIGHT.
The manufacturers quote a maximum distance of 914mm
(3ft)
ONDULINE ROOFING.
The manufacturers quote. Roof pitch of 5 - 10
degrees, support on decking. Roof pitch of 10 - 15 degrees, 450mm
(18") centres, maximum. Over 15 degrees 600mm (2ft) centres maximum.
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