The Readability of CAD Drawings

May 1, 2005
When architects create computer-aided-design (CAD) drawings, they use certain layers of the drawing for certain things. One layer might be used exclusively

When architects create computer-aided-design (CAD) drawings, they use certain layers of the drawing for certain things. One layer might be used exclusively for walls, while another layer might be used for room numbers. There is almost no limit to the number of layers one can use on a single drawing. Layers can be turned on and off, color-coded, and externally referenced (XREF'd) by other drawings. The entities on a XREF'd layer can become an underlying template for the drawings created by other disciplines, such as structural, mechanical, and electrical engineers. Any changes that an architect makes to the layers will be reflected automatically in the drawings that reference (or use) this layer. For example, if an architect decides to change the location of a wall or add another closet to a room, the change will be made automatically to any other drawing that externally references that layer. Only the architect has to revise the wall drawing. This is a great labor-saving method, but has its drawbacks.

Consider the example of an architect who creates a layer that includes only room numbers and square footage. This layer is controlled by the architect and XREF'd in the mechanical engineer's drawing of the air-distribution system (which also is overlaid on the XREF'd floor plan). The room numbers take up so much space on the drawing that the mechanical engineer does not have enough space to put in his drawing entities. The mechanical engineer asks the architect to move some of the room numbers to free up space, adversely impacting the readability of the electrical drawing made by the electrical engineer, who uses the same XREFs. This turns out to be a difficult move to coordinate. In real life, it can lead to heated discussions, gnashing of teeth, and even fisticuffs. As a result, engineers and architects adopt a devil-may-care attitude. Like a bull in a china shop, they plod ahead, putting their notes, lines, circles, and arrows wherever they please. So what if these drawing entities happen to fall smack dab on top of someone else's?

Architects and engineers looking at CAD drawings on a computer screen are not as adversely affected because they can turn off layers that interfere with their view. But what happens when these drawings are printed in black and white, with all of the layers turned on? Answer: Everything gets garbled and hard to read, especially if the drawing is plotted at a scale of ⅛ in. to the foot. How do you fix this and make the drawing more readable?

You might be able to highlight the most important layers when you print a drawing. For example, a mechanical drawing that shows ductwork overlaid on a floor plan can be printed with heavy lines for the ductwork and lighter lines for the floor plan. This visual effect may work for the first generation of paper copies, but when you start making copies of copies or scanned images of copies, the distinctions start to lose their distinctiveness.

Another solution to the readability problem may involve plotting the drawings at a scale of ¼ in. or ½ in. to the foot. But if your floor plan is so big that these enlarged scales do not fit on a blueprint-sized sheet of paper, what do you do? You break up the floor plan into tiled segments. Now, let's say you have to make nine tiled segments — you have just created 10 pages of drawings to represent the information that should have been presented on one drawing page. Furthermore, assume that the project involves a new building with five floors. Now, you are going to need 50 mechanical sheets (10 sheets per floor multiplied by five floors). The plumbing engineer needs 50 sheets, the electrical engineer needs 50 sheets, and so on.

Dealing with something like this is a hindrance that discourages use of the drawings and leads to other problems. Builders may not look at the drawings because they are just too cumbersome to handle. Contractors, subcontractors, and suppliers may inflate their bid prices to cover the additional overhead costs, such as phone calls and e-mails, brought about by unreadable drawings.

When I look at the drawings that firms made for some of our older buildings, I am amazed by how much useful information they were able to put on a single sheet of blueprint-sized paper. When I compare the size of the old drawing sets and that of the new drawing sets, it's no surprise that it takes a lot more paper to make the modern drawing sets marginally readable.

Figure 1 is a section of a scanned engineering drawing. Notice the overlapping lines and text. The lines are hard to follow. The text is hard to read. Labels are placed on top of labels. Text is placed on top of lines. Both lines and text are indistinguishable. This is a mechanical drawing that is supposed to detail the ducted air distribution of an HVAC system. The ductwork is to be installed above a suspended ceiling and include separate supply and return ductwork. There are room numbers, room names, and structural grid lines on the drawing. There are labels for the air-supply diffusers, return-air grilles, and variable-air-volume (VAV) boxes, as well as dimensions for the ductwork. There are lines that represent walls, lines that represent furnishings, and arcs that represent the swing of a door. Why does all of this need to be included on a drawing that is supposed to be used by mechanical contractors? Why not turn off some of the layers? Everything does not need to be on every sheet.

Look at the mechanical information. Notice the labels for the diffusers. The labels show a box with three lines (one each for a tag), a duct-connection size, and airflow. This is too much information. If the airflow data and duct-connection sizes are included in the mechanical schedules already, there is no need to repeat this information. Look at the tags for the VAV boxes. They are large hexagons with two lines of text. Why not make a smaller tag with only one line of text? Look at the volume dampers with the VD tag. Why bother repeating the tags if you use standard symbols from your mechanical legend? Now, take a look at object placement. Notice how frequently you see text overlaid on other text or lines? How hard is it to reposition the text to avoid interference?

So, the gauntlet is down. I challenge all architects, engineers, and designers to make their drawings easier to read. Before you start indiscriminately creating vectors, visualize what your drawings will look like in a rasterized, black-and-white world of third- or fourth-generation paper copies and scans. I challenge you to make your drawings more concise and to use less paper.

Michael J. Rocchetti, PE, is a mechanical engineer for the University of Connecticut. Having recently completed a tour of duty in Iraq, he is serving with the U.S. Air Force in Washington, D.C.

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