Modeling roofs for trusses???

[Anonymous]

I’m curious how the rest of the ArchiCAD community models roofs when they are structured with trusses? ArchiCAD’s roof tool is fairly powerful but does not have a good interface for “properly” defining roof planes relative to trusses and/or heel heights.

Modeling roofs for “stick frame” or basic rafter construction is very straightforward. Set the thickness of the roof to match the rafter dimension + roof sheathing – something like 5 ½” + ½” for a simple roof. Now, simply set your reference line to the inside of the stud wall and model away. The plate height (to story x) is easily set at something like 8’ (or 8’1 1/8” for more precision).

However….when modeling roofs for trusses, it’s not so straightforward. Composite thickness does not apply. I set the roof thickness to match the top cord dimension + roof sheathing – something like 3 ½” + ½”. I set the reference line to the outside of the wall stud. Now I need to adjust my (to story x) height if there is a special heel or raised heel condition. Nothing in the roof tool dialogue represents this kind of adjustment so I have to manually calculate the proper (to story x) every time – arghhhh! Its’ never a clean number so its always a problem if I am trying to properly represent the actual roof in relation to the rest of the building.

Note: If I am using a cantilevered truss with the bearing point/height equal to the eave/soffit height, I might place the roof reference line all the way to the outside edge of the roof.

This has always perplexed me…I’m curious what others are doing when modeling roofs for truss conditions.

Thanks,
Dan K

[Jere]

i'm barely considered and intermediate archiCAD user (i'm trying!) but here's what I do.

It's already been mentioned, but I model just the roof sheathing. I take the time to trim it nicely so there's no overlapping areas. I set the pivot lines to the outside face of the sheathing/stud and set them on top of the wall. Then in section I just move it up exactly to the heel height i need. Or you can simply set the height in the roof settings. Easy as can be. I haven't figured out my preferred method for modelling rakes yet, but i've used the roof tool, rafters, and created .gsm parts (the best way i think). On a hip roof, i'll just make a custom profile for the fascia (i usually do double fascias) and draw it with the wall tool.

For the gable ends, i model small roofs for the lookout rafters then trim the gable walls to these roofs. Looks good in section and model. Usually i'll just draw in the trusses in 2d for the construction documents.

[Jefferson]

Here's my response to you points:
1. I agree, ArchiCAD gives us some powerful tools to explore/document design issues.
2. Again, I agree...better to explore and find discrepincies before they arise.
3. Hmmm, I don't know if this applies to everyone...I get paid to do builder's set... anything beyond is money out of my pocket. I think this gets back to your first point about modeling it because "we can" If I'm not paid to do it I can't afford to do it. I know some/most clients expect it, but I guess it centers around what your contract actually requires.

Good point Dan! Perhaps the reality is I'm over-working it to cover my a-s-s [censor that!], as best I can, to insure everything really will work. Puts me in the position to point to the plans, show it was all figured out, clearly represented, and ask what happened? Really beats being held accountable for a discrepancy.

I do everything on a hand shake and most of the builders are friends............small community where your reputation is everything. If I were to have a couple of catastrophes, that reputation is toast Then there's the whole pride thing............