Jeff's last installment in the Making Mistakes series.

This month, we’ll conclude our ongoing and lengthy discussion of making mistakes in the estimating process.

In Part 9, I began to describe the kinds of conditions encountered and overlooked/ignored that can and do lead to mistakes being made—mistakes that cost money sooner or later. We discussed in detail three of the most common:

• Open-to-below

• Within shafts

• Difficult access

Aside from these three, there are many others, too numerous to outline here in our limited format. The following is a sampling of some of the more commonplace conditions that are very often overlooked during the quantity survey.

New/Existing Interface: Common whereby new construction materials meet existing conditions/construction elements such as walls and ceilings. Typically, control/expansion joints are required. Also, integration/matching of materials and surfaces is often called for. In many instances, this is a costly undertaking and may not be practical.

Adjoining Stairwells: It is inherently difficult for a tradesman to work within a stairwell. The rake of the stair itself denies the tradesman a level platform to work from that in turn creates hazardous and difficult working conditions—this has a detrimental effect on production. As well, the installation of framing and gypsum board panels may interface directly with the stair stringer thus the framing/board must be adapted to suit the rake of the stair and a casing bead installed to provide a break between the stringer and the bottom of the gypsum board panel.

On the other hand, very often one or more layers of a rated gypsum board assembly facing the inside of the stairwell will run past the slab edge at each floor line. This forms a monolithic surface of gypsum board within the stair itself from the lowest floor to the highest. As such, relief is required lest the gypsum board panels crack and possibly fail from the natural movement of the structure. Typically, a horizontal control joint, placed at every floor line around the perimeter of the stairwell, provides the expansion/contraction relief required due to structural movement and temperature expansion/contraction within the building. Even if it is not detailed on the drawings, I always include a control joint within the stairwell as described—it is good construction practice and a job-site reality. The control joint should always be installed in such a way as to maintain the integrity of the wall assembly’s rating (check manufacturer recommendations/details).

Curved Surfaces: Curved surfaces inherently hinder production and put a premium on material costs. Plywood templates, track-bender machines, flexible track, framing 12 inches on center maximum, bendable gypsum board panels, wetting/scoring the back of gypsum board panels, etc., are commonplace when curved surfaces are encountered. In some instances, such as round or oval column enclosures detailed to be framed and rocked, it is more practical and cost effective to provide alternate pricing for custom or stock size glass fiber reinforced gypsum pre-fabricated column enclosures. This is particularly true whenever small radii surfaces, such as column covers, are encountered.

Structural System: A building’s structural system is a condition that must always be considered since it directly affects the “Head-of-Wall.” A flat-plate “green” concrete slab is at one end of the difficulty factor spectrum—a “waffle” slab is at the other. Commonplace are corrugated steel “fluted” decks typically found in structural steel framed commercial buildings. Since rated wall assemblies must maintain their integrity from floor to the underside of the floor/deck above, the deck type determines the difficulty of achieving this code requirement. For a waffle (two-way) or rib (one-way) slab (most difficult), the framing and gypsum board panels must follow the contours of the rib/deck. This typically requires use of a template to cut the board to shape (an expensive endeavor). For the more typical corrugated steel deck, a flat steel plate secured across the deck provides a level surface to install the top track. Rated assemblies typically require safing insulation to be stuffed into the void of the flute. As well, a “slip-joint” detail is typically required using a long-leg top track and cutting the stud +/- 0 feet, 1 inch, short at the top to allow for vertical contraction and expansion of the wall due to structural movement and temperature variations.

Deck Height Variations: Most often, a floor is a uniform floor-to-deck height for its entirety but not always. For example, a lobby area or ground floor may have a mezzanine above for part of its floor area. This means that there are two heights for this floor to consider: ground/lobby to underside of mezzanine and ground/lobby to the underside of the second floor. Such a differential in heights is significant and must be recognized and distinguished in the quantity survey. A properly prepared schematic height diagram will highlight such a condition.

Dissimilar Material Interface: When two materials of different chemical composition meet, something’s got to give. In the drywall trade, this something is most often in the form of a casing “J,” “L,” or reveal bead. If it isn’t detailed, include a finish bead at the return to the sill, head and jambs of exterior wall windows. Exposed calcium sulfate (gypsum) will corrode metal if in direct contact.

Patch and Repair: This is perhaps the most vague of term in the estimating lexicon. If I have to deal with “P&R,” I do so in the form of a labor and material allowance that is a separate line item on my bid proposal letter.

Lastly, in the realm of making mistakes in the estimating process, you will very often encounter design errors, omissions, ambiguities and inferences that, if not dealt with in the course of producing the estimate, can lead to problems on the job or can cause you to lose a bid. It is the purpose of tools such as exclusions, inclusions, qualifications, notes, alternates, unit prices, allowances, etc., to deal with the mistakes of others (those not of your own making).