Thursday, August 15, 2013

Service Life Predictability

John A. D'Annunzio
August 15, 2013

The most heated debate in the roofing industry centers on the issue of roof removal vs. roof repair.  It is not uncommon that 3 or 4 roof evaluators of a given roof installation would reach 3 or 4 different conclusions relative to the roofs condition, maintenance requirements, and service potential.  The evaluation of the sources of available maintenance options, and their economic benefits to the building owner, would likely yield additional varying conclusions.  Their condition exists because roof maintenance is often conducted in the absence of a standard set of measurements, values, or decision-making guidelines.

For example, a 2-inch high ‘ridge’ in the roofing membrane is an entirely different problem on a six-year old roof than it is on a twenty-one year old roof.  It is also different when it occurs within an organic felt system rather than a fiberglass felt system, and so on.  How many authorities or experts would agree on its ultimate impact on the serviceability of a roof system?

The standard of measurements and values referred to would establish the specific problems or potential problems existing within the roofs, their severity, their density, and their impact on the remaining serviceability of the system.  Depending on the defect type and membrane type, its age, and other considerations such as climate and building occupancy, a ‘decision tree’ process could guide the user to the most technically and economically sound course of action.

The intent of a program developed in this manner is to reduce costs, minimize maintenance requirements, and establish a level of quality assurance that would result in predictable and controllable roof service.  The program should be dynamic so that it can be upgraded, revised, or remolded to reflect changing roof technology, in house experience, or specific user requirements.

Developing a system to rate a roofs condition, estimate its service life, and to provide a basis to make decisions or select repair alternatives is a difficult task.  Ideally, the system would be based on the instrument-measured impact that each situation (such as a defect, weather) has on the roofs integrity and condition.  Each measurement would include combinations of problem type severity level, and identify the membrane type, the climate, test sample analysis, and thermal performance of the insulation component.  Such an approach would require both roof investigation and material forensic analysis in a laboratory.

Considering the complexity of roof systems, and the state-of-the-art in roofing technology, an empirical approach is necessary to establish a procedure that will provide a disciplined and effective management tool for optimizing the service life of a roof system.  Following are suggested procedures for instituting the rating and decision process.  This process eliminates the subjectivity of the evaluator and is based solely on objective analysis and evidence.

Forensic Analytical Serviceability Tracker (FAST)

The goal of the Forensic Analytical Serviceability Tracker (FAST) program is to remove all subjectivity from the formulation.  The service life prediction is based solely on objective evidence.  All roof system components should be analyzed, inspected, and tested.

In the Forensic Analytical Serviceability Tracker (FAST) procedure a service life prediction is established within the following parameters:

1.                  The age of the existing roof system

2.                  The industry average service life of the roof system
3.                  Roof Condition Evaluation:

a.                   Roof Inspection

b.                  The identified distress factors of the existing roof system

4.                  The on-site forensic analysis of the existing roof system, based on:

a.                   Moisture analysis

b.                  Attachment – bonding or wind uplift

5.                  Material Forensic Testing (Laboratory Analysis)


Forensic Analytical Serviceability Tracker (FAST) Calculation Factors

The following factors are used in the calculation of the FAST method of determining the existing serviceability of an existing roof system:

1.                  Provide the age of the existing roof system.

2.                  Identify the existing roof membrane system in the chart below and determine the industry average service life.  (The National Roofing Contractors Association (NRCA) developed the estimated service life chart).
                                   Roof Membrane System                                 Mean Life Years
Natural Slate                                                               60.3

Clay Tile                                                                      46.7

Metal Panels                                                                26.5

Coal-tar Organic BUR                                                23.0

Coal-tar Glass BUR                                                    11.2

Asphalt Glass Shingles                                               17.7

Asphalt Organic Shingles                                           17.5

Asphalt Glass BUR                                                    16.7

SBS Modified Asphalt                                               15.9

Asphalt Organic BUR                                                14.7

EPDM                                                                         14.2

PVC                                                                            13.8

APP Modified Asphalt                                               13.7

CSPE-CPE                                                                  12.8

EP-TPO                                                                       12.7

Polyisobutylene                                                           10.6

3.                  The roof condition investigation should determine the existing defects of the roof system.  All components of the roof system should be investigated: membrane, flashings, penetrations, and metal terminations.  All defects should be noted.

All roof membrane systems have defects that decrease the service life of the roof system.  The major defects that have a direct influence on the service life of the roof system should be identified.

4.                  The on-site forensic analysis of the existing roof system:

a.       Moisture Analysis: Proper analysis includes a combination of both non-destructive and destructive methods of testing.  Investigations completed using only one of these methods are insufficient and lack creditability.  The equipment used to conduct non-destructive tests provides analysis (a snapshot) of the overall roof conditions of large expansive areas in a quick and efficient manner.  Destructive testing – coupled with gravimetric tests – are required to verify the conditions observed by the moisture analysis equipment.

There are three types of non-destructive testing equipment:

  1. Impedance or Capacitance
  2. Infrared
  3. Nuclear 

b.      Attachment: System attachment is the most critical element of roof application.  Improper attachment results in the increased probability of wind blow-offs and contributes to membrane strain created by differential movement of the system components.  Testing can be conducted with bonded pull test or wind uplift (dome) tests in compliance with  

5.                  Material Forensic Analysis of the existing system (Laboratory Testing):

a.                   Insulation:  determine the condition of the existing insulation by completing the following tests:

·                     Gravimetric Moisture Content

·                     Volumetric Moisture Content

b.                  Membrane: determine the condition of the existing membrane by completing the following tests:

Built-Up Roof System and Modified Bitumen:

·                     ASTM D 2829 Standard Practice for Sampling and Analysis of Built-up Roofs

·                     ASTM D 4 Standard Test for Bitumen Content

·                     ASTM D 1670 Standard Test Failure End Point in Accelerated and Outdoor Weathering of Bituminous Materials

·                     ASTM D 2523 Standard Practice for Testing Load Strain Properties of Roofing Membranes

·                     Microscopic Examination

Thermoset Membrane Systems (EPDM):

·                     ASTM D 4637 Standard Specification of EPDM Sheet used in Single Ply Roof Membrane, to include

·                     ASTM D412 Tensile strength

·                     ASTM D412 Elongation

·                     ASTM D816 Factory Seam Strength

·                     Mil thickness of existing membrane

·                     Seam strength

·                     Microscopic Examination

Thermoplastic Membrane Systems:

·                     ASTM D 4434 Standard Specification for Poly (Vinyl Chloride) Sheet Roofing, to include;

·                     ASTM D638 Tensile strength at Break

·                     ASTM D638 Elongation at break

·                     ASTM D638 Seam Strength

·                     ASTM D638 Overall Thickness

·                     Seam strength

·                     Microscopic Examination

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