Saturday, December 03, 2005


On the roof, the only place I know
Where you just have to wish to make it so
Let's go up on the roof (up on the roof)

I really dont know why Revit keeps bringing up these old classics ! First it was Zeplin and now The Drifters are doing it !

In case you havent guessed, were going to be looking at Roofs in Revit.
Revits roofs can be wonderful things - fast and simple to model. They can also be a problem, if you dont know some of the trade secrets.

In this first article, were going to look at how roofs are created, or to be more precise, what rules does Revit apply. In a future article, well look at how to change the shape of a roof that doesnt match the geometry you expected.

One of the simplest and fastest ways to build a roof in Revit is Roof By Footprint.
Using this tool, Revit draws roofs based on the footprint of the outer edge and determines roof planes based on a slope parameter. The footprint sketch must contain a closed loop. Inner loops will define openings. All sounds simple and straightforward doesnt it ? Well, it is, but be aware of the process you use to create your roof.

Lets look at a roof with a 30° pitch drawn and a 500 mm overhang with different options.

Option 1 ( Fig 1 ) Roof drawn using Pick Wall, Truss and Extend to Core not checked.


Option 2 ( Fig 2 ) The same roof, same conditions , except that Extend to Core is selected. Note that the roof overhang is now measured from the core walls outer edge. The pitching point is located at the roof level and the outside face of the core wall.


Option 3 ( Fig 3 ) This time we changed the roof from truss to Rafter and unchecked extend to core. Note that the roof pitching point has moved to the intersection of the roof level with the inside face of the wall.


Option 4 ( Fig 4 ) Same as Option 3, except that Extend to Core has been selected. Note that the overhang is measured from the outer face of the core and the pitching point is on the outer face of the core.


So far we have seen the four possible combinations for constructing a Roof by Footprint using the Pick Wall method. The Pick Wall method is the one I recommend to use most of the time, for two reasons. First, the roof construction is what most users expect, since it uses a wall to define the roof pitching point. Second, if you move the wall in the design, the roof will move with it maintaining the same relationship to the wall. Hips and valleys will remain in place relative to the wall junctions if the walls are moved. If the roof sketch is modified, the hip will remain where it was and the roof eaves height will be changed.
Another thing, the Align eaves tool can be used to adjust the roof setout either by matching eaves heights or eaves overhangs if the roof was sketched by Pick walls.
This Align eaves tool can only use matching eaves heights if the roof was sketched by lines.

Lets have a look now at constructing the same roof using the Lines method.

Option 5 ( Fig 5 ) Same roof, same pitch, sketch using offset 500 mm. Note that you are no longer able to select Rafter or Truss, nor is there a choice for Extend to Core.


The most glaring difference is that the roof is now pitched from its outer edge, not from the wall.

So I hope that now Everything is all right up on the roo-oo-oof ).


Anonymous said...

Thanks for clearing up why on some roofs I've not been able to change from truss to rafter.

F Blome

Anonymous said...

Thanks Bruce for explaining this aspect of footprint roof creation.

I have wondered in passing many times where the spring point / where the roof was judged to be pitched from and how it could be (wittingly) altered when desired.
Your blog clears that up in a succinct and useful manner - cheers