There is a C Purlin family available in the OOTB content. "Light Gauge-Joists.rfa"
Use a Beam System for the purlins and either use the level as the workplane for the system, or if its offset from the level place a reference plane at the appropriate level and name it, then set that as the work plane. In that circumstance I would also have 4 different beam systems as the purlins obviously won't intersect the rafters.

HTH.

I'm guessing the Rafter is sloped? If it is, create a workplane following the slope in section or elevation view. Name it, and use it as a workplane for the Purlins. Remember though: When you start the Beam System command, you FIRST need to assign the Workplane before closing the sketch. Once you've closed the sketch you cannot change the workplane anymore into an angled workplane (or at least it'll be very difficult)

When I model something like this, I use a reference plane.

Usually, I draw this as the "Top of Rafter" reference plane. When I place my rafter I make sure that it places associated to the top of beam (which you can set in properties before placing)

Now when I place my purlins (or floor joists) I know that I will cleat them 10mm above my rafter, so simply add in +10 above reference plane when you place them. Additionally you want to place these relative to the BOTTOM of the beam - as opposed to your rafter which you placed from the top.

This way, should my roof level move my purlins move when I alter the reference plane location, along with my rafter and reduces the amount of work I need to adjust.

Additionally, by setting the rafter to model DOWN from your reference plane, if for any reason later the engineer changes this rafter from (for example) a 410UB to a 610UB, my rafter depth adjusts, but will not push my purlins around (as it would had you modelled your rafter from the underside)

Revit is often about thinking up the best method that suits your situation, think 2 steps ahead and think how things will be affected should a level, or member size change. You can save yourself alot of rework by doing this.

Hope this makes sense.