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Question DetailsAsked on 11/23/2015

How far apart do you space furing strips when putting on a metal roof.

When putting on a metal roof how far apart do you space the Furring strips. Thank You.

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1 Answer


You need to check the installation instructions from the manufacturer - depending on whether the purlins (firring strips) are 1x or 2x material and how wide, thickness (metal gage) of roofing, steel or aluminum, type of seams, design wind and snow loading, whether designed to be walked on or not, etc I have seen it vary from as little as 4 inches clear gap between firring strips on lightweight aluminum to as much as 4 feet on very heavy gage steel standing seam roofing - commonly around 1-2 foot for many residential applications and brands.

Also pay attention to how they present the data - the maximum allowable gap between firring strips relates to the ability of the roofing material to resist buckling or crumpling under snow, wind, or walking loads. However, the screw placement diagrams commonly are based on the on-center spacing (center to center of the purlin) rather than the gap between purlins, to determine the length and spacing of the screws needed to prevent wind blowoff for various design wind speeds, so pay close attention to that difference.

Also pay attention to whether they are talking the screws just going through the purlin, or also into the underlying sheathing - some provide both options in their tables, some only assuming the purlins are holding everything - which if using 1x material can make for very close spacing, because the pullout resistance of screws in 1x material is pretty low.

My personal recommendation having seen too many blowoffs - never use 1x material less than 4 (nominal) inches wide (splits too easy when screwed into) and preferably use true 1" or 5/4 material if possible. Best of course, especially in severe Chinook/Williwaw/Santa Ana/valley funneling wind areas and in thunderstorm/tornado/hurricane areas (and required by code in some areas) is using 2x4's.

One other thing that is ignored by most installers, is by putting in purlins you are blocking downhill flow of any water that penetrates and gets on top of the water barrier - you should cut water slots in the bottom of the purlins every 4-6 inches. Just a saw kerf will do though I prefer a smoother, less stress-concentrating broad dish-shaped router cut - but with a 1/8" or so cut into the wood that additionally reduces the wood strength and screw holding-power of nominal 1" think (3/4" actual) wood.

Pay attention also to the fastening of the purlins to the roof structure - if they are not fastened down well enough to carry the design uplift load the roof can come off purlins and all ! And the purlins need to be fastened securely into the rafters/trusses too, and for some wind loadings the decking/sheathing needs to be screwed down to the trusses/rafters too, to be able to handle the uplift loads without lifting up, so check your local/state code requirements.

Answered 4 years ago by LCD

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