Scholarly article on topic 'Comparison of Different Roof Types in Housing Projects in Turkey: Cost Analysis'

Comparison of Different Roof Types in Housing Projects in Turkey: Cost Analysis Academic research paper on "Civil engineering"

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Abstract of research paper on Civil engineering, author of scientific article — Serdar Ulubeyli, Aynur Kazaz, Bayram Er, M. Talat Birgonul

Abstract As essential elements of buildings, roofs traditionally correspond to about 3-8% of total project cost. Despite this considerable share, in the literature there have been only a few researches that investigate roof types preferred by owners/designers and make a cost analysis to reveal cost differences between alternatives. Toward this aim, in this study, twelve types of roofs installed in construction projects in Turkey were first examined. In this context, practical answers of the following questions were investigated briefly: (i) how different are roof types from each other, (ii) by which criteria are roofs chosen, and (iii) which roof types are selected in which buildings (such as, housing and industrial). Among twelve roof types, reinforced concrete flat roofs (RCFR) and free standing wooden roofs (FSWR) were determined as the most used roofs in housing projects. Finally, a real- life building project was considered. Its roof plan and cross sections of above-mentioned two roofs were given, and their detailed measurements and cost estimations were made. As a result, in terms of initial investment cost, a RCFR which has similar insulation conditions with a FSWR was found to be 35.46% more inexpensive than a FSWR. In conclusion, designers/constructers can manage projects more efficiently by directing their clients towards a more inexpensive option. Thus, potential building owners can allocate lower project budgets by decreasing their roof costs. As a research implication, future studies can compare life cycle costs of these roofs, which will likely provide a broader perspective for better cost management practices.

Academic research paper on topic "Comparison of Different Roof Types in Housing Projects in Turkey: Cost Analysis"

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Procedía - Social and Behavioral Sciences 119 (2014) 20 - 29

27th IPMA World Congress

Comparison of Different Roof Types in Housing Projects in

Turkey: Cost Analysis

Serdar Ulubeylia*, Aynur Kazazb, Bayram Erb, M. Talat Birgonulc

aDepartment of Civil Engineering, Bulent Ecevit University, 67100, Zonguldak, Turkey bDepartment of Civil Engineering, Akdeniz University, 07058, Antalya, Turkey cDepartment of Civil Engineering, Middle East Technical University, 06531, Ankara, Turkey

Abstract

As essential elements of buildings, roofs traditionally correspond to about 3-8% of total project cost. Despite this considerable share, in the literature there have been only a few researches that investigate roof types preferred by owners/designers and make a cost analysis to reveal cost differences between alternatives. Toward this aim, in this study, twelve types of roofs installed in construction projects in Turkey were first examined. In this context, practical answers of the following questions were investigated briefly: (i) how different are roof types from each other, (ii) by which criteria are roofs chosen, and (iii) which roof types are selected in which buildings (such as, housing and industrial). Among twelve roof types, reinforced concrete flat roofs (RCFR) and free standing wooden roofs (FSWR) were determined as the most used roofs in housing projects. Finally, a reallife building project was considered. Its roof plan and cross sections of above-mentioned two roofs were given, and their detailed measurements and cost estimations were made. As a result, in terms of initial investment cost, a RCFR which has similar insulation conditions with a FSWR was found to be 35.46% more inexpensive than a FSWR. In conclusion, designers/constructers can manage projects more efficiently by directing their clients towards a more inexpensive option. Thus, potential building owners can allocate lower project budgets by decreasing their roof costs. As a research implication, future studies can compare life cycle costs of these roofs, which will likely provide a broader perspective for better cost management practices.

©2014TheAuthors. PublishedbyElsevierLtd. Selection and peer-review under responsibility of the IPMA.

Keywords: Cost; flat roof; wooden roof; housing projects; Turkey

* Corresponding author. Tel.: +90-372-257-4010; fax: +90-372-257-4023. E-mail address:serdar.ulubeyli@karaelmas.edu.tr

1877-0428 © 2014 The Authors. Published by Elsevier Ltd. Selection and peer-review under responsibility of the IPMA. doi:10.1016/j.sbspro.2014.03.005

1. Introduction

Roofs are the components installed at the top of buildings to protect them against adverse weather conditions such as high and low temperatures, rain, snow, and wind. In other words, they cover the top floors of buildings. With this essential characteristic, they traditionally correspond to approximately 3-8% of total project cost (Ocal & Pancarci, 2010; Toydemir & Bulut, 2010).

Roof surfaces are constructed with slope to let water on them flow immediately. This slope can vary generally according to climate, environmental conditions, type of roof covering material, and aesthetics of building. In the literature, roofs are classified by their slopes, shapes, and construction materials (MNE, 2011). In order to describe a roof completely, it is necessary to employ all of these groups to some extent. In building projects executed in Turkey, owners and designers mostly prefer some roof types to others in these groups. Up to date, only a few research studies investigating mostly preferred roof types in building projects and cost differences between them have been carried out (Worth, Boyle, & McDowall, 2007; Coffelt & Hendrickson, 2010a; 2010b; 2012). Moreover, they mainly focus on roof costs from the life-cycle perspective.

Therefore, in this study, main roof types in practice were first examined. In this context, mostly preferred twelve roof types in Turkey and their particular characteristics were considered. These characteristics were investigated by means of the following questions: (i) how different are roof types from each other, (ii) by which criteria are roofs chosen, and (iii) which roof types are selected in which buildings (such as, housing and industrial). A detailed cost analysis that compares mostly preferred two roof types in housing projects in Turkey was then made. Toward this aim, a real-life housing project was taken into account. Its roof plan and cross sections of two possible roofs were presented, and their detailed quantity measurements and cost estimations were performed. Thus, as the objective of the present study, cost-based comparison of mostly preferred roof types in housing projects in Turkey was carried out.

2. Main roof types in Turkey

In Table 1, main roof types installed in construction projects in Turkey and their demand conditions are given by classification. In the following subsections, common roof types in this classification are explained in terms of definition, building types, and causes of widespread use.

2.1. Flat roof

A roof, which has a slope angle of maximum 5 degrees, is called as a flat roof. In general, such a roof can be constructed as the ceiling floor of top story in a large-area building such as hotel, office, apartment and shopping mall. It is not suitable for buildings located in heavy rainy or snowy geographical areas. It is widespread especially in housing buildings in many regions of Turkey. Avoiding costs of a detailed roof construction, installing solar energy system, and constructing additional stories in the future are among the causes of its widespread nature in Turkey.

2.2. Middle-sloped roof

Slope angle of this roof vary between 5 and 40 degrees. Because four seasons are seen in many regions of Turkey, it is the most frequently constructed roof type in buildings.

2.3. Lean-to roof

It is also called as single surface roof because of the fact that it has flow towards only one direction. Its construction is easy and inexpensive. In general, shingle or decorative tile is used as cover material. This type of roofs is constructed in small-width buildings and low annexes that lean to a building or a wall. In Turkey, there are many examples of this roof type in garage and farm structures.

Table 1. Classification and use of roofs in Turkey.

Roofs by

Roofs by slopes Widespread use Roofs by shapes Widespread use construction Widespread use _ materials

Flat V Lean-to V

Middle-sloped V Saddle V

Steep x Hipped V

Mansard x

Tower (sharp) V

Lantern x

Saw-tooth x

Built-up x

Dome V

Butterfly V

Winged x

Cylinder V

V: common; x: uncommon.

2.4. Saddle roof

This roof has two surfaces and is covered mostly by tiles or aluminum panels. Its surfaces are tied to each other by a ridge. Triangular surfaces constituted by front and back walls of building are covered by gable walls. When the slope of roof is high, attic is formed automatically. Mostly, it is constructed on low-rise buildings and rectangular housing structures. Because it is easily constructed, it can be listed in the most popular roof types in Turkey.

2.5. Hipped roof

This type of roofs possesses the sloped surfaces as many as the edges of building. This multi-surfaces roof is usually covered by tiles. Slope is almost the same for all surfaces. Eaves are horizontal and on the same plane. Since it can cover wide areas in general, it is built especially on middle- and high-rise residential buildings in Turkey.

2.6. Tower (sharp) roof

It is built on small-area buildings in different forms such as square, polygon, or circle. In terms of structural materials, reinforced concrete or metal is used. Since it has no frame, load-bearing system is designed by rafters on a pier in the middle of the roof. As an aesthetic architectural design, it is built on towers and religious buildings in general. Clock towers erected in many cities around Turkey are among typical examples of structures with such a roof.

Wooden V

Steel V

Reinforced ^ concrete

2.7. Dome roof

This roof is constructed on circle or regular-polygon buildings. It is usually applied to religious and historical buildings for utilizing aesthetics and acoustics together. Its covering materials are the same as those of tower roof. Because of the fact that it can easily adapt to different adverse conditions in four seasons, it is widespread in Turkey.

2.8. Butterfly roof

It is essentially more suitable when the roof height is designed as small. Aluminum sandwich panel is mostly preferred as a covering material due to its lightweight and inexpensive nature. The sloped gutters are naturally constituted on the convex roof surfaces. This roof is almost always built on industrial buildings as it can cover extremely large areas by means of its steel framework. It is rapidly erected on factory buildings in many regions of Turkey except the regions where the snow precipitation is intense in winter.

2.9. Cylinder roof

In general, this type of roofs is constructed on rectangular buildings. It has cylinder surface. In order to give such a flexible shape to this roof, steel is used mostly in its construction. In terms of the structural design, it is usually erected on industrial buildings that have large spans without using many columns. Similar to butterfly roofs, it is not suitable for the regions that receive intense snow precipitation.

2.10. Wooden roof

It is also called as free standing wooden roof (FSWR). Tile is the typical covering material of this roof. Such a roof is erected on load-bearing walls, beams, or a reinforced concrete floor. When it is erected on walls or beams, span between supports should be a maximum of 4 metres. This type of wooden roofs is built on detached house or farm structures in rural areas. The other type (i.e., a wooden roof erected on a reinforced concrete floor) is preferred in the majority of low- or high-rise housing buildings in Turkey as it is easily and economically constructed. It is mostly applied together with middle-sloped and hipped roof types.

2.11. Steel roof

Because steel has adequate strength against high tensile stresses, roofs of buildings that have large spans between supports are constructed by steel. Covering materials of this type of roofs are lightweight materials such as sandwich panels manufactured by aluminum, zinc, PVC, or rock wool. Mostly, it is used on industrial buildings and not suitable for the regions that receive intense snow precipitation.

2.12. Reinforced concrete roof

This type of roofs is constructed to provide a perfect thermal insulation and to have a strong and long-lasting roof structure. It can be built in different forms such as framework, arch, and shell. However, the most popular type of such roofs in Turkey is reinforced concrete flat roof (RCFR).

3. Case study

As construction material is the dominant work item in calculating the cost of a roof, it is reasonable to compare the roof types classified according to construction materials. This means that initial investment costs of other roof types (i.e., roofs by slopes and shapes) can vary in an extremely large interval, considering the chosen construction materials and methods. Thus, it is very difficult to constitute a specific design standard. Similarly, it cannot provide

a meaningful cost perspective to compare three roofs by construction materials together, since they are preferred in different building types. Reinforced concrete and wooden roofs are usually built on similar building types such as housing and school structures, while steel roofs are typical components of industrial buildings such as factories and warehouses. Therefore, only housing buildings were taken into account in this section of the study. Toward this aim, initial investment costs of RCFR and FSWR, which are the common roof types in residential projects in Turkey, were compared with each other.

In order to facilitate numerical comparisons that may be performed by future studies, the example real-world case was taken as a FSWR that had a sitting area of about 100 m2, as shown in Fig. 1.

Tasks in cost estimates performed below were directly taken from Construction Unit Price Analyses (Akcali, 2012). In Turkey, these analyses are published every year as a book since all input and unit prices of activities are updated. Unit prices in this study were expressed in US$ for easy understanding of readers.

In cost estimates, high-quality insulation materials were chosen, and thus, both thermal insulation and waterproofing characteristics of roofs were satisfied.

The first roof in the case study was RCFR. It had 12.1 metres in length and 8.1 metres in width. The net area of the top floor or RCFR was 80.64 m2 and is given by a dashed line in Fig. 1.

Although illustrated, chimney was neglected in both roof calculations as it is a natural part of a housing building and does not make a clear difference between two roof types in terms of numerical computations of dimensions and cost.

3.1. RCFR

F " " 1

L _ . J

Fig. 1. Roof plan.

Multiple layers of this roof are shown in Fig. 2. They were on a reinforced concrete ceiling floor of 12 cm thickness. On this floor, there was a sloped concrete layer which provided an inclination to naturally discharge precipitation. Plaster on it constituted a smooth surface between concrete and membrane. Vapor membrane prevented water vapor from penetrating concrete. Over this membrane, there was a thick thermal insulation material against low or high temperatures. For a better thermal protection, two overlapping layers were additionally laid on this material. Geotextile felt was for waterproofing. The top layer was covered by ceramic floor slabs of 45x45 cm2 to be accessable for foot traffic. Adhesive mortar layer was prepared for this floor covering.

— Floor covering

— Mortar

— Geotextilefeit

— Upper layer ofpolymer bituminous coating (with polyester felt)

— Lower layer ofpolymer bituminous coating (with glass wool)

— Thermal insulation material

— Vapor membrane

— Prime plaster

— Sloped concrete

— Reinforced concrete floor

Fig. 2. Layers of RCFR.

Cost estimation for this roof is presented in Table 2 in a detailed manner. While measuring quantities of activities, rules in the construction conditions section of each task in Construction Unit Price Analyses were taken into account. In this point, it should be noted that the thickness of the mortar layer in 10.013/MK was accepted as 3 cm and that the thickness of the sloped concrete layer in Y.16.055/A was taken as 5 cm. Because the cost estimation is solely based on the calculation of the roof budget, cost and quantity computations concerning regular reinforced concrete ceiling floor were not performed.

Table 2. Cost estimation for RCFR.

Task no

Definition

Unit Quantity

Unit price Total cost (US$) (US$)

04.601/2G Geotextile felt (500 gr/m2)

105.64

Unlaminated polymer bituminous coating with glass wool 04.605/A covered by double-side polyethylene film for the bottom layer m2 of the sloped roof covering materials

139.51

Unlaminated polymer bituminous coating with polyester felt 04.606/A covered by double-side polyethylene film for the bottom layer m2 of the sloped roof covering materials

241.92

483.84

10.013/MK Morter produced by cement of 500 kg

100.60

v 1 Pouring C8/10 ready-mixed concrete by pump (including

Y .16.055/A , . .

shipping)

195.86

19.044/5

19.049/5

Under thermal insulation materials, vapor membrane production by polymer bituminous coating with elastomer glass wool covered by one-sided metal folio of 3 mm thickness

Making thermal insulation by rock wool of 5 cm thickness and spreading one-layer bituminous board on it (density of rock wool is 150 kg/m3)

796.72

833.82

Floor covering of joint by smooth-surfaced ceramic floor slab 2 26.005/037A . .. 2 • , i • ^ , m

of 45x45 cm with glue-fixing method

931.39

Prime plaster by cement-lime based thin plaster material on 2 27.560/4 m 80.64 1.73 139.51

interior surfaces

Total cost

3726.89

Overall (including VAT)

4397.73

VAT: value-added tax.

3.2. FSWR

The second roof in the case study was a middle-sloped and hipped FSWR. The roof had overhangs of 50 cm in every side. Lengths of ridges are given in metre in Fig. 1. Layers of this roof are illustrated in Fig. 3 in detail. Slope of the roof was 10% in every direction.

Ventilation

1: tile; 2: waterproofing; 3: bituminous coating; 4: coating board; 5: rafter; 6: attic; 7: thermal insulation; 8: reinforced concrete floor; 9: ceiling plaster.

Fig. 3. Details of FSWR.

Cost estimation for FSWR is presented in Table 3 in detail. Lengths needed for measuring ridges was taken from Fig. 1 directly.

Both roof covering and ridge material was chosen as common corrugated tiles. For waterproofing, two overlapping layers of bituminous coating were laid under tiles. To be an extra waterproofing service, ventilation blanks were designed on overhangs. As a practical application of such roofs in the house-building industry, thermal insulation material of 10 cm thickness was put onto ceiling floor.

Table 3. Cost estimation for FSWR.

Task no

Definition

Unit Quantity

Unit price (US$)

04.608/1

Waterproofing plate with bituminous organic fibres under tiles

m2 118.14 2.86

337.88

18.211 Roof covering by corrugated tiles

m2 100.04 8.13

813.33

18.231/MK Ridges by corrugated tiles

m 32.20 3.23

104.01

18.246/07

Single-layer waterproofing by polymer bituminous coatings of 3 mm thickness with elastomer-based polyester felt under 2 the roof covering (where the roof slope is between 5% and

118.14 9.34

1103.43

Thermal insulation by expanded polystyren foam (EPS) of 19.054/3 10 cm thickness and 15 kg/m3 density on the floor in the attic

m2 80.64 6.13

494.32

Free standing rough wooden framework with board coating 2 21.210 , ^ . ° m 100.04 29.20

under the roof covering

2921.17

Total cost 5774.14

Overall (including VAT) 6813.49

VAT: value-added tax.

3.3. Findings

Considering the results of cost estimations in Tables 2 and 3, it was found that RCFR costs US$4397.73 while FSWR costs US$6813.49. In fact, they had almost the same thermal insulation and waterproofing conditions due to the chosen materials in this study. This means that there was no clear difference between their protection qualities and characteristics against adverse weather conditions. High-quality materials were used in the construction of both roof types. This feature made these roofs very strong against negative environmental conditions. Keeping this fact in mind, the findings in the study clearly point out that RCFR is more inexpensive than FSWR. This result corresponds to a difference of US$2415.76 for a housing building with a top floor area of 80.64 m2. It should be importantly noted that the more the roof area is, the more the cost difference becomes. From another numerical perspective, RCFR's cost is equal to 64.54% of FSWR's cost. In other words, RCFR results in a cost saving of 35.46%.

Consequently, given the most preferred two roof types in housing projects in Turkey, it is evident that FSWR has no superiority over RCFR in terms of cost and insulation conditions. On the contrary, RCFR was found to be clearly a more cost-effective and advantageous roof type than FSWR. In addition to this advantage regarding cost saving, RCFR also allows residents in warm or hot regions to install their own solar energy systems. In essence, these systems need wide and flat areas in installation and RCFR provides such an adequate space for solar energy equipment. Moreover, RCFR facilitates various green roof applications that can be made in almost every type of climates.

4. Conclusions

In this study, the number of the most frequently constructed roof types in Turkey was determined as twelve. These are flat roof, middle-sloped roof, lean-to roof, saddle roof, hipped roof, tower (sharp) roof, dome roof, butterfly roof, cylinder roof, wooden roof, steel roof, and reinforced concrete roof. Of course, only one of these roof types is not enough to describe a roof completely. In practice, most roofs are composed of some of these roof types.

RCFR and FSWR investigated in this study are among such composed roof types. They are the most popular two roof types built on the top of housing buildings in Turkey. Considering the detailed cost estimations of these roofs, a RCFR that has similar insulation conditions with a middle-sloped and hipped FSWR was found to be 35.46% more inexpensive in terms of initial investment cost. Therefore, it can be claimed that a RCFR provides a cost-effective solution against adverse weather conditions.

As practical implications, based on the results of this study, architects and contractors can better design, build, and manage housing projects and direct their customers towards a more inexpensive roof type. Thus, potential building owners can allocate lower project budgets and thereby decrease their roofs' costs. As a research implication, future studies can compare life cycle costs of these roofs, which will likely provide a broader and a more straight perspective for better cost management practices.

References

Akcali, U. (2012). Construction unit price analyses. Ankara: Safak Publishing.

Coffelt, D. P., & Hendrickson, C. T. (2010a). Life-cycle costs of commercial roof systems. Journal of Architectural Engineering, 16, 29-36. Coffelt, D. P., & Hendrickson, C. T. (2010b). Inspection, condition assessment and management decisions for commercial roof systems.

Journal of Architectural Engineering, 16, 94-99. Coffelt, D. P., & Hendrickson, C. T. (2012). Case study of occupant costs in roof management. Journal of Architectural Engineering, 18, 341348.

MNE (Ministry of National Education) (2011). Drawing of roof plan and details. Ankara: MNE. Ocal, M. E., & Pancarci, A. (2010). Technical drawing for construction-II. Istanbul: Birsen Publications. Toydemir, N., & Bulut, U. (2010). Roofs. Istanbul: YEM Publications.

Worth, Z., Boyle, C., & McDowall, R. (2007). Combined life-cycle cost assessment of roof construction. Engineering Sustainability, 160, 189198.