Beware the roof spec numbers when configuring unequal pitches to have common fascia height

[GeneDavis]

Happy Easter, everyone!

 

I have been having a dialog with the truss designer for the project plan attached here.

 

A cross-gabled roof has two side bays of slightly lower pitch intersecting a center bay, the center bay's ridge just 2-1/4" higher.  Fascia heights are to match, and the fascia boards are 1x8s.  3/4 x 7 1/4 to be precise.

 

I gave Mr Truss Guy the heel heights, pitches, and overhangs I derived from the Chief model I made, and I used all the Chief roof spec tools to get a model in which the fascia heights match exactly. 

 

I did not give him the fascia height.

 

He built his truss model using his software, and came back saying the fascia heights did not match, and to study things, I used Chief CAD to see what's happening.

 

Well, here seems to be the problem.  Listen carefully, as there is some detail here.  I always thought the Chief roof plane dialog, showing us the numbers for heights at ridge, baseline, and fascia, that the dialog is returning us points in 3D space that are co-linear.  We all know about how a roof plane is manipulated with Chief, using the lock function.  You lock fascia height, and that becomes the pivot as you change one of the other variables, either pitch, baseline height, or ridge height.

 

Or you lock pitch, and change one of the other variables, either facia height, baseline height, or ridge height.  Whatever your preference, the roof plane is rotating as you change a variable.  And I always presumed all those height points were co-linear.

 

And it was with that thought that I used CAD to see what Mr Truss Guy was saying.  I drew the two cross sections of my building in CAD, and using the roof overhang distance, fascia height, and ridge height, the two heights right from the Chief model, sought to proof the heel heights.  And that is when I saw the bust.

 

When Chief generates the fascia and places it in 3D space, it is using the roof sheathing spec for thickness, and the fascia spec for thickness, to place the fascia so its outer top edge is co-linear with the top surface of roof sheathing.  Chief is returning the true fascia height when it shows the figures in the roof plane DBX, and the thicker the sheathing, and the thicker the fascia, the more not-co-linear that fascia tip becomes, relative to the roof plane line.

 

Mr Truss Guy did not tell me what fascia height his software generated with my specs, but now that I understand what Chief is doing, I'm going back to him and specifying only heel height and ridge height for the two roofs, and explaining that with my thicknesses of sheathing (5/8") and fascia (3/4") and logic that roof overhang is to outside face of fascia, he can be assured that we will have the common fascia height we need.  

 

 

 

Fawn_Ridge_Downsized.plan

[robdyck]

It's very important for anyone using the Chief roof dbx to remember to distinguish between "sub-fascia" and "fascia". I never look at that POS setting, baseline height and pitch only.

The "fascia" top height shouldn't even be a spec in the roof dbx...it should be "sub-fascia". 

In the meantime, enjoy adjusting your roofs based on the height of the finish material!

[GeneDavis]

I disagree.  It's fascia, the finish, that you want to match, and I am glad it is there.

 

If Chief did not give us the ability to lock fascia and rotate from that point by setting either pitch, or one of the other elevation points (baseline or ridge), we would have to work harder to do the match we need when dealing with roof configs with differing pitches.

 

You just have to realize, when doing the lock on fascia, how it will affect, say, baseline, if you change pitch or ridge.

[robdyck]

I'll split the difference with you and say that a sub-fascia height lock should at least be a choice. 99.9% of houses in Canada have aluminum fascia trim; from a dimension standpoint its essentially non-existent. And when utilizing different roof pitches, it is imperative that the sub-fascia height be aligned. The soffits are then level, as they are under the sub-fascia.

For houses where there is no finish fascia, like so many houses in the southwest that just have the sub-fascia painted, the sub-fascia height would also be more logical.