New job

Starting October 8, 2007 I will be working at:

Dressler Consulting Engineers, Inc.
4425 Indian Creek Parkway
Overland Park, KS

913.341.5575
www.dressler.net

Please update your file.

My e-mail and direct dial number will stay the same.

You Can’t Always Get What You Want

You Can’t Always Get What You Want

Moral: You don’t get what you expect, you get what you inspect. (Or, you don’t get what you specify, you get what you observe.)

Situation

In the preparation of Construction Documents, we spend a great deal of time determining the proper components for the building systems. We apply a great deal effort writing specifications and making drawings and cross checking between them.

Mistake

Rather than giving consideration to the perfect set of Construction Documents, we often overlook who will use them and how they will interpret them.

For example, for our roofing projects it is not uncommon for the foreman to have all of his instructions about how to assemble the roof system on a single 3 by 5 note card. He carries this card in his pocket.

All of the wonderful specification writing and drafting work in the Construction Documents is reduced to a single 3 by 5 note card!

Result

Some times the construction crew assembles their portion of the work by habit or “area practice”, rather than what was so carefully researched and specified.

Maybe they cannot do their work the way it was specified because of interference with other work. Maybe they were told to do their work at the wrong time in the construction sequence. Maybe the wrong materials were delivered to the project site.

The result will be a building not in conformance with the Construction Documents.

Lesson learned.

There is no substitute for timely jobsite visits. This means being at the place of work at the correct time.

For example, the roof is one of the largest field-assembled systems in a building. It is typically the assembly of multiple components. Looking at the finished roof does not show which components were used or how they were installed. For that, the Owner’s representative needs to be on the roof as it is being assembled.

Some clients forgo the expense of roof warranty premiums and use the saving to pay for a roof construction observer instead.

On Being Flexible

On Being Flexible

Moral: Be especially careful when changing roof system type.

Situation

A large retail building was being converted for a different use. In the process of the remodeling, it was decided to change the roof system type. The existing roof was a ballasted EPDM (rubber) membrane. The new roof was a coal tar built-up roof.

The roof deck was corrugated metal on bar joists. The building exterior walls were tilt-up concrete.

Mistake

Little (or no) consideration was given to the movement of the roof deck relative to the walls. Since the roof deck was attached to the supporting bar joists, it can only more in the same direction as the bar joists. Since the bar joists span from one side of the building to the opposite side, there can be a differential movement between the deck and the adjacent wall.

Specifically, the walls at the ends of the bar joists will stay in the same relative location since they were fastened. However, the walls running parallel to the bar joists can move relative to the bar joists/deck since they were not fastened together. As the building moves with temperature expansion or contraction, or wind loads, or foundation settlement, the roof connection from the roof deck to the wall can be strained.

Result

The roof base flashing wrinkled noticeably along the parapet walls.

The roof membrane actually split in the field of the roof.

The EPDM roof provides a flexible membrane system that accommodates building movement. A coal tar built-up roof with fiberglass ply felts does not.

Both are good systems, with unique performance attributes.

Lesson learned

Changing the type of a system should be based on relevant factors. Swapping one type for another is not always as straightforward as it may seem.

Particular care must be paid to the interface between systems, i.e. between the building structure and the tilt-up walls or between the roof and the walls.

Self-inflicted Roof Blow-off

Self-inflicted Roof Blow-off

Moral: Don’t override the safety system. It’s there for a reason.

Situation

We were asked to design a roof replacement for a small building that housed electronic switchgear and computers. The HVAC system was oversized with a backup system, because of the necessity to keep this equipment at its proper operating temperature.

Mistake

The mechanical system operator decided to test the system. In preparation for the test, the pressure relief dampers were blocked off. These dampers were spring loaded and calibrated to vent excess pressure from the building interior in the event of a problem with the HVAC system.

The HVAC system could operate by recirculating the interior air, or by pulling in air from outside the building.

With the relief dampers blocked, the HVAC system was turned on with full outside make-up air.

Result

The rise in the interior air pressure resulted in the roof system being “blown” off the building. It actually did not leave the building, but rose off the deck and came back down when the HVAC was shut off.

Lesson learned

The safety system is there for a reason. Someone has determined the need for the system and designed it for that purpose. The system may be designed for situations that may not be obvious.

Overriding the safety system can lead to unintended consequences.

Don’t Get Ahead of Yourself

Don’t Get Ahead of Yourself

Moral: Don’t take off more than you can replace.

Situation

Note: This is not one of our projects. We don’t do residential work.

There was a residential roof replacement project being done by a friend of the homeowner. The scope of work included removing the existing cedar shake roof and replacing it with an asphalt shingle roof. No problem.

The “roofer” had ordered a dumpster to be placed in the driveway. As the existing roof was demolished it could be collected and hauled away at the end of the job.

Mistake

Rather than continuing to pay the daily charge for the dumpster, it was decided to completely remove the existing roof and have it hauled away. No temporary roof was installed.

A quick pop-up thunderstorm overnight dumped an inch and a half of rain in about 30 minutes.

Result

Most of the gypsum board in the house was ruined. Some ceilings dropped. The carpeting was ruined. The electrical system was damaged, etc.

Lesson learned

Although it seems obvious when we are writing re-roofing specifications, we always instruct the roofing contractor to tear off no more of the existing roof than can be “dried-in” or replaced each day. Apparently, this instruction was not obvious to the amateur “roofer”.

No weather forecast is perfect all the time. Make allowances for the worst-case possibility. Take control of the variables you can, because there is often another variable beyond your control.

Wrong place at the wrong time

Wrong place at the wrong time, or Leaks at the Roof-to-wall Interface

Moral: Proper construction detailing is essential between building systems.

Situation

A new retirement home was built a couple of years ago. It was a three story building. The roof consisted of asphalt shingles on a plywood deck. The octagonal front entryway had stone masonry cladding. The General Contractor was anxious to get the building “dried-in” so that the interior work could be done without concern of getting wet.

Mistake

The roofer was told to complete the application of the shingles before the stone mason laid up the wall. As a result, where the stonework extended above the roof, the mortar and stone was actually laid on top of the previously installed shingles.

Result

When it rained, the rainwater ran down the slope of the roof, under the stone and mortar and into the building. A gypsum board ceiling collapsed.

Lesson(s) learned.

There is no substitute for proper sequencing of construction work. The stone mason should have completed the wall before the roofer laid the shingles.

There is no substitute for proven construction details properly installed. The flashing between the roof and the wall cannot be left out simply because the roof is sloped.

Particular care must be paid to the interface between systems, i.e. between the roof and the walls. Each trade is responsible for their own work, but not for others’. Although there were several opportunities to prevent or correct this failure, no corrective steps were taken.