Roof layouts

I recently started a new supermarket renovation project and came to the point of putting the existing roof onto the building. I’ve done flat roof’s before but this time I realized I still had gaps in my knowledge about how Revit goes about this. I always got the roofs just close enough to look right but still had some errors in the model that could cause problems down the line. So as any technologist would do I searched the internet for any tutorials I could find. The best tutorial I found was at CAD Clips, but it was for Revit 2009 and still didn’t cover a few points that I was missing. In fact, most of the tutorials I found were missing the few points I needed.

I’ll be using RAC 2011 for this post as I work in the architectural field, not structural. So if I miss any important structural items, you’ll know why. Why do I bother modeling the structure of the building? The first reason is; the structural engineer on this project doesn’t use Revit so I can’t get him to do it. The second reason is; between reflected ceiling plans, wall sections, building sections and interior elevations there are many views which will be showing the roof structure and so taking the time to model it means you don’t have to worry about drafting it into a dozen views later on. Remember, if your only going to see it in one view, it’s not worth creating a complex model for it.

How to layout an OWSJ system….completely.

Step one – layout the supporting beams. You don’t need to start with columns first. In fact, in this case it is desirable not to because the columns need to attach to the beams, not the other way around. This is because the beams cantilever past the columns, a trick used by structural engineers to reduce the span and thus the required size of beams. See the image below.

Roof plan

Step two – layout the beam system for the OWSJ’s. However, instead of using OWSJ, I’m going to begin with 2×10 dimensional lumber. This is because the computer can render the simple geometry of a 2×10 faster than the complex geometry of an OWSJ. Later on I can set the joist type to an OWSJ. The screen-shot below shows a section through our building. Nothing special so far, the roof is indeed flat (too flat) and the top of the structure is all at the underside of deck level where I laid it out.

Building Section

Note: when I laid out the beam systems, I used the “Pick Supports” tool to select the beams and walls that the joists would be bearing on.

Step three – lets add some slope to our roof. I’m going to add a 6″ slope to the roof with the low points at the beam ends. The first time I laid out a roof like this I would end the beam over the column and use the “Start\End Extension” parameter to set the cantilever distance. However this makes setting the slope more difficult as the “Start\End Level Offset” lowers the beam at the column point and not at the end of the beam like I wanted. It is also important to take into account the seat depth of the OWSJ at this point. The beams don’t sit at the underside of the deck, they sit below the joists and the joists sit at the underside of deck level. I’m going to use the DLH Series Bar Joist that comes with Revit. It has a 5″ seat depth. So I need to drop the beam end at the concrete block wall 5″ and the beam end near the columns 11″ (6″ slope + 5″ seat depth). See image below.

Roof framing plan

Hints: You can modify the Level Offset of 2 beams at the same time if the beams have a common end point. Just select both beams and the offset will appear at the common end. Also sometimes you will have to adjust the beam cutback when you are adjusting the level offset, to avoid the condition shown in the image below. The beams should end where the arrow indicates but a beam cutback has forced on beam back. Just activate the “Column\Beam Join” tool in the Modify -> Geometry panel. You’ll see arrows appear next to the beam ends, when you click on one of the arrows the beams should act normally. See image #2 below.

Image #1

Image #2

Step four – once the slope has been added select the beam systems and change the joist types to the OWSJ. Looking at a section through half the building (see image below) now we can see that the beam end near the wall is at the level of the underside of the deck but the other end is at the same height as the beam. This is incorrect because OWSJ’s sit on top of the beam. To fix this select the beam system and add the seat depth (5″) to the elevation parameter. Now the OWSJ’s sit on top of the beam but the end at the wall is 5″ above the underside of deck level. Still not quite right. The last step required is to select all the OWSJ’s in the beam system, unpin them from the system and deduct 5″ from joist level offset at the wall end. Looking at the section again shows the OWSJ’s now slope as they should. See image #2 below.

Image #1

Image #2

Note: It is more efficient to wait until the end of step four to change the joists from 2×10 to OWSJ’s. Even this little project took a minute to regenerate when the beam system slopes were adjusted.

Step five – add the columns. Just drop them at the grid line intersections and then attach the top to the beam above them and the column height will adjust automatically to reach the bottom of the beam.

That’s it for now, next week I’ll put a sloped roof assembly on the OWSJ’s and show how to add a deck profile so all your wall sections show the metal deck without having to draft it in after.

PS – if anyone wants a copy of the project file I created in today’s post, send me an email.

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Revit and Building Information Modeling