Purlin is the horizontal structural member of a roof, either timber or steel. In steel construction, the term purlin refers to parallel roof framing members that support the roof decking or sheeting, which are supported by walls or rafters.
There are three types of purlins, these are:
- Purlin plate
- Principal purlin
- Common purlin
Before discussing them, it is important to consider the concerns when renovating an existing roof, fixing a new replacement, or putting in a new one. The most vital factor to be considered is the pitch of the roof. There are four major types of roofs, namely:
- Flat roofs
- Low-pitched roofs
- Conventional roofs
- High-pitched roofs
Not only does the pitch of a building’s roof determine the water runoff and accumulation of snow, but it also affects a building’s structure. Calculating a roof pitch is important as it greatly reduces the risk of leaks. The Multiform Roofing offers a full-range of roofing services.
Installation of Purlins
Purlins are placed on top of rafters and a vapour barrier is placed on top of them. They are 2 by 4 feet in size and are built in the same way as metal roofing is. They provide additional roof support by serving as a nailing barrier for the drip edge and end panels.
A rafter should support all purlin joints and laps, be spaced no further than 1.2 meters apart when using sheets that are 0.7mm and 1 meter when using 0.5mm sheets, though the requirements will vary.
Sheets should be 50mm wide to be easily fixed to the purlins. It is important to ensure that the purlin framework is aligned and levelled; adjustments can be made if not.
The purlins must be spaced correctly and within specified tolerances; otherwise, the weight of the roofing sheets will damage them, rendering them useless. Steel purlins can be connected using either the sleeved connection or the butted connection.
Supporting roof purlins correctly begins long before new members are installed. During refurbishment projects, experienced roofers often discover that movement elsewhere in the roof structure, such as ageing rafters, distorted ridge lines or previous alterations, has affected how loads are being transferred through the roof. Simply replacing or reinforcing the purlins without addressing these underlying structural issues can leave the roof vulnerable to continued movement, uneven loading, or recurring defects. Drawing on over 30 years of roofing experience, Multiform Roofing assesses the condition of the surrounding roof structure before recommending repairs or replacement, ensuring that any new purlins work as part of a stable, properly supported system rather than treating a symptom in isolation. This practical approach helps deliver a longer-lasting repair while reducing the likelihood of future structural problems developing.
Purlin Spacing
Purlins assume the shape of the roof. As a result, the spacing necessitates careful planning to match the nodal pattern of the supporting trusses.
This means that purlins should not be mounted on the members themselves, but rather at the nodes of the trusses. Purlins in steel buildings are made of ZED, which is cold-formed Z, and CEE (C) segments.
Compared to thicker hot-rolled shapes, cold-rolled shapes offer stiffness, lightness and high strength, easy packaging, transportation, fabrication, and installations.
