Purlin support or not?

On the below pic. is an attic space over a carport. Since there are no support walls should purlin braces be installed or not? This is a 1989 home in the stix.


this one may have come out clearer.


A purlin is used to “reduce the length of the rafter”. So if the span of the rafter is within the allowable limits no purlin is needed. If a purlin is used to “reduce the rafter span” it must be supported at least every 4’ and the purlin must be equal in size or larger than the rafter. The purlin in the picture looks like it it is not being used for support but as a “hanger”!

Richard, from the first picture the rafters don’t look overspanned and so, is not in the need of purlins. But they do need rafter ties in this frame to prevent ridge sag and bowed out exterior walls. The joists aren’t serving as rafter ties since they are not running parrellel with the rafters and tied to them. So at every 4’ a minimum 2x4 should be run parellel with the rafters in the bottom third of the roof frame area and fastened to the rafters. Might as well set them on top of the joists (on edge), fastened to also serve as strongbacks to help prevent twisting of the joists.


Looking at the spacing of the ceiling joist I am estimating that your clear span exceeds 10 feet (I counted 8 spaces at 16" on center which give you 10’-8")…typically 2x6’s can have a clear span of 8’ - 9’ depending on slope of roof and your dead and live load. If my guestimation is correct then purlins should have been installed.

Also, I see a 2x4 supporting the ridge which I suspect is at a ridge joint…that 2x4 should be a T-brace if its under a joint…if not under a joint than it was probably used as a temp while framing.



Jeff, good point with your span calculation. I didn’t think it was that much of a span. I guess because of how steep the roof is in the pic makes it not look as wide of a span. Will you site the source of your 8’9" max span. Because with a comp roof (10 psi dead load) on this structure it would have a 11’- 9" allowable span for #2 SPF, 2x6 w/ 24" on center spacing. Table R802.5.1(1) 2006 IRC . Are you figuring heavy snow load? Because here in San Antonio and Houston where Richards at we don’t get snow.

Joe, which IRC are you using? When they talk about span they do mean the rafter run don’t they.
Never rellized before that the book calls a rafter span. Span is total width of both runs of the rafters.
Isn’t that confusing or is it because it is not in French. :mrgreen::wink:

Hey Marcel buddy, you’re trying to make me dizzy again. Or is it that I make you dizzy. :mrgreen: Isn’t it that the span of the roof is the total of the 2 rafter runs. And each rafter can span as per- span charts. ??? Help !
The IRC that I am looking at and was only to verify what I thought , is the 2006 IRC pg 251 and the table is as mentioned above. So wheater it’s run , ran or span the rafter in this case can go unsupported 11’-9".

That is what I said. Why do they call it rafter span when span means total width of the roof and 1/2 of that is called rafter run.
Isn’t that confusing or what?


Tornado warnings for the Northeast right now. Hold on to your hats. :wink:
Hope your weekend is off to a great start.

Spans are based upon size of lumber, grade of lumber, species of lumber, spacing distance, dead/live loads for your area and type of covering and slope. You can do a google search using the search parameters of: rafter span chart. The numbers I gave I believe were for SPF # 2 at 16" O.C. with a dead load of 10…live load of 40 with a slope greater than 6/12.


Marcel, I agree thats always been confusing (very) ??. Jeff, why would you calculate a live load for rafters/roof? also you forgot to mention snow load in your last post(#10) which we do not have in south Texas. You went to google why not go to IRC or AWC ? I stand by my point, that 8’9" is way to short of a span allowable for rafters especially since you say 16" oc, as these I figured at 24" oc for 11’9". 16 " inch on center would be 14’4" span, run, ran :). Just sayin.
Marcel, I did fib to you the other day, saying we were going to get 100 degrees this week. So far only low 90’s. Weather men, their like inspectors, giving their opinion.

Maybe Jeff dosen’t have one. :wink:

Span charts off the Internet are all different and to many variables to deal with.
Just pulled one out for chits and giggles and it is different than the IRC.

Design Specifications:
Slopes Less Than 4:12, Lightweight Roofing
Dead Load = 10 psf, Live Load = 20 psf (Residential)
Douglas Fir - Larch Framing, Deflection of L/240 maximum.
Allowable Spans for Douglas Fir Roof Rafters Size of
Roof Rafters
(inches) Spacing of
Roof Rafters
(inches) No. 2 Douglas Fir No. 1 Douglas Fir
a ceiling Not supporting
a celing (-2.5#) Supporting
a ceiling Not supporting
a ceiling (-2.5#)

2 x 6 12 13’-6" 13’-10" 13’-9" 14’-2"
16 12’-3" 12’-7" 12’-6" 12’-10"
24 10’-8" 11’-0" 10’-11" 11’-3"

2 x 8 12 17’-9" 18’-2" 18’-2" 18’-8"
16 16’-2" 16’-7" 16’-6" 16’-11"
24 13’-10" 14’-5" 14’-5" 14’-10"

2 x 10 12 22’-8" 23’-4" 23’-2" 23’-10"
16 20’-7" 21’-3" 21’-0" 21’-8"
24 16’-10" 17’-8" 18’-0" 18’-10"

The dead weight or dead load on the average of a standard roof is about 6.3 lb.'s. and 10 is used or 20.
Considering the safety factor on dead loads and live loads, framing would have to be very deficient structurally before I make note or recommend an SE which I never do anyways.
This is way beyond the duties per the SOP anyways, but always fun talking about it.
One would be surprised how much load a framing member or roof member can carry before failure.

Plus we are not Code inforcers or engineers right.?
I just like to do it like the engineer wants and then tell him he _ucked up. :mrgreen::wink:

Naw, I wouldn’t do that;)

Which still would work for this. Marcel do you still have your 50’s rafter span little blue book that framers kept in their back pocket. I’ve seen one of those. :slight_smile:

You mean this one?:):wink:

Marcel, thats great, can,t stop laughing. That actually looks pre WWII.:mrgreen::mrgreen:. But actually when I first got out of college in the very early 80’s and became a construction superintendent, the framers really did have a little blue book. I think it was rafter and joist spans, but they never let me see it. They always did invite me to walk the top plate with them though. I didn’t walk the top plate:mrgreen:


I use 10psf for dead loads (loads that are static, ie lumber, shingles rafters, etc.) and 40psf for live load (loads which are not permanent, people, snow, furniture etc).

I have a span book for American and Canadian lumber which I carry in my truck… haven’t really used it that much simply because I use SPF to build houses… don’t need to mess with other species.

As to google, I am simply stating that there are many charts and calculators… the website I used was AWC.
I used 10 as a dead load, 40 as a live load (which doesnt just mean snow)…2x6 at 16" O.C., SPF (southern), 360 deflection, grade 2 (which is the most common grade used), no exterior exposure. It calculates out to 8’-7"

Again, I did not inspect it and simply taking a swag based upon your limited pictures and not knowing the type of roof covering, sheathing, etc.

:slight_smile: :):slight_smile:

Joe, that little blue book belonged to my father years ago. That is all he had at the time. I used it for years as a young framer until calculators came out and gained the education to use the darn things. :wink:
Just keep it for sentimental value now. Makes me laugh everytime I see how much he paid for it. About the price of a cup of coffee today. :mrgreen:

AWC use to give a small hand book away to GC’s… I got several from them.

Jeff, that must have been during the days when GC’s new how to build, now they are all Brief Case Contractors, so they have no need of those books. :mrgreen: