Revit to CAD - Nothing to Fear

We are well on our way to our goal of 50% DD on our new 'GA Apartment' BIM project, with a well developed model, fifty sheets set up, and the first round of CAD backgrounds issued to consultants. 80 hours.

Our largest BIM housing project so far was nearly derailed during a kick-off meeting with the owner's MEP consultants. I was not present (perhaps I should have been) but it was reported that these CAD based designers were upset about the extra effort involved in working with CAD files exported from a Revit model. One claimed that they would need to add 4 weeks to their schedule because of problems they have had with some other architect's BIM output. Of course this is just fear the unknown - were have not yet provided anything. (Owners take note - if the work were being done with BIM, this would be a non-issue.)

It was stipulated that unless we can provide backgrounds and sheet layouts that look exactly like our standard 2D files, the project will revert to CAD. I did not hesitate to guarantee that will not happen. I further guaranteed that the export will be better than, not equal to, the typical 2D output.

Internally, we had previously discussed that the 'OOTB' Revit export setup produces files that conform to the AIA Layer Standards, which is also the basis for our office standard. To placate those who cannot seem to conceive of walls with finishes, we had further decided modify the export setup to place the finish material line work on separate layers, allowing these items to be hidden or deleted.

The main problem with CAD standards is they DO NOT WORK. We have a very well documented office standard that defines layer names for every type of "object" in the 2D world. We also have a table listing all of the approved line colors and their corresponding line weights. I randomly chose one base plan file from the project's "xref" directory and easily discovered violations of both these "standards". Non-standard layer names and colors.

With the typical "x-clip/xref" methodology, there are literally hundreds of layers to manage - one set for every instance of every unit plan, plus many other random layers - more than six hundred layers in all.

The CAD exported from the Revit model is completely consistent and reliable. The options for CAD export are explained very well by Steve Stafford in this Revit OpEd post,

"It is important to understand that the intended purpose for exporting to DWG is to create files that can be used as a background for other trades. It was never really intended to be a better way to make DWG files."

In the export dialog Revit categories are mapped to layer names and colors in CAD. I actually had to "dumb down" the standard export setup, combining items that would normally be on separate layers.



For export to our specific office layer standard, line weights are controlled using color by layer:

In the Visibility Graphics dialog, Cut Line Styles allows adjustments of lines weights of host objects (floors, walls, ceilings & roofs) which affect both display and export.


While some fine tuning remains, we achieved the goal of providing CAD base drawings with separate layers for finishes (which do not exist in the normal CAD world!) Overall the output is better. 689 layers in a typical CAD floor plan base are reduced to thirty-one - yet CAD is the "industry standard" that is defended, against the elegant, simple, consistent output created by Revit.

Strange but true.

Multi-Family Housing Concepts

When I became immersed in the housing industry and began studying its processes, my original strategy for implementation was to emulate the methods being used with CAD, to make the transition to BIM easier. That strategy has evolved over several months, and each new project is an opportunity to improve the the process and the outcome.

Housing is similar to high-rise hospitality, where repetition of the individual units within a structural bay is the key to efficiency, in the modeling process as well as in actual construction. This repetition allows the unit mix within a hotel or condo tower to be adjusted late in the design process, or perhaps during construction, in response to project conditions. In tower BIM the individual unit models typically do not include the building enclosure (although this has been done).


Low-rise, Type V mixed-use and multifamily housing differs in that, unrestricted by a structural grid, the unit shapes and sizes vary greatly, and in design the pieces must be assembled like a jigsaw puzzle, to achieve the desired unit mix. Within the individual unit types, there may be variations based on story level or exposure, including windows, railings, balconies and other interior and exterior features.

In construction, these buildings are "stick framed" with bearing walls, simple span floor joists, and roof trusses. They are very simple buildings.

• The benefit of BIM to builders is the refinement of the design to eliminate surprises in the field. Surprises lead to changes, delays, and loss of efficiency.
• The benefit of BIM to designers is that a building prototype may be rapidly assembled with many variations tested to achieve the optimum design configuration.
• The benefit of BIM in production is rapid composition of construction documents that are coordinated and graphically consistent.

For the RW Apartment project, in less than 40 hours, using previous projects as a resource, we already have a prototypical CD package that could pass for 50% completion. 40 sheets in all. Starting a residential BIM project from CAD and Sketchup files is quick and easy. Anyone with Revit construction knowledge can do it. Imagine how productive and profitable it is when these techniques are successfully pushed "upstream".

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Modeling Notes~

It is tempting to try assembling an entire building from groups or linked files. While this is a great way to start, for a "stacked flats" project it must eventually be abandoned - especially when using groups.

A residential building's exterior walls vary in their height, materials, and openings. Once the basic form is generated, it seems best to remove these items from their groups. We start with "core only" walls for layout, but these are quickly replaced by components with specific materials and finishes.

At the building level, the largest repeatable segment was identified as the entire building enclosure, consisting of roof, walls, floors, windows, doors, stairs and railings.

At the unit level, the largest repeatable segment consists of interior elements and exterior features, including windows, that do not vary from floor to floor. The key with these interior groups is that all objects must share the same reference level. Interior groups are independent; they are not nested into the larger building group.

Door and window style schedules developed on previous projects allowed rapid assignment of those components based on room type.