COST MODELLING FOR ROOFING MATERIAL SELECTION

During recent decades new building techniques have been developed and many new materials have been introduced to the construction market. The material market has become more profitable for contractors and construction material producers than for the client. Not having being a functional and performance oriented roofing material selection system, domestic clients are in a very tight situation when selecting a suitable roofing material according to their requirement, without they being misled from sales exaggerations. This paper elaborates preparation of a cost model which can be used for selection of roofing materials for domestic purposes, giving adequate consideration on cost and some other performance factors. Cost comparison of roof covering, ceiling and insulation materials those are available in the Sri Lankan market are investigated along with the other performance factors of materials. Further, the current status of the roof construction and design in the Sri Lankan context is also explained


INTRODUCTION
Shelter has been one of the three basic needs of human beings ever since evolution.The roof is a main component of any shelter, which gives protection from natural elements and climatic conditions such as sun, rain, wind, heat and cold.
There are many debates over the relative merits of roofing materials selection for domestic construction.The client and designers could not select the most suitable roofing materials for domestic construction due to the fact that they do not have sufficient sources and very little reliable published data in existence.
Having identified the requirement of a cost model to analyse roofing materials; a cost model was developed by analysing some commonly used roof covering, ceiling and insulation materials in terms of cost and some other performance factors.The roof design complexity was adequately taken in to consideration for the purpose of modelling.
This paper describes a cost model for roofing material selection for house construction purposes.A critical review of the literature on cost modelling and the roofing systems is also carried out in this paper.Further performance factors of roofing materials and analysis of roofing cost are also provided.

METHODOLOGY
Having identified the need for the development of a cost model to select roofing material for house construction a model was developed to provide basic cost and performance information to the user.Since one roof type does not represent all the features of a roof and the fact that the design of a roof affects its cost it was decided to model costs based on three types of designs of varying complexity.These designs represent Simple, Medium complex and Complex types of roofs.Most commonly used roofing materials were selected for comparison purposes.Adequate consideration was given for roof frame, covering, ceiling, insulation, rainwater goods, painting or polishing of ceiling and all the other cost items that can influence the total cost of the roofing system.
In the Sri Lankan construction industry in practice it is often found that either small-scale contractors or individual craftsmen carry out house construction works (except in the case of large housing schemes and super luxury mansions).In such type of construction it is found that the labour cost is little higher when compared to the large-scale building construction works.Therefore, prevailing labour rates in the domestic labour market in Colombo district were used for the purpose of roof cost calculations.Cost of different roofing systems were estimated separately under few categories viz: Basic cost, Ceiling cost, Insulation cost and Total cost for the benefit of different types of users of the model to select roof covering, ceiling and insulation systems separately, according to their requirements.These are defined as given below.
Basic cost: represents the roof covering, structure, rainwater goods and all the other items of a roof, excluding ceiling and insulation cost.
Total cost: represents the overall cost of the total roofing system (i.e. the cost of structure, covering, ceiling, insulation, rainwater goods and all the other items of a roof. -11-

R. S. Perera U.L.A.S.B Fernando
Roofing materials were ranked according to the knowledge through literature, observations, interviews with material manufactures, users (house occupants) and by analysing product data.
The subsequent sections of the paper briefly analyses the cost modelling techniques and explains in detail the model construction and the main findings.

INTRODUCTION TO COST MODELUNG
A model can primarily be physical, mathematical or statistical.Cost Modelling is defined as the symbolic representation of a system, expressing the content of that system of the factors, which influence its cost (Ferry &Brandon, 1991).Brandon (1987) states that the purpose of cost modelling seem to be to provide a reference point where the cost adviser can refer before applying his own experience and to increase understanding and/or predict or influence the future.He further states that the role of a cost model is to predict a future situation and to give an economic dimension to the problem.Brandon (1991), a good cost model shall provide better cost information quickly, more in quantity with more reliability at an early stage in the design process and in a more understandable form as possible.Further, using a good cost model shall provide an economic assurance to the client with regard to the cost of his project, and will link the cost control at the design and the construction stages and may be helpful to cost advisers for cost advising purposes and for representation of building in such a way that its cost can be tested and analyzed.A computer integrated cost model shall fulfill most of the above mentioned requirements successfully.Cost modelling can be classified considering its use, purpose and the stage at which the cost model is used (Refer Figure 1).Several methods can be used for cost modelling purposes as empirical method, Algorithmic methods, Simulation, Heuristics and a combination of these methods.Emperial models: are symbolic models, which are derived from observation, experience and intuition.They have been developed largely on the basis of "right feeling" (Ashworth, 1982).Algorithmic methods: are developed using statistical relationship between design variables and cost.Here, the regression analysis technique is used to find a formula or mathematical model (Ferry & Brandon, 1991).Simulation method: tries to simulate a series of future events.This method allowes sampling a large number of possible solutions to discover the best solution (Ferry & Brandon, 1991).Heuristics: are essencially rule of thumb procedure, which enables a near optimum solution to be produced once the model has been built.These approaches are an attempt to discover the qualities of the expert and to incorporate these into the model (Ashworth, 1982).

The Need of a Cost Model
Having abundance of material options for roof covering, insulation and ceiling materials in the construction market, selection of materials according to requirement has been a difficult task for domestic client as well as for construction professionals.There is hardly much product information available at hand to the domestic client or to the construction professionals to advise a building client.Information available is piece meal and hardly any comparative analysis available.Therefore, it is obvious that there should be a proper advisory system to select the most suitable materials that best satisfy client requirements in order to gain the maximum benefits.

Advantages of Having a Cost Model for Roofing Material Selection
Developing a cost model to select roofing materials for domestic purposes will provide numerous advantages to domestic clients, designers and cost advisors in various ways.The cost model will be helpful to select suitable roofing materials giving adequate consideration on cost factors as well as for other performance factors like thermal performance, water tightness etc.
• The cost model will be helpful to get a decision of suitable roofing material by giving consideration on cost factors as well as for other performance factors like thermal performance, etc. • By having the model a laymen shall not need to depend on sales exaggerations.He will be able to form an independent decision on selection of roofing materials.Source : Demographic Survey 1994, Release 4

Colombo
Figure 2: Roof • The designers and cost advisors shall put adequate consideration on cost factors when selecting a suitable roofing system for domestic purposes.• The client and cost advisors shall be aware of allinclusive average cost figure for a roofing system.

THE ROOF
Roof is the building element most exposed to the climatic conditions.The impact of solar radiation, loss of heat by long wave radiation during the night, the rain, and other climatic elements affect the roof more than any other part of the structure (Givoni. B, 1976).The primary function of the roof is to protect the inside of the building from the hostile elements (Lakshman K. D, 1992).Break up of elemental cost shows that the roof cost takes a considerable portion of total cost of a house construction.It is approximately 10-20percentage of the total cost (Perera J, 1992).The roof is one of the main elements exposed to the out side.Consequently its contribution to the aesthetics of a house is also greater.Therefore, to make the house more attractive, it is often required to have an attractive roof.
The roof is not a contemporary product, but it is a result of many experiments and experiences (Lakshman, 1992).New roof systems have been introduced from time to time with additional features like greater thermal comfort, less cost, greater durability, easy of fixing, greater weather tightness and many other aspects of performance to satisfy the requirements of the customer (Coates, 1993).
The uses of roof covering materials are subjected to change from time to time.A greater part of roofs in Sri Lanka had been constructed using cadgen in the early days and it has now gradually converted wards the use of more durable materials.Figure 2 below shows the usage of roofing materials within Sri Lanka by year 1994.Usage of Asbestos cement sheets is more common within the city of the country and Calicut tiles in rural areas.

Thermal Comfort
Roof modifies the internal temperature, when appropriate properties are chosen.It will be possible to achieve and maintain comfortable internal temperature over a wide range of external conditions (Lakshan, 1999).The roof is the only element, which is unprotected against the radiant heat load from the sun.Increase in the temperature of the ceiling is transmitted to the occupants of the room mainly by radiation as shown in the Figure 3 below.

Table 4: Durability And Maintenance of Different Roofing Materials
Calicut tiles • There is a high possibility for calicut tiles to get damaged, so replacing of tiles from time to time is needed.• Fungus are grown on tiles, so, periodical cleaning is needed.

Colorcon tiles
• Cleaning of tile is needed to maintain the appearance and thermal properties of the roof.• Re-painting is not needed.
Asbestos sheet • External surface becomes dull gray colour, so, periodical painting is needed to maintain the thermal properties and the appearance.

•
If white washed on roof surface to get thermal performance, re-painting is required at least once in two-years.

Asbestos with covering tiles
• Covering tiles are subject to slip over the time, so.periodical re-arrangement of covering tile is needed.

MCR tiles
• Color tiles are subjected to discolour, therefore re-painting is required once in five years time.• There is a possibility for growing of fungus on colour-less tiles.

Zn/Al
• Damage to outer coat during transport and erection lead to corrosion of the base material.

•
Regular cleaning of surface by hosing and removal of debris is essential to preve nt corrosion.• There is high possibility to corrode the cladding in corrosive environment like co astal areas.

•
No re-painting cost.

Concrete flat roof
• Reinforced concrete is usually of excellent durability provided adequate care is taken with the design mix, choice of ingredients, mixing, placing, compacting and curing.Further, the care must be taken to provide adequate clear cover and proper waterproofing system.(Sperling.R.

Erection, Strength and Weight Table 5: Erection, Strength and Weight of Different Roofing Materials
Clay tiles • Fixing method is familiar to workers.
• Fixing takes a considerable time, due to the complexity of roof structure and tile laying method.
Colorcon tiles • Fixing method is familiar to workers.
• Tiles are little heavy and not easy to handle as calicut tiles.
• Fixing of tiles using nuts is possible.
Asbestos sheet • Method of fixing is familiar to workers.1994).However, the Asbestos roofing and ceiling sheets are widely used in Sri Lanka.There are no records, research nor conclusive proof of any ill effects of the use of asbestos roofing products found in Sri Lanka.Reasons for this may be that the low or minimum quantity of asbestos fibres being used along with a high proportion of cement in the asbestos products manufacturing process.
The roof cost not only changes according to the type of roofing material used but also according to the design of the roof.Three different roof designs as simple, medium complex and complex those represent pitched roof, hip roof and roof consist of several levels respectively were selected for the analysis.Basic cost, ceiling, insulation costs and cost of total roofing systems are shown separately.Basic costs represent the cost of roofing systems excluding ceiling and insulation costs.

Basic Cost of a Simple Type Roof
It was found that in terms of basic cost of a simple of Asbestos roofing system is 11%, 21%, 27%, 29% type roof, the Asbestos roofing system cost was the and 42% lower than the MCR non-colour tile lowest.It was due to its less material requirement for (600x600mm

Total Roof Cost
The total roof cost refers to the total of basic roof cost, ceiling cost and insulation costs.Different roof covering, ceiling and insulation material combinations gives different total roof costs for different roof designs.Analysis of total roof cost shows that the lowest cost combination (total cost) for all simple, medium complex and complex type of roof design is given by the Asbestos roof covering with slope rafter exposed asbestos ceiling and 3mm thick Poluthaleen insulation.When analyzing roofing materials on its performance it become evident that none of roofing materials perform to the highest levels.The importance of performance factors differs from one person to another and is governed according to the client requirements and their income levels.However, thermal comfort, water tightness and economy were identified as performance factors considered important by the majority.Asbestos cement roofing system was to be found to be having lowest cost over all the other roofing systems considered in each of simple, medium complex and complex type roof designs.It was found that the cost of using other materials reduces considerably with increase of design complexity of the roof.This can be seen as one of main factors to consider in designing roof and selection of roofing systems by the architects.

3.4Summary of Roofing Materials Performance
Another important factor that came in to light was that the cost of Asbestos and Plywood ceiling appears to be identical and is considerably lesser when compared with traditional timber paneled ceiling (eg:-Lunumidella).Further, the use of the method of exposed rafters for ceilings reduces the structural cost of ceilings considerably with a net saving of percentage overtraditjonal flat ceilings.
The use of 3mm thick Polyuthaleen provides thermal insulation of a roof at a lower cost.Adequate consideration should be given to achieving optimum roof pitch as it has a significant effect on the roof cost The selection of low roof pitch will cut down the roof cost by a greater portion particularly, where rafter exposed ceilings or insulation is used.
The roof system comprising of Asbestos cement sheet covering/Asbestos or Plywood ceiling/ 3mm thick Polyuthaleen insulation gives the lowest cost roofing system cost for a domestic roof.The cost reduction is in excess of percentage compared with the next lowest alternative.
The selection of roofing material must be based on not only the cost factor, but also on other performance factors like thermal comfort, appearance etc.Such a cost model will be helpful to laymen, designers and cost advisors in advising and selection of roofing system for domestic buildings.With the use of this cost model it is planned to develop a knowledge based system that would advise on roofing system selection.This part of the research is in progress at present.
World Wide Web is a place, which anyone can access information from anywhere in the world.A web based information dissemination method on roofing material selection is sure to bring numerous advantages to many stakeholders in the construction industry.It will be an informative decision support tool for the people involved in house construction in their decision making process on roofing material selection.
at late design stag, to bring contractor's estimate to the design stage.

Figure 3
Figure 3 Proportion of heat flowing down wards from ceiling

Figure
Figure 3: Thermal Performance of Roofing MaterialsPerformance Reduction.

Figure
Figure 4: Water Tightness of Roofing Materials Performance Reduction • Figure 5: Durability of Rooting Materials Figure 7: Erection of Roofing Materials

Figure 12 Figure 13 :
Figure 12: COST BREAKDOWNS OF DIFFERENT ROOFING SYSTEMS IN COMPLEX TYPE ROOF 1400

Figure
Figure 15: Ceiling Cost of a Medium Complex Roof 30001

FigureFigure
Figure 18: Insulation Cost vs. Pitch -Simple Type Roof

Table 6 :
Availability of Different Roofing MaterialsCalicut tile• Availability is little low in Colombo area, but commonly used in rural areas • There are no options for colours and different sizes.