Roofline Details


Basics: Generally, in the post frame world, trusses tend to follow posts, that is, if posts are set to 8' o.c. then trusses are typically placed atop the posts. This isn't set in stone, however. It's perfectly acceptable to use girders (truss carrying members) such that trusses are spaced at say every 6' o.c. even if posts are set to 8' o.c. This can happen for engineering reasons, or even because you got a great deal on 6' o.c. trusses.

While this may not be the everyday practice you have, there's another reason why we control spacing. If your customer wants a building that is a particular size, say 42' long, and your normal way of doing things is to use 8' spacing... why, that's a problem. How do you handle this? In a perfect 40' building you'd have five 8' bays. But in this case you don't; you have an extra 2'. How this gets handled differs by builder.

What the system allows for is a variety of ways to space trusses so that you can get as close to your desired target as possible. You can choose to evenly space all the trusses with an "optimum" of say 9' o.c. which then gives you 5 bays of roughly 8'5". That assumes you can get trusses of 9' spacing made. If not, you'll use 8' o.c. trusses divided into the 42' length and have 6 bays of roughly 7' o.c. each. Then again, you can always elect to to enforce the 8' o.c. standard and push the remaining 2' leftover space to the end. (This works well for a 6' leftover, which is why this is in there.) Or, depending on openings you're going to place, you can enforce the 8' standard at each end and divide the remaining 10' space leftover in the center into a pair of 5' bays for rigidity at that point.

Additional: As we started out this section noting, trusses tend to follow posts. There are myriad ways to attach trusses to posts (or to truss carriers/girders as the case may be.) The common ways are all covered:

Where it concerns the first and last truss in a structure, there are again multiple ways of handling this:

NOTE: Spacing may not be as straightforward as it seems. Let's say you have a perfect 40' long structure and nominal 8' spacing. Because the structure is 40' you have to account for the size of the outer girts and skin, which generally results in the corner posts being inset by 1.5" The upshot is that the actual working length is 39'9", not 40'.


Purlins of course are spaced and sized according to the engineering, e.g. how much snow load needs to be dealt with, etc. This system allows all of the basics to be dealt with accordingly, and adds some nuances. The primary methods of supported purlin placement are:

These ought to be reasonably self-explanatory.

There's also an allowance for dropping the outer trusses by the purlin height for placing endwall overhangs (typically in hung purlin systems.) This is to be expected.

In addition, you can manipulate the fascia setting such that the fascia board is also the first purlin. And of course you can always denote whether the purlin lies on its face or its edge.

Probably what you'll want to know though is how purlin layouts are approached; that is, what philosophy is used to automate this. Obviously in hung purlins this is reasonably obvious and doesn't matter as much. But there is a great deal of attention paid to this for standard purlins.

Essentially, in any given purlin course, we try to make the first and last purlins the same length, then vary the lengths in between. As much as possible, we try to use as many of the same sizes as we can. This helps when ordering and in the field when constructing the building. As a rule we also try to use the longest possible piece of lumber; this helps keeps the piece count minimal.

The system also automatically places Ridge purlins. Doesn't matter if your building uses ridge venting; you still need a reliably positioned cap assembly on any gable roof.

Here's an example of purlin layout:

Note the staggered layout scheme starts one course with a longer length, the next course starts with a shorter length, and so on. In this drawing the truss Top Chord centerline is shown in green; the dotted lines on each side indicate the size. The purlin actual lengths are shown along with the nominal length needed. In the picture shown, also note that the purlins are egde-on and the ridge purlins (towards the top) are laid on their face.


The system isn't optimized one way or another for material types; it can handle metal panels as well as asphalt shingles (or whatever else you may choose.) The use of sheathing etc depends entirely on your preferences and local code; e.g. in some areas it's acceptable and common practice to put steel panels directly on purlins with no sheathing required.

Let's discuss steel panels. In this product this is the most prevalent roofing type, mostly because of the types of buildings that this product is used to create. Refer to the following picture; the light blue lines are positions of the roofing purlins.

Many roofs will be longer (apex to eave) than the longest steel panel, and you can control the method by which you apply the panels. Yes, the system knows the position of the purlins and can size the panels and their meeting positions accordingly. The top figure assumes that you would use the longest possible piece. The lower figure assumes you will divide the length in half (if so, the nearest halfway point purlin is adjusted to position itself accordingly.) This is all controlled in the file PANELS.INI which is in the program folder. Here's the contents of PANELS.INI:

Layout Scheme: 
--- 0 = full panels when you can, split if chord > max and chord < (2 x maxlen)
--- 1 = full panels = use longest avail panel, add next panel(s) to fit
--- 2 = split evenly = assume extra purlin or moved purlin, split in long. center
--- 3 = split best = split according to where purlins are closest to long. center
Layout Scheme of reduced length panels:
--- 1 = follow main layout scheme
--- 2 = use longest available
Underlap, in inches:
Overlap, in inches:
Lap position:
--- 1 = purlin low edge
--- 2 = purlin center
--- 3 = purlin high edge

Underlap and overlap are used as per the following picture:

And then the "Lap Position" variable is handled thusly:

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