There are several factors to keep in mind when matching overhangs (fascias) on the job. In order to obtain an even fascia band when more than one roof pitch is used on your home, there are tricks involved. Roof trusses and conventional framing follow different rules, so in the following calculations, we will use roof trusses for the example. Further, we will assume all match up situations incorporate a 2×4 top and bottom chord and that the overhang extensions equal 12 inches on all set ups. The top chord size is critical in matching fascias, so if a top chord size changes for any reason, adjustments must be made accordingly. We will not take the width of the fascia board into account for this example. As a rule "1x lumber" is used for fascia board and is 3/4 thick.
Trusses are cut with a 'lug' on the bottom chord to allow for a proper scarf cut. The lug cut in addition to the 'plumb' of the stick equal the 'over-the-wall height' (OWH). The standard lug is 1/4 (*) and will adjust as required to raise the heel of the trusses in your match up scenarios. (**) If the lug exceeds the depth of the bottom chord, wedges or webs may need to be added to make up the difference. If this is the case, then a lug is not actually cut on the bottom chord at all. For the sake of illustration we will refer to the lug as the difference between the plumb dimension and the OWH regardless of the circumstances.
The 'match point' is usually (but not always) determined by the lowest pitch roof you will be working with as is the case in this example. This example also indicates that the match point (the top tip of your top chord) falls below the top plate line, and therefore, is a negative number. For our example we will use 3 different roof pitches. All bearings will be at the same height which we will set at zero for this example. In reality, plate heights may vary on the job. If so, these differences must be addressed in the vertical height differences of your match-up scenarios.
The 'plumb' of the stick = the vertical offset distance between the top and bottom edges of the lumber used for the top chord. It will vary depending on your roof pitch (slope). The higher your pitch, the deeper the plumb cut.
Plumb + Lug = OWH
Drop – OWH – match point = lug
OWH = Plumb + Lug
Drop = Y / 12 (when over hang = 12, Y = roof pitch). Otherwise Y / overhang distance = Pitch / 12: where Y = the overhang drop.
For our example here we will use 6/12, 8/12, and 12/12 pitches.
Begin with the 6/12 scenario (lowest pitch).
Plumb (+) Lug (-) Drop = Match Point
3-15 / 16 "(+) 1/4" (-) 6 "= (-) 1-13 / 16" (*)
Now that we have the match point the other match ups will be worked backwards, with the lug becoming the unknown factor.
Plumb (-) Match Point (-) Drop = lug (*)
4-3 / 16 (-) -1-13 / 16 "(-) 8" = 2 "
Plumb (-) Match Point (-) Drop = lug
4-15 / 16 (-) -1-13 / 16 "(-) 12" = 5-1 / 4 "(**)
* Used to determine the match point. (Note that the match point falls below the top plate in this example resulting in a negative number, therefore it is added to the plumb, rather than subtracted from it.
** This heel needs a wedge between the top and bottom chords for plating purposes, and so that the two members meet. The square of the stick (depth of a 2×4 horizontally) is only 3 ½. This leaves a gap between chord members. Your truss fabricator will determine the size of the wedge needed to transfer the loads through the adjoining members. In the meantime, you just need to be aware that top and bottom chords do not always meet.