To top it off

Back to the roof of the Big Box Blog Store. In last week’s post, the structure for the roof was setup and properly sloped to future drains so that sections would display properly. In today’s post, we’ll add a roof assembly, sloped of course, then I’ll show how to add skylights without affecting the slope lines on the roof , adjust the roof structure for the skylight and finally add a clerestory skylight. I’m going to skim over the easy stuff. I’m assuming everyone reading this post knows how to add a basic flat roof by footprint, for example. Instead I’ll focus more on the extra steps it takes to get the roof looking like it should. Those steps usually not taught in the Revit courses.

Step #1 – Adding a Roof

  1. Select the “Roof by footprint” tools and use the “Pick walls tool” to select the walls. Make sure defines slope checkbox is unchecked. This is because flat roofs are not defined by slope (4 in 12) but by datum elevation points that are relative to the underside of the deck (-6″).
  2. Make sure roof assembly has no “variable” layers, unless using sloped insulation. Otherwise only the variable layers will change and not the entire roof assembly like we want in this example.
  3. Select the roof and use the “Add point” tool in the “Shape editing” panel. Specify a -6″ elevation that is relative to the current level and place a couple points where the beams along gridline ‘B’ end.

In the images below I show the final roof layout. The elevations I show are noting the top of the roof assembly instead of the underside of the deck but you can see there is a 6″ slope. You can always reselect the points you added and change the elevation at a later time too.

The Roof Plan

Building Section

Step #2 – Adding the skylight opening

There’s not much to this step but it is important to use the “Vertical opening” tool instead of adding a hole in the roof boundary. This is because the roof slop should continue past the skylight uninterrupted. By adjusting the roof boundary, Revit will make the border of the skylight at 0″ elevation which will mess up the slope lines. The “Vertical opening” tool is applied after the roof is created and so doesn’t affect the roof slope (it acts like a void extrusion). The images below demonstrate what I mean.

Skylight opening by roof boundary (wrong way)

Skylight opening by vertical opening (right way)

Step #3 – Adjusting the roof structure

You probably noticed that the skylight opening exposes 2 joists below the roof. The architect doesn’t want these joists blocking the light so we’ll need to adjust the structure supporting the roof here. Revit allows for this without too much effort or MATH. The last thing we want to do is to be using trigonometry to calculate beam vertical offsets on a slope that is less than 2%. Well put away the calculators you don’t need them.

In this step we are going to put W16 beams around the skylight opening and then replace the OWSJ’s that run through the skylight with W12 beams. See the image below.

Final skylight structural layout

  1. Add W16 beam’s to support the end of the joists at the skylight. Make sure you check “3D snapping” when placing the beams, Revit will attached the beams to the joists at wither end of the skylight opening. However, we need to raise the beams 5″ to match beam system offset which we added last week to adjust for the OWSJ seat depth. No problem, just use Z-direction justification parameter on the W16’s. Set the justification to “Other” and then set the amount to +5″. See no math required. The top of the W16 beams now matches the joists on either side.
  2. Now we need to edit the beam system so the joists don’t run through the skylight. Before starting change the joist type to 2×10 joists (this will increase regeneration speed) in the beam system properties. Next edit the boundary to create a boundary hole around the skylight. Use the “Pick supports” tool to select the W16 beams and connect these lines with boundary lines to create a rectangle. When you click finish you’ll see the 2 joists at the skylight will become 4 joists, 2 on either side of the opening.
  3. However something is not quite right. If you look at the section below you’ll see the 4 shorter joists come in at the default height, not sloped like our other joists. Toggle 3D constraint on beam system to return joists to proper seating, that is check the checkbox and click apply and the uncheck the checkbox and click apply again. Your 4 joists will then look like the second image below. Still not quite right.
  4. Simple to fix this just unpin the 2 joists at the wall and drop end at wall -5″ again. Revit pins these joists when boundary is edited. See still no math required.
  5. The final step is to switch the 4 joists to W12 and switch joists at ends of opening to W16. The you can select the beam system and change the default joist type back to the OWSJ. The joist’s you changed to W12’s and W16’s will remain unchanged.
Joists at default height.

Joists at default height

Joists after 3D toggle

Step #4 – The clerestory skylight

The clerestory skylight is nothing special. I just added four walls around the skylight opening and then another sloped roof aver the walls then I attached the top and bottom of the walls to the appropriate roof assembly and added a window in one side. One thing I wanted to point out when I attached the bottom of the wall to the main roof the insulation on the roof was automatically cut back for me by Revit. The structural layer of the wall is connected to the structural layer of the roof assembly. The image below shows the finished skylight.

The finished clerestory skylight in section


In last weeks post, I mentioned that I would also be adding a decking profile to the roof assembly so all the sections would show this correctly. You can’t add deck profiles to roofs yet, only to floor assemblies. Hopefully, Autodesk will add this soon.

See you next week.

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Revit and Building Information Modeling