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Metal Roof Purlins Spacing and Rafter Distance Details

metal roof purlins spacing and rafter distance details

A metal roofing system is a great choice for any building that needs protection from the elements. One of the main components to this type of system is the purlins and rafters.

There are many things to consider when you choose your spacing and distance, and we will go through them in this post!

What are Metal Roof Purlins?

Metal roof purlins are the members of a metal panel system that run perpendicular to the panels.

They provide support for the roof by transferring loads onto the rafters or trusses below them which can then distribute these forces further down through their own structure.

Architecture, Metal Roof

What is a Rafter?

A rafter is a structural member in a roof which runs from the ridge or hip to the wall plate.

It carries much of the weight of the roof, along with stiffening it against wind loads and helping maintain its shape under all kinds of weather conditions.

The rafters sit on top of your purlins and transfer the load from above to your structure below.

They are covered in drywall or siding so you cannot see them, but they help distribute loads through a building’s walls and ceiling!

roof structure with rafters and beams

The Types Of Metal Roof Purlins & Rafters

There are three different types of metal purlins and rafters. Each has their own benefits, but it is important to understand the differences between them when you choose your system!

1. Welded Rafter Purlins – If you want something that is durable and easy-to-install, then welded rafter purlins could be your best choice.

This type of purlin is typically installed with a hidden clip and the rafters are welded to it on site, creating one durable unit!

These types of metal purlins and rafters come in either cold-formed steel or aluminum, depending on your needs. You can even get them galvanized for an extra layer of protection!

2. Open Metal Rafter Purlins – If you need something that is lighter and easier to transport, then open metal rafter purlins could be the best choice for you.

These are made of interlocking channels which are easy to install with a hidden clip system! Most people choose these because they offer good strength at an affordable price.

Open purlins are also easy to cut and customize, so you can get the exact size that you need!

3. Customized Rafter Purlins – Finally, if you want something made completely custom for your project then customized rafters could be a great choice.

These metal roof purlins come in many different materials including aluminum or steel and you can get them in a variety of finishes.

This type of purlin is customizable and easy to install with either fasteners or adhesives, depending on the material that you choose!

Customized rafter purlins are also incredibly durable and will last for many years without any problems.

The Spacing Of Metal Roof Purlins

The spacing of metal roof purlins is another important detail that you need to consider before choosing your system.

In general, the larger and heavier a metal roofing structure is, the more space it needs between rafters or purlins! 

The standard distance for most buildings ranges from 12″ up to 24″. However, this can be customized depending on what you need!

Many building codes and metal roof purlin manufacturers recommend the following:

“Metal purlins shall not be more than 30 inches (762mm) on center when supporting steel framing and 38 inches(965.8mm) when carrying sheathing only”.

Good judgment should be used when spacing metal roof purlins for long spans on light-frame construction.

Here are a few tips to help you out:

  • If the number of purlins used in a span is more than 5, then the center-to-center spacing should not exceed 30 inches (762mm). In this case it’s advisable to use additional purlins.
  • In case the roof spans over 50 feet(15.24m), then the purlins should be designed in pairs by using a single higher purlin at the center of the span to support two lower purlins spaced 30 inches (762mm) apart.
  • For very long spans, it is recommended that one end post every 200 feet (60.96m) be fitted with a bearing plate to support the purlins.

Rafter Distance Details

Many of the metal purlins are used to hold rafters at specified distances apart.

But where the roof has a very heavy snow load, then it’s advisable that you space the rafters closer than what is normally required by the manufacturer.

However, if the building codes allow it, you can always increase your spacing according to your requirements.

Here is a list of recommended rafter spacing distances for metal roof purlins:

  • 3 inches (76.2mm) on center – For roofs having snow loads less than 20 pounds per square foot (psf)(9.1kg/sq.m).
  • 4 inches (101.6mm) on center – For roofs having snow loads between 20 and 40 psf (9.1-18.2kg/sq.m).
  • 6 inches (152.4mm) on center – For roofs having snow loads in excess of 40psf (18.2kg/sq.m).

* Rafter spacing distance generally applies to metal purlins with a nominal thickness of 0.0187 inches(4.8mm) and a minimum yield strength of 40,000 psi (275kg/sq.cm). All dimensions are in inches (millimeters).

Closure

The reason for specifying purlins at set distances is that it allows the weight of the roof to be supported by the full length of each member without overstressing or buckling.

If they are not, then some load will be carried only by the first and last purlins on either side of a span. This can lead to deflection and will ultimately lead to the metal purlins breaking.

Roof Rafters Close-up 2