Development of a Quantitative Roof Maintenance Management Model

Transcription

1 International Journal of Architecture, Engineering and Construction Vol 4, No 3, September 2015, Development of a Quantitative Roof Maintenance Management Model Danny P. M. Cheng and K. C. Lam Department of Civil and Architectural Engineering, City University of Hong Kong, Hong Kong, China Abstract: Sustainable property maintenance is becoming an important issue in public organizations owning or managing large portfolio of properties in view of the increasing maintenance and repair costs and awareness of sustainability. With the ageing property stocks and limiting budget, it is an important task for these public organizations to ensure funding is allocated to the property mostly in need. The current maintenance management approach adopted by many of these public organizations is based on individual facility manager s assessment of property condition to decide on the works priority. Such qualitative approach is fraught with problems of subjective judgment and inconsistency. Targeting at roofing system, this paper aims to develop a generic, quantitative and objective model of maintenance management which combines the use of mathematical methods and opinions of experts and stakeholders. To ensure a balanced outcome is obtained, two sub-models namely Condition Assessment Model and Asset Priority Model are developed respectively. On one hand, the Condition Assessment Model will investigate the different sub-components of roofing system and provide appropriate weightings to reflect their influence to the overall condition of the roofing system. On the other hand, the Asset Priority Model will involve maintenance experts and stakeholders to look into and allocate weightings to the different major factors which influence the importance of asset toward achieving the missions of the organization. The combined use of these two sub-models provides a quantitative and objective means to prioritize the ranking of different roofing maintenance projects strategically. With this maintenance management model, not only the problems of subjective judgment and inconsistencies in decision making are minimized, but also the most cost effective and efficient budget strategy can be achieved. Keywords: Building condition assessment, roof maintenance, questionnaire survey, analytic hierarchy process, asset prioritization DOI: /IJAEC INTRODUCTION According to British Standard ISO (ISO 2002), service life performance audits are undertaken to ensure that performance over time has been adequately considered in the pre-briefing, briefing, design, construction, life care management (including refurbishment, alteration) and disposal of a constructed asset, and to provide a reasonable assurance that the required service life performance will be achieved. Condition assessment for the roof system during its normal operation period is one type of service life performance audit within its asset life cycle. The consistent, objective and reliable results of the condition assessment is of significant importance for the facility managers to accurately predict the performance of roof systems, properly choose the maintenance action and reasonably make budgeting decisions in both long-term and short-term planning horizon. Roof system and its components, deteriorating over time under the combined effect of usage, climate and unpredictable accident, protects the occupiers and building from the serious climate (e.g. water ingress or penetration, intensive etc.) and reduce the energy consumption of the whole building to some extent. Throughout the life of a building, the roof will inevitably undergo a serious of patch repairs or planned re-roofing (i.e. complete replacement of roofing installation) at certain time intervals. In order to ensure that all the roof systems could function satisfactorily, a notably large sum of resources in roof maintenance management are required. However, it is commonly *Corresponding Author. dannyatsowers@gmail.com 140

2 found that the condition assessment for the roof system is mainly based on individual maintenance staff s experiences and professional judgment. Such a qualitative and subjective approach in condition assessment does not ensure an optimal use of maintenance resource which needs to be carefully planned in view of the continual aging and deteriorating building conditions. 2 CURRENT ROOF MAINTENANCE PRACTICES It is common practice that condition surveys are conducted at regular intervals to obtain information on the current condition and performance of the existing roofing system by the maintenance staff. Such condition survey is meant to give a general overview of the condition of the roofing system and help to develop works plan for future maintenance actions if required. The condition survey record form, if adopted, is normally designed in a simple but practical format that the overall condition of the roofing system is classified into basic categories such as good, acceptable and poor. There is seldom any specific requirement to inspect and consider the criticality, severity, intensity and extent of deficiencies identified. In deciding whether patch repair or re-roofing is to be carried out, although water seepage records of top floor flats are considered, other subjective factors such as local political pressure and strong complaint cases will also play an influential role. This could easily lead to the situation that some roofs are re-roofed pre-maturely while others are kept repaired again and again in a patchy manner without receiving adequate resources for the launch of re-roofing as needed. In terms of prioritization of identified maintenance works, factors such as severity of defects, potential hazards to users, damage due to usage, location, importance and usage frequency of the asset, and whether the asset is subject to complaints or performance pledge are supposedly taken into consideration by the responsible maintenance staff, nevertheless the relative importance of these factors is often judged by the knowledge and experience of staff concerned. There may be a lack of consistency when different staff are involved. With an aim to develop a quantitative-based method for roof condition assessment, an extensive literatures review on building condition assessment as well as roof maintenance management has been carried out as background study for a good understanding of the subject problem as described in next section. 3 LITERATURES REVIEW Around 1990 s in US, the US Army Construction Engineering Research Laboratory (USACERL) developed a decision support system for roofing system (ROOFER) by combining engineering, architectural and management methods with data base management technology to facilitate maintenance managers in determining an optimal level of maintenance and repair for roofing system. A comprehensive set of functional condition attributes, rating scales, and method of weighting the attributes was integrated and developed into a decision support model to help army facility managers to measure the functionality of a facility objectively. USACERL had also developed an inspection and distress manual for roofing systems which included standardized information needed to conduct the visual inspection survey. Procedures for distress density calculations were also provided. Roof inspectors could use this information to objectively determine the indexes that reflected the ability of the roof membrane and flashing to perform their functions, needed level of maintenance, and waterproof integrity. Similarly in Canada, the National Research Council Canada (NRCC) developed the Building Envelop Life Cycle Asset Management (BELCAM) project which embraced technologies of maintenance management, life cycle economics, service life prediction, user requirement modeling, risk analysis and product modeling. The project was applied to roof maintenance management and assisted asset managers in selecting a proper maintenance, repair or replacement solution. Moreover, Straub (2002) suggested that objective and reliable information about performance loss and defects of building components could be obtained by adopting the methodology of condition assessment of building components in Netherlands (which was later developed into the Dutch Standard NEN 2767) using a six-point condition scale with consideration to the criticality, intensity and extent of defects. He further added that by using the Dutch standard for condition assessment, building inspectors could provide property managers with objective data about the condition status of building components. Aggregated condition data could be used for setting condition targets for built assets and for benchmarking (Straub 2009). Other similar condition assessment methods were developed such as the European TOBUS project which aimed to assess the degree of physical degradation, extent of any degradation, extent of the necessary works and costs for selecting office building upgrading solutions (Wittchen and Brandt 2002) and the five-point colour-coded rating system developed by Abbott et al. (2007) with value addition including converting condition assessments into condition-based maintenance budgets, eradication of backlog maintenance, performance assessment of effectiveness of maintenance actions and service life prediction. In Queensland of Australia, the Maintenance Management Framework (MMF) is the whole-of-government policy for managing building maintenance. By adhering to the policy requirements in the MMF, departments will have a consistent approach to the management, planning and delivery of building maintenance. In UK, recently 141

3 many local authorities have adopted a Housing Asset Management Strategy in order to ensure their housing stocks are able to meet the need of the community whilst ensuring best use of resources available (Charnwood Borough Council 2009). It is noted that building condition assessment practices differ from one country to another but one emerging theme coming up strongly is the adoption of quantifiable and grading approach in rating the performance standards. It is observed in many of the public organizations in Canada, Netherlands (Straub 2002), USA, Australia (Kenley et al. 2010) and UK (Jones et al. 2007) where more and more resources are being deployed to develop an objective condition assessment method for their property asset management systems. 4 IDENTIFICATION OF CRITICAL INSPECTION ITEMS For the purpose of this paper, roof system is selected as the targeted building component under consideration for the development and application of a generic quantitative-based condition assessment model. To start with, common inadequacies in roofing systems identified during inspections and audits were studied in order to obtain a comprehensive list on the critical inspection items in roof condition assessment. 4.1 Common inadequacies Water Ponding Inadequacy and unevenness of fall of the waterproofing membrane can cause ponding of stagnant water which becomes a source of water leakage. The prime causes of the deficiency are summarized as follows: No specification of degree of fall of finishes Incorrect orientation of horizontal waterproofing membrane Presence of efflorescence at concrete curbs blocked water flow Blockage of rainwater outlet by accrued debris Poor workmanship and inadequate design Detachment of Horizontal Waterproofing Membrane Detachment of waterproofing membrane can exist in the form of blistering, buckling, wrinkling, cracks and open seam. The major causes of the defect are believed to be: Dust particles and demolition debris are not thoroughly cleaned during primer application Poor torch-flame control in immaturely melted bituminous substance on waterproofing membrane mat, which cannot adhere strongly onto the primed substrate Building up of water vapor pressure underneath waterproofing membrane due to exposure of screed receiving waterproofing membrane and inadequate drying time for newly laid screed Detachment of Vertical Waterproofing Membrane at Skirting Detachment of vertical waterproofing membrane and backing rendering from substrate is one of the most common deficiencies. Detachment usually occurs at the interface either between vertical waterproofing membrane skirting and backing rendering, or between backing rendering and concrete wall surface. In addition to the same prime causes of deficiency as horizontal waterproofing membrane as mentioned above, other causes are: Tuck-in groove not deep enough and irregular in cross section Sealant pointing at tuck-in groove so thin that it deteriorates at a much quicker pace than a full sealant pointing Waterproofing membrane skirting too tall hence easily gets buckled, bulged out or fallen down Use of inappropriate sealant at tuck-in groove Corroded metal lathing at skirting Missing of angle fillet to prevent cracking at the junction between horizontal and vertical waterproofing membrane Missing of flashing to protect the skirting Joints of roof tiles Water will penetrate through the layer of roof tiles if the joints between tiles are not properly designed and constructed. The prime causes of the deficiency are: Depth of expansion joint not reaching the bottom of bedding screed, most probably due to bedding screed not divided into bays when being laid Undersized backer rod is used for filling up the joints Waterproofing at rainwater outlet and pipe penetration through slab Areas around rainwater outlet and penetration through slab are vulnerable to water leakage due to: Poor workmanship in sealing up the gap between the pipe or rainwater outlet grating and surrounding concrete Blockage of rainwater outlet by debris causing water ponding and seepage Missing of pipe collar 4.2 Major sub-components From the review on the above findings, a total of 8 major sub-components of roofing system which should be carefully examined during the condition survey are identified as follows: Concrete tiles Cement sand screed bedding (fall of finishes) 142

4 Waterproofing membrane Skirting Flashing Pipe penetration Expansion joint Rainwater outlet 5 DEVELOPING A NEW MAINTENANCE MANAGEMENT MODEL The proposed decision support model for roof maintenance management is underpinned respectively by two quantitative sub-models, i.e. condition assessment model to analyze the physical condition of the roofing system and asset prioritization model to determine the priority of asset. 5.1 Condition Assessment Model (CAM) An accurate assessment on the condition of the asset is vital to the decision making on maintenance management. To achieve an accurate assessment, there must be quantifiable criteria and performance standards which can be evaluated by building inspectors objectively and consistently. A condition assessment model for roofing system is developed as an analytical tool to provide objective evaluation with the use of standardized inspection process and established performance standards to ensure consistent condition index ratings. The prime objective of this condition assessment model is to collect the necessary amount of information required both to define the overall condition of a roofing system and its sub-components by a Facility Condition Index (FCI). The development of the Condition Assessment Model is divided into the following stages: Identification of major sub-components Allocation of weighting to each major subcomponent Assessment of each major sub-component with defects classified according to their criticality, intensity and extent Calculation of weighted condition scores Establishment of a Facility Condition Index A pilot questionnaire is prepared to obtain the practitioners opinions towards the relative importance of the above mentioned major sub-components when assessing the roof condition. The questionnaire is divided into three parts as shown in Appendix 1. Part 1 will request the maintenance experts or practitioners to provide the information of their experiences in handling roof repair or replacement projects. This information will be useful for identifying the experts to be interviewed at the second stage questionnaire survey. Part 2 will request the practitioners to rate the importance of the 8 major sub-components using the 7-point Likert Scaling method. Part 3 are open-ended questions to obtain the practitioners comments on the improvement areas in roof condition assessment. The second-stage questionnaire survey as shown in Appendix 2 is designed to calculate the weighting of each major sub-components based on the use Analytic Hierarchy Process (AHP) approach. The AHP was developed by Satty (1980) in the early 1970 s to deal with multi-criteria decision problems and allow decision makers to evaluate a complex problem using a hierarchical structure. It provides decision makers with an intuitive and common-sense approach to evaluate the importance of every element of a decision through a pair-wise comparison process by incorporating both subjective and objective data into a logical hierarchy framework. Application of this methodology can be found in maintenance prioritization in the building sectors (Shen et al. 1998), determination of the relative significance of maintenance priority ranking factors (Ramadhan et al. 1995) and selection of the maintenance policy (Bevilacquaa and Braglia 2000) etc. The first step of the AHP process is to breakdown the problem into its component parts and develop a hierarchy. The structuring of hierarchy developed obeying to the following guideline: (a) an overall goal is positioned at the top of hierarchy; (b) a set of criteria (if any) that relate the alternatives to the goal are located at midlevel; (c) and a set of alternatives are at the bottom. The second step is to establish the priorities by determining the importance of the objectives in achieving the goal through pair-wise comparisons, which is represented by the pair-wise comparison matrix A, which allows for subjective and objective data to be used. The third step is the calculation of the global priority order and global consistency for each alternative by synthesizing the results. Saaty s eigenvector method is the most popular method to derive the priorities of the alternatives. The eigenvector method computes w as the principal eigenvector that corresponds to the largest eigenvalue, called the principal eigenvalue λ max, of the comparison matrix A. Aw = λ max w (1) where λ max n, w = [w 1, w 2,..., W n ] T. Normalizing the principal eigenvector w results in a priority vector w is established for each criterion and sub criterion, as well as the alternatives under each sub criterion, V i represents the overall priority weight of alternative i and is computed as follows: V i = j W j X ij (2) where W j is the weight assigned to criterion j and X ij is the weight of alternative i given criterion j. Discrepancies are common during pairwise comparison for consistency in preference judgments; hence AHP allows for 10% consistent in human judgments. 143

5 The consistency ratio CR is used to examine the consistency of the entire matrix A as follows: CR = CI RI where the consistency index CI is given by: CI = λmax n (4) n 1 where n is the size of the matrix and RI is the random consistency index, obtained by computing the CI value for randomly generated matrix. Hence, a matrix is considered consistent if CR 0.1. Interviewees were asked to complete the questions by choosing the relative importance of major subcomponents through the pairwise comparison exercise. From the questionnaire results analysis, a weighted geometric mean complex judgment matrix is constructed based on the weighting of the pair-wise comparison matrix as shown in Figure 1. (3) The weightings of importance level of the major subcomponents are obtained by rounding up or down to the nearest 5% as presented below: Waterproofing membrane 30% Flashing 15% Pipe penetration 15% Expansion joint 10% Rainwater outlet 10% Skirting 10% Concrete tile 5% Cement sand bedding 5% Besides the identification of relative importance of sub-component, the criticality, extent and intensity of the defects shall also be taken into consideration during the condition rating for each sub-component; the condition rating is carried out according to the schemes as shown in Table 1-4 (Straub 2009; NEN 2006): Table 1. Condition rating matrix for critical defects Extent Intensity <2% 2-10% 10-30% 30-70% 70% Low Middle High Table 2. Condition rating matrix for serious defects Extent Intensity <2% 2-10% 10-30% 30-70% 70% Low Middle High After the rating to each sub-components under assessment is given by the inspector, total score representing the Facility Condition Index of the roof system could be calculated by summing up the weighted scores of all the sub-components. The higher the score of FCI, the poorer the condition and the higher priority to carry out maintenance works. A worked example is depicted at Table 5 for illustration. Table 3. Condition rating matrix for minor defects Extent Intensity <2% 2-10% 10-30% 30-70% 70% Low Middle High Table 4. Six point scale for condition assessment Condition rating General condition description 1 Excellent (score 0) 2 Good (score 20) 3 Fair (score 40) 4 Poor (score 60) 5 Very poor (score 80) 6 Fail (100) 5.2 Asset Prioritization Model (APM) To overcome the shortcomings of considering the asset s physical condition only, an asset prioritization model was developed as an analytical tool to determine the importance of an asset towards contributing to the achievement of organization s missions. The development of asset prioritization model is divided into the following stages: Identification of major factors Allocation of weighting to each major factor Assessment of each major factor Calculation of weighted scores Establishment of an Asset Priority Index Major factors affecting the importance of an asset towards contributing to the organization s mission were identified through a focus group discussion with facility managers and other stakeholders including internal and external customers as follows: Leakage history Use of the building under the roof area Age of the roofing system Disturbance to occupiers Durability of waterproofing membrane Constructability of roofing Susceptibility to damage Each major factor was assigned with a weighting through the discussions of focus group so that its relative importance towards achieving the organization s mission could be established. The allocated weightings, rating scale and judgment reference were decided by the focus group. The assessment process for the major factors was structured in a way that rating scale for each factor was set out clearly with judgment reference for the 144

6 Figure 1. Weighted geometric mean complex judgment matrix (major sub-components) 145

7 Sub-component under assessment Concrete tiles (including tile joints) Cement sand screed bedding (fall of finishes) Waterproofing membrane Table 5. An Example of Assessed Roof Condition Checklist Allocated Weighting % Importance of defects Intensity of defects Assessment Results Extent of defects Condition rating Condition score 5 Serious Low <2% Critical Low 10-30% Critical Middle 2-10% Skirting 10 Minor High <2% Flashing 15 Serious High 10-30% Pipe penetration 15 Serious Middle <2% Expansion joint 10 Critical Middle 70% Rainwater outlet (including channel) 10 Serious Middle 70% Facility Condition Index (FCI) 27 Table 6. An Example of API Calculation Weighted Score A B C D E F G Factor of assessment Leakage history (% of flats underneath the roof have leakage) Importance factor (use of building under roof area) Age of the roofing system (expected useful life of roofing) Disturbance to tenants (number of tenants affected) Durability (Existing membrane toughness) Constructibility (Ease of repair or replacement) Susceptibility to damage (Ease of damage) Allocated Rating Notional Weighted Weighting % Score Score Asset Priority Index (API) 65 assessor to follow. The most relevant judgment reference would determine the score for the factor under assessment. A weighted score for each factor would be obtained by combining the assessed rating and weighting of the factor. In this way, a consistent judgment process was established to ensure that meaningful assessment could be made. After assessment and with all the weighted scores of major factor summed up, an Asset Priority Index (API) was established to provide facility manager with a quantitative and objective priority of asset. A worked example is depicted at Table 6 for illustration. The priority of asset is represented by the Asset Priority Index (API). The API is a balanced scorecard approach that allows maintenance managers and other stakeholders to compare the relative importance of the asset in relation to one another. The higher the score of API for a roof, the more importance of that roof for the organization, i.e. the higher priority to carry out maintenance works. 5.3 Prioritization of Works with Combined use of FCI and API In an environment of limited funding, it is important to focus funding on the asset with the most impact to the organization s mission in relation to customer s satisfaction, building operation, building sustainability, safety and cost effectiveness. The adoption of combined use of FCI and API could transform facility managers evaluation process into one where strategic decisions were supported by data and information rather than being made as a reaction to a particular need. The overall framework of the maintenance priority methodology for roofing system is shown in Figure

8 Table 7. Prioritization based on FCI and API Priority FCI API Descriptions 1 F CI i F CI mean AP I i AP I mean Important asset with worse condition 2 F CI i < F CI mean AP I i AP I mean Important asset with better condition 3 F CI i F CI mean AP I i < AP I mean Unimportant asset with worse condition 4 F CI i < F CI mean AP I i < AP I mean Unimportant asset with better condition Note: F CI mean = 50 AP I mean = 50 Priority of maintenance projects can then be allocated according to the sequence in Table 7. Table 8. Project Prioritization Project FCI API Priority A P4 B P3 C P1 D P2 E P2 F P3 Through the adoption of an objective and quantitative approach with a step-by-step process of assessment, subjective elements in the decision making process of the surveyor are minimized and assessment standards can be aligned to a consistent level across different teams and districts. 7 CONCLUSION Figure 2. Framework for roofing maintenance works prioritization 6 APPLICATION OF THE NEWLY-PROPOSED MODEL In order to investigate the applicability and effectiveness of the proposed roofing maintenance management model, case studies of six planned re-roofing projects were conducted. The responsible facility managers for the six projects are requested to conduct the condition assessment with the use of the inspection checklist to obtain the Facility Condition Index (FCI) and evaluate the priority of asset with the use of Asset Priority Index (API) assessment form as developed above. Results of the case studies are summarized in Table 8. The proposed maintenance management model is proved to be a useful tool for condition assessment and works prioritization. Based on the quantitative and systematic process of proposed model, maintenance managers are enabled to give objective assessment about the condition status of building components in a consistent manner. Being capable of quantifying the condition status of a building component through the proposed model, the performance-overtime for that building component can be measured and its rate of deterioration can be ascertained. With the predicted serviceable life, the maintenance requirements of the building component can be forecasted and necessary works program and resources can be planned ahead to meet the requirements. To enhance the reliability and consistency of the condition assessment process, specialized training with measuring equipment and pictorial guides for identification of defects will be useful for the inspectors to align their assessment standards. Another merit of the proposed model is that it can give an objective and quantifiable condition rating as the desired maintenance level to be performed by the building component. Any drop in the maintenance level will trigger off necessary maintenance and improvement actions to bring back the performance of the building component to the required level. When maintenance budget is limited and several projects are competing for funding to proceed, it is necessary to set the priority of different projects so that 147

9 resources are utilized in the most cost effective manner. In the past, the priority of maintenance works is set by the professional judgment of facility managers. With the support of Asset Prioritization Model, the decision made by facility managers would likely be more objective and contributive towards the achievement of the mission of the organization. In conclusion, the establishment of the newly-proposed quantitative maintenance management model for grading the condition of building component and determining priority of asset can provide the decision makers an effective tool to benchmark with different projects and assist the priority setting of maintenance works. REFERENCES Abbott, G., McDuling, J., Parsons, S., and Schoeman, J. (2007). Building condition assessment: a performance evaluation tool towards sustainable asset management. CIB World Building Congress, International Council for Research and Innovation in Building and Construction, Cape Town, South Africa (14-18 May, 2007). Bevilacquaa, M. and Braglia, M. (2000). The analytic hierarchy process applied to maintenance strategy s- election. Reliability Engineering and System Safety, 70, Charnwood Borough Council (2009). Housing Asset Management Strategy. Loughborough, United Kingdom. ISO (2002). Buildings and constructed assets service life planning part 3: Performance audit and reviews. Report No , ISO, Geneva. Jones, K., Cooper, J., et al. (2007). The role of routine maintenance in improving the sustainability of existing social housing. Proceedings of the European Network for Housing Research W07 Housing Regeneration and Maintenance, Rotterdam, Netherlands. Kenley, R., Chiazor, M., Hall, J., and Heywood, C. (2010). Good practices for managing australia s public and community housing assets. Report No. Final Report Series, No. 148., AHURI, Melbourne. NEN (2006). Condition assessment of building and installation components part 1: Methodology. Report No. NEN 2767, NEN, Delft (in Dutch). Ramadhan, R., Wahhab, H., and Duffuaa, S. (1995). The use of an analytical hierarchy process in pavement maintenance priority ranking. Journal of Quality Maintenance Engineering, 5(1), Satty, T. (1980). The Analytics Hierarchy Process. McGraw-hill, New York, United States. Shen, Q., Lo, K., and Wang, Q. (1998). Priority setting in maintenance management: a modified multiattribute approach using analytic hierarchy process. Construction Management Econmic, 16, Straub, A. (2002). Strategic technical management of housing stock: Lesson from dutch housing associations. The Journal of Building Research and Information, 30(5), Straub, A. (2009). Dutch standard for condition assessment of buildings. Structural Survey, 327(1), Wittchen, K. B. and Brandt, E. (2002). Developm ent of a methodology for selecting office building upgrading solutions based on a test survey in european buildings. Energy and Buildings, 34(2),

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