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Purlin Question

Started by wng-2, April 25, 2017, 03:23:33 PM

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wng-2

I want to build a barn with a metal roof. How wide should I mill the purlins, and at what distance apart should they be installed on the trusses? Thanks very much.
Rich
Berwick,Maine
Woodmizer LT-15
Kubota L-4400

scsmith42

The short answer is "it depends".

On my farm I have pole barns with trusses installed on 24" - 30" centers.  These structures use 1 x 4 purloins (we also call them "skip sheathing" on 24" centers from the ridge to the drip line.  Typically the uppermost and lowermost purloins are 1 x 6 doubled up (side by side) to provide more strength at the ends of the tin sheathing.

A different barn utilizes reclaimed scissor trusses from a poultry house.  These trusses are installed on 10' centers with 2 x 4 purloins installed on edge on top of the rafters (24" centers).  Around here that is considered an "agricultural pole barn" style of roof.  Since we don't get a lot of snow, we can get by with it.
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wng-2

Thanks, I should add the plan is for the barn to be 36' X 72', with 12 pitch attic storage trusses 2' apart.
Rich
Berwick,Maine
Woodmizer LT-15
Kubota L-4400

scsmith42

Quote from: wng-2 on April 25, 2017, 03:40:25 PM
Thanks, I should add the plan is for the barn to be 36' X 72', with 12 pitch attic storage trusses 2' apart.

With 24" on center you should be fine with 1" x 4" purloins installed on 24" centers from top to bottom.  I'd triple them at the top's and bottoms, or mill some wider ones for those locations.
Peterson 10" WPF with 65' of track
Smith - Gallagher dedicated slabber
Tom's 3638D Baker band mill
and a mix of log handling heavy equipment.

wng-2

Quote from: scsmith42 on April 25, 2017, 04:03:10 PM
Quote from: wng-2 on April 25, 2017, 03:40:25 PM
Thanks, I should add the plan is for the barn to be 36' X 72', with 12 pitch attic storage trusses 2' apart.

With 24" on center you should be fine with 1" x 4" purloins installed on 24" centers from top to bottom.  I'd triple them at the top's and bottoms, or mill some wider ones for those locations.

Thanks!
Rich
Berwick,Maine
Woodmizer LT-15
Kubota L-4400

Larry

Quote from: wng-2 on April 25, 2017, 03:40:25 PM
Thanks, I should add the plan is for the barn to be 36' X 72', with 12 pitch attic storage trusses 2' apart.

I feel sorry for the guy putting that metal down....he won't walk it.
Larry, making useful and beautiful things out of the most environmental friendly material on the planet.

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KirkD

Out here the preferred method is to do a double truss every 12' on the poles with 2x6 Purlins on 2' a center and then sheet the roof with OSB and tar paper before the metal. We have winds here and also it keeps the bottom side of the metal from collecting moisture and freezing / dripping all winter inside.  I would think you would have that problem in Maine also?
Wood-mizer LT40HD-G24 Year 1989

wng-2

Quote from: KirkD on April 25, 2017, 06:21:34 PM
Out here the preferred method is to do a double truss every 12' on the poles with 2x6 Purlins on 2' a center and then sheet the roof with OSB and tar paper before the metal. We have winds here and also it keeps the bottom side of the metal from collecting moisture and freezing / dripping all winter inside.  I would think you would have that problem in Maine also?

You certainly make some good points. I'm trying to avoid sheathing that whole beast though....
Rich
Berwick,Maine
Woodmizer LT-15
Kubota L-4400

paul case

We used some foil backed bubble rolled insulation under our metal roof. It sure helps keep the metal from heating the whole building in the summer. It would no doubt keep sweat from dripping everywhere under your roof and it can be put over your purlins no problem.

PC
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Chuck White

To insulate from the heat and to prevent sweating and dripping during cold weather, I would suggest solid boarding the roof, then put down felt paper, then put the steel on!
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Don P

We do "skip sheathing" pretty often up here in 1x6. Like scsmith double up top and bottom then put down a 1x6, leave a 6" gap, then another 1x6, 6" gap, etc to the top. Tarpaper running up and down the roof as you go between purlins and metal.

Peter Drouin

Quote from: Chuck White on April 26, 2017, 02:57:59 PM
To insulate from the heat and to prevent sweating and dripping during cold weather, I would suggest solid boarding the roof, then put down felt paper, then put the steel on!


That's what I did.


 
Only the part where the mill is. Where the lumber is piled is 1x4 2' oc. no heat over there.
A&P saw Mill LLC.
45' of Wood Mizer, cutting since 1987.
License NH softwood grader.

bluthum

As has been said it depends. Big time.

In my world a purlin is a 2x installed on edge usually between trusses spaced from 4-12 feet o.c.

Boards laid flat across rafters or trusses are called lath and are either 2x or 1x.

In agri buildings purlins have fallen out of favor [I'm glad to say] due to their propensity to leak. It's tough to consistently sink  fasteners through a roofing panel into a 1.5'' wide framing member you can't see. Seems that banks have even figured this out these days as all the agri buildings here abouts are lath [mostly 2x4] these days.

Generally speaking my preference is  a full 1'' board on rafters or trusses up to 24''o.c. 2x4 lath to 3' at most and 2x6 lath to about 4'. Maybe a little more with really good 2x6. That's mostly a worker safety issue rather than final roof load requirement. Step on a lath board with a big knot over spanned and it could break. Not good.

Lath spacing up the slope is another issue and depends on the roofing type but maybe 3'.

I always look this stuff up before doing a job rather than relying on my memory or judgement.

wng-2

Lots of good information, thank you.
Rich
Berwick,Maine
Woodmizer LT-15
Kubota L-4400

newoodguy78

It might be worth seeing what the metal manufacturer recommends. Different metal requires different spacing. Screw down corrugated they usually recommend no more than two foot on center. Standing seam they usually recommend one foot on center.

With a roof that steep I would cut the purlins at least an in and a half. Makes walking on them while installing incredibly easier.

As for the metal condensating and dripping install a vapor barrier directly under the metal and you won't have that problem. I use a roofing underlayment. If you use a 4' roll and 4 inch purlins space them at 22" on center. That way you'll be able to get it good and tight. Especially if it will be seen from below.

What brand of metal are you going with?

wng-2

Thanks, good info. Not sure what brand metal yet. Will have to check with my builder, still in the planning stages. Just trying to look ahead so I can mill the appropriate size materiel.
Rich
Berwick,Maine
Woodmizer LT-15
Kubota L-4400

sandsawmill14

we always use full 1x6 every 24" and the truss or rafters are 24" i have went 30" on centers but probably will not again  :) you never now with the big snows we get every few years some times we will get 4-5 inches :o :D :D :D
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Remle

Quote from: sandsawmill14 on April 28, 2017, 07:00:35 AM
we always use full 1x6 every 24" and the truss or rafters are 24" i have went 30" on centers but probably will not again  :) you never now with the big snows we get every few years some times we will get 4-5 inches :o :D :D :D
The problem with THIN/WIDE purl-ins is that they ten to cup up or down and leave raised or dimple spots on your roof as time passes. My preference is 2X4, it has more strength to resist sagging and less chance to cup for a little more BF of material. As they say each to his or her own method.

jaciausa


Purlins- Using the design you have chosen of 12 pitch trusses 24' oc, snow or load will not be a problem. I would go with 2x4's flat. That should make walking and screwing the metal very easy when doing the installation. I believe they should be kiln dried or air dried before applying metal.

The sweating will be a problem if you are heating lower level with no insulation between floors. I would check with suppliers and builders in area for a plan that has worked in your area.

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