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Creating a parametric rafter

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One of our Forum members recently asked how roof rafters could be modelled in Revit. He wanted the ability to be able to model the roof structure in detail and also produce schedules directly from the model. Modelling roofs and their structure is an interesting topic. There are various methods of representing roof constructions within Revit- and just like anything else, it depends on how much detail you need and what you wish to do with the information. For some, just drawing the roof construction using detail lines will be adequate. For others, a full 3D model of the roof structure (and all the inherent data that goes with it) is required. Anyway, back to the focus of this article. The parametric rafter was relatively straightforward to produce. It was formed using the Family Editor and was based on a “Generic Model” template.

The first thing I did was set up the Reference Planes that would control the geometry of the rafter

The reference planes are the green dashed lines. These form the skeleton that the geometry is fixed to.

In the image below, you can see the parameters that I have defined for this component.

All these parameters are Instance Parameters, which means that we can change their values for each instance (ie each separate rafter) in the model.

Now I add the geometry- there are two parts to this. The first is a solid extrusion to form the rafter itself…



And then a Void Extrusion to “cut out” the birdsmouth notch from our (previously created) rafter.



With regards controlling the thickness of both the solid extrusion and the void extrusion, I set up a reference plane (vertically)- in which I sketched the profiles of the solid and void. I could then set the extrusiuon properties to start the extrusion at 0 (which means the start of the extrusion is coincident with the reference plane it is sketched on) and the "Extrusion End" can be set to the parameter Rafter Thickness



Look carefully at the Extrusion Start and Extrusion End properties. Notice how the value for Start (0) is shown in black, while the value for End is shown greyed out- also notice the small = sign at the end of the "Extrusion End" line. This is because I have set the value for Extrusion End to a parameter, rather than an absolute value. Extrusion Start will ALWAYS be 0 (ie will always be on the same reference plane in which it was sketched), while Extrusion End will always be the value of the parameter "Rafter Thickness"



OK, so that’s the rafter. Now let’s load it into a project and see how it performs…



Here’s our rafter sitting onto of a wall. There are 6 instances of our component. If I pick one of the instances and look at the Element Properties for it….



You can see that we have access to all the parameters that we created. Just to prove how flexible the component is, I will change the values of these parameters for each instance. Here’s the result…..



I can’t imagine a roof formed like this! But is demonstrates how flexible our family is. Now, the only thing I haven’t done here is use Shared Parameters. If I had, we could have then produced a schedule of rafters- automatically extracting the values for each shared parameter, for each instance of the component.


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