THE
PERENNIAL ROOFING PROBLEMS OF THE COLLAPSE AND THE RIPPING OFF OF ROOFING
SYSTEMS DURING RAINSTORMS IN GHANA - A DEMAND DRIVEN SHELTER DELIVERY RESEARCH
PUBLICATION
Ing. Samuel Addai, M.Sc., MGhIE
BRRI (CSIR), Kumasi, Ghana
Published in The Ghana Engineer, March 1998
Reprinted with GhIE
permission by the African
Technology Forum
ABSTRACT
The persistent media reports on rainstorm disasters for the past few years throughout the country and the plight of the affected people prompted the Optimum Shelter Design and Construction research group in the Building and Road Research Institute (BRRI) of the CSIR (Council for Scientific and Industrial Research) to undertake a Roof Design Verification Survey of the affected roofing systems in the Ashanti region and elsewhere in the country. The aim of the research studies was to find out the main causes of the perennial roofing problems affecting the housing industry and the national economy as a whole.
The studies revealed that, environmental degradation poor roof design and supervision, lack of maintenance, poor construction, poverty, ignorance and petty savings are the main causes of the perennial roofing problems. The cost effect of the annual rip-offs with an assumed rip-off rate of about 0.6 houses per 1000 houses was estimated to be about 1.8 billion cedis, sufficient to build about 200 new average fast-track low cost houses. Analysis of the annual rip-offs also indicated that about 90,000 man-days are wasted annually in reinstating the ripped-off roofs. The total homelessness effect from the annual ripped-off roofs has been estimated to be about 3600 persons and from environmental considerations, 300 logs are wasted on the reinstatement of the ripped-off roofs annually. (ATF Editor’s note: Please note that all costs quoted herein are in 1998 values for the cedi, unless otherwise noted.)
Some bad roof construction practices which were identified during the survey have been catalogued and t heir corresponding recommended procedures have also been indicated to help improve the situation. A roof construction manual on common roofing system in Ghana for improving roof construction technologies has already been produced at the BRRI, slide films and video documentation of typical roof design and construction errors detected during the survey that led the collapse and rip-off of most of the roofing systems have also been produced to help roof designers to improve on the design and construction of our roofing systems.
General
Effects of Rainstorm Disasters
Cost Effect of the Roof
Rip-Offs
The Roof Design Verification
Causes of Roof Failures
The Effects of Environmental
Degradation
Design Problem
Poor Construction
Practices and Supervision
Lack of Maintenance
Poverty, Ignorance, and Petty
Savings
Identified Bad Roofs Construction
Practices and Recommended Procedures
Recommendations
References
Acknowledgements
The roof is that structure which covers the top of any building. Its main function is to provide comfort and adequate protection to everything under it. Roofs in the tropics are generally designed to-insulate buildings against heat penetration and also shade external walls. They are also designed to shed off rainwater very quickly from the roof surface so as to prevent leakage. Roofs may also be used to collect rainwater in areas where water is precious.
The roof basically consists of two components, namely, the roof covering material which is directly exposed to the atmosphere and the roof structure which is an understructure supporting the roof covering material.
For the roof covering material to function effectively, it should be very durable and be able to resist the violent solar radiation, oxidation, the effects of adverse weather conditions such as rainstorms, wind pressure, high daily and seasonal temperature and moisture variations and so on. In order to get a very reliable roof, not only should the roof covering material be of high quality but also the roof covering material and the roof structure must function as a coherent system and thus special care must be taken in the preparation of the roof structure.
Structurally, roofs must be sufficiently strong and rigid enough to withstand the anticipated wind and imposed loads and should span between structural supports without any noticeable deflection.
In general, design, construction and environmental factors do affect the roof performance after it has been constructed and thus the roof can be kept in its functional form through planned maintenance.
EFFECTS OF RAINSTORM DISASTERS
The collapse and the ripping off of roofing systems during rainstorms especially in the rural areas is a common occurrence in Ghana throughout the rainy months. During these rainstorm disasters, a large number of our people are rendered homeless and personal and public properties worth several millions of cedis are always destroyed.
A typical percentage analysis of elemental building cost of a normal average house shows that the roof component including the ceiling alone takes about 30% to 40% of the total cost of the building, with the labour component ranging from 7% to 10% of the total labour input. This clearly shows how expensive roofs in Ghana have become and with the ever increasing cost of roofing timber and roof covering materials, it is becoming increasingly difficult to replace collapsed roofs and thus any improper construction leading to the collapse of built-up roofs brings about untold hardships to the affected people. The situation is even worse in rural Ghana where replacement of damaged roofs becomes almost impossible due to their low incomes.
Worst off, any delayed response to replace these collapsed roofs during the rainy season can lead on the total collapse and erosion the mainly earth wall structures that are supporting the roofs. In this case the total house can be left to be eroded and may not be replaced even in the distant future, thereby reducing the national housing stock and worsening the housing situation in the country.
COST EFFECT OF THE ROOF RIP-OFFS
With an assumed annual rip-off rate of about 0.6 houses per every thousand houses and if it is estimated that about 80% of our net housing stock of about 1,260 x 103 houses are houses with conventional pitched roofs, then an average of about 600 houses is estimated to have their roofs ripped off annually.
Assuming the cost of reinstating each roof structure and the covering material is put at ˘ 3.0m from 1994 computations then the total cost of reinstatement amounts to about ˘ 1.8b which is sufficient to build 200 new average low-cost fast-track houses. It is also estimated that 90,000 man-days, amounting to a rate of 150 man-days per roof reinstatement in terms of labour input is wasted annually.
The homelessness effect throughout the year assuming 2 months reinstatement period, with an occupancy rate of 12 persons per house is estimated to be 1200 persons and considering the deprived 200 new houses that could have been built the total homeless-ness effect throughout the year amounts to 3600 persons.
At an achieved utilization rate of 0.5 average logs per roof for an average fast-track house, the effect on the environment in general was also found to be the loss of 300 average logs annually from our forests for reinstating rip-off roofs, and these logs would have been sufficient to roof 300 new average fast-track houses.
Random surveys carried out on the collapsed and ripped off roofing systems in the Ashanti region and elsewhere in the country for the post two years indicated that our perennial roofing problem is not only with old roofs but also with newly built ones, in both rural and urban areas.
There were situations where newly uncompleted conventional houses in the Kumasi Metropolis have had their roofs blown off in the same way as those in some rural areas.
The surveys also revealed that the perennial roofing problems are with all types of buildings, both single and storeyed, including schools, offices, churches, factories and residential buildings. Most of the roofing systems which collapsed or were ripped off were mostly pitched roofs with lightweight timber roof structures covered with a variety of roof covering materials like corrugated galvanized iron, aluminum, cement asbestos roofing sheets, roofing tiles, thatched roofs on earth or on sandcrete wall structures.
A careful analysis of the roof design verification surveys revealed that the main causes of the perennial roofing problem in the country are:
-
the effects of environmental degradation
- design problems
- poor construction practices and supervision
- lack of maintenance
- poverty, ignorance and petty savings
THE EFFECTS OF ENVIRONMENTAL DEGRADATION
If we consider the fact that most of the old roofs, especially those in the rural areas have been functioning for the past forty to fifty years in a special environment without those problems, even though some were not properly designed or constructed, then the sudden problem of frequent ripping off of these roofing systems can only be attributed to the destruction of the environment.
In the past, the trees in the forests around most of the affected villages were acting as wind breaks and were protecting these villages from the full impact of the wind loads. Thus having cleared much of our forests, most of our buildings in the villages are now exposed to suffer the full effects of the wind loads coming on their roof surfaces. The effect of environmental degradation was seen during one of the random surveys in one of the new estate sites in Kumasi where about ten new estates houses had their roofs blown off. In that particular case, the affected site was an open one on an uphill with no natural wind breaks in the form of trees. All the trees on the site were possibly uprooted when the site was initially cleared to make way for the construction of the estate houses. The natural wind breaks were not replaced with fast growing trees after the completion of the estate houses and as a result some of the buildings which were exposed to the full impact of wind loads had their roofs blown off.
During the surveys also revealed that, the constant and severe erosion around the bases of most of our buildings especially those in rural Ghana were proper drainage systems are absent, had seriously weakened some of the buildings and that greatly contributed to the collapsed and the rip off of some of the roofing systems in some areas.
Structural inadequacy was found to be one of the major causes of the collapse and ripping off of some of the roof structures in some of the areas visited. The result of the random surveys of roof design verification, undertaken by our research group indicates that over 98% of our roofs are basically designed using the growth design approach which relies basically on experience, intuition, common sense and the rule-of-thumb. In such a situation, those artisans who possess these qualities are able to size structural roofing members without having to resort to design codes, theories and rigorous computations since they have the understanding of what the correct sizes of the roofing members should be. On the other hand, for those artisans who are less experienced, it is not possible to have that sense to size structural roofing members, so they rely mostly on the rule-of-thumb which unfortunately resulted in very defective roofing systems which in most cases also contributed to the collapse and ripping off of roofing systems in some of the areas visited.
It was also noticed that, due to the high cost of the standard structural roofing members, the would-be house owners who usually adopt direct labour techniques in their projects realization normally go in for off-cuts and undersized structural timber members which are relatively cheaper but structurally inadequate. The roof structures formed from these timbers are usually not built to any design specifications and in such situations the whole construction is left to the discretion of the carpenter to decide on the design solutions. The lightweight nature of most of roof structures formed this way from smaller timber sections makes them highly susceptible to damage from suction caused by strong winds and that also contributed greatly to the collapse of some of the roofing systems in some of the rural areas surveyed.
It was also noted that the principle of triangulation which gives rigid structural systems was completely ignored in most cases by the carpenters and that, where ceiling joists could have been used as ties to restrain the spreading out of the feet of rafters in some of the roof structures, they were rather fixed to the wall plates at different places, and thereby turning the whole roofing structure into a mechanism which can easily collapse under any serious windstorm.
POOR CONSTRUCTION PRACTICES AND SUPERVISION
The random surveys that were carried out revealed that ignorance of some of the carpenters also contributed very greatly to the total collapse of some of the roofing systems which were poorly constructed in some areas.
For there were situations where the roofing members were structurally adequate, but due to poor construction, the roofing systems became very defective and hence were easily blown off under serious windstorms. In such situations, it was detected in most cases that either there was no anchorage of the roofing systems to the rest of the building structure or the anchorage provided for the roof was not adequate.
In other situations where the purlins were not firmly secured to the rafters or trussed-rafters we have had the purlins ripped off together with the roof covering materials leaving the rest of the roof structure behind. Also in situations where rafters are not firmly secured to wall plates we have had the roofing systems being blown off leaving the wall plates behind. Wrong or inadequate fasteners on the roof covering materials and at the critical stressed areas on roof surface also led to situations where the roof covering materials were ripped off from the roof structure.
The random surveys also revealed that nailing was done during construction of the roof structure without detail reference to the end and edge distances in some cases and thus where the nail does not get good holding in timber roof joints without cover plates on both sides, the timber roof joints were easily distorted under any serious windstorm which resulted in the failure of such joints and consequent collapse of some of the roofs. Also the lengths of laps at roof timber joints in some cases were found to be inadequate leading to premature deterioration when the roof is subjected to constant vibrations and that might have caused some of the distortions and failures in some of the laps at some roof joints in some of the collapsed roofs. In other situations where the end or side laps were not adequate or the corrugated roofing sheets laid in the wrong direction with respect to the direction of the prevailing wind, we have had water back splash and the wind forcing water through the edges of the sheets and thus causing leakages which were not rectified immediately, and that resulted in the deterioration of the timber roofing members and the rusting of the fasteners in the roofing systems thereby weakening the entire roofing system and that might also have contributed to the collapse of some of the roofing systems.
Non-compliance with design specifications and manufacturers requirements for the use of roof covering materials have also resulted in poor performance of some of the roofs and are the major causes of roof leakages, especially where roof slopes have been constructed below the manufacturer's recommended minimum level. Failure to stop these leakages immediately resulted in the deterioration members and the loosening and rusts on roofing nails used in the timber joints and when this continues, the roof structure deteriorates and eventually fails.
A survey of roof construction practices in general in the country, has revealed that there is complete lack of supervision during roof construction by experts. The general practice has been that the would-be house owners, and even some of the functional roof designers, most of the time rely on the experience of the carpenters and roof tiles in the system during the construction of the roof. In most cases, these artisans are usually not challenged and they are allowed to construct the roof the way they think is right. This over reliance on these "so-called" experienced roof artisans is a major cause of the perennial roofing problems facing the housing industry, since most of the roofs constructed by them are usually defective and can easily collapse under any serious rainstorm.
The random surveys showed that most of our roofing systems have undergone continuous deterioration through leakages in the roofing. After long time exposure to various environmental factors in the course of their service lives, roof covering materials deteriorate and may be damaged at some points which may cause leakages in the roof. It was realised that lack of effective maintenance of such roofing systems especially in the rural areas to combat the effect of time and that of the hostile environment as well as the repairs of accidental damages in roofs also contributed to the final collapse of some of the roofs in some of the areas visited.
POVERTY, IGNORANCE AND PETTY SAVINGS
The roof design verification surveys revealed that either due to poverty, ignorance or in an attempt to make petty savings, non-standard low-grade roofing timber members were used in most of the roofs. Out of ignorance, green timber which were not seasoned or treated were used which led to the early deterioration of such timber and consequent rip off of such roofs during rainstorms.
It was realised that thin gauged corrugated roofing sheets were mostly used in rural Ghana where income levels are relatively low and that resulted in the tearing of the sheets through the roofing nails in some of the ripped off roofing systems.
It was also observed that, out of ignorance, the carpenters did not make use of the basic principle of triangulation during the construction of most of the roof structure systems to increase the stability and rigidity of the roofing systems even though the sizes of the roof timber available were structurally inadequate and that led to the collapse of some roofing systems.
Most of the collapsed roofs were also constructed without compliance to the manufacturer's specifications and requirements, and that resulted in inadequate slopes, inadequate fasteners at the highly stressed areas in the roofs like the eaves and ridges and poor and off-nailing which resulted in defective roofing systems and consequent collapse and ripping off of such roofing systems.
It was also noticed that either out of ignorance on the part of the carpenters or poverty on the part of the would-be house owners anchorage systems were not provided for most of the roofs and that led to the collapse of most of the roofing systems.
IDENTIFIED BAD ROOF CONSTRUCTION PRACTICES AND RECOMMENDED PROCEDURES
The general performance of timber roof structure is often connector dependent and thus any timber roof structure is as strong as its weakest joint. During the random roof design verification survey it was revealed that, as it is always with other construction procedures there were some wrong ways by which some of our roofing systems were constructed.
To help improve the quality of our roofing systems, some typical wrong procedures of some roof-structure construction details as identified are shown in Table 1 together with their recommended or right procedures based on the authors' judgment and experience in roof construction.
The recommended procedures are just recommendations and the roof designer is at liberty to make modifications provided they are in accordance with good roof construction practices.
It is recommended that specialist advice be sought in checking maintenance requirements of existing roofs in the country and also during the design and construction of new roofs. In any case B.R.R.I. has prepared a map indicating wind speed regions that should be consulted by architects and engineers who are engaged in functional roof design and construction.
In areas of deforestation, where settlements are exposed to full impact from windstorms, tree planting programmes should be established to grow fast growing trees which will grow and serve as wind breaks to protect these settlements.
The use of green timber for roof construction must be regarded as a bad practice and as much as possible should be avoided. In any case well seasoned timber should be used where possible or at worst the green timber meant for the roofing could be put on the building for at least one month before laying the roof covering material.
Realising the need for good anchorage of the roofing structures to the rest of the building structures, some of the people in the affected areas are now using wire straps and 6 mm. diameter 'U' shaped mild steel anchor rods to anchor the roofing structures to the external walls. The anchorage rods must at least come down to about 500 mm. down the external walls and should be spaced at most on 2000 mm. centres. In any case a good anchorage must be provided to hold the roof against wind damage and must be inspected before the roof covering is fixed.
The openings in a building should be positioned so as to allow through ventilation so that during a windstorm, the wind forces could easily blow through the building to avoid the concentration of uplift forces on the underside of the roof which will tend to uplift the roof.
The critical stressed area on any roof surface are usually located on the eaves, verges and ridges, therefore the fastening of the roof covering materials to the roof structure should be such that the spacing of the fasteners are closer at these places than the other portions of the roof.
Well seasoned timber wall plates, liberally treated with creosote-bitumen solution should be embedded preferably on the inner side of the wall leaf.
The rafters should be birdsmouthed over the wall plate and should be cut at the top to fit against the ridge board. The birdsmouth cut must not be taken more than halfway into the rafter and must be well cut to make good contact with the wall plate. A tie member which can also act as a ceiling joist should be nailed across the feet of the rafters flush with the top of the birdsmouth to help to restrain the spreading of the feet of the rafters due to the action of the resulting outward thrust at the eaves from roof loads. In straight-purlin or gable roofs, the purlins must be anchored down in the gables before grouting and low pitches in such roofs must be avoided. Truss rafters must be anchored using bolted mild steel angle clips to roof beams and avoid using main reinforcement bars from columns to tie them.
To ensure effective performance of any roof covering material, the use of such material must comply with the manufacturer's specifications or requirements. In any case the optimum shelter design and construction research group in the Building and Road Research Institute has produced a roof manual which provides the basic guidelines for effective utilization of common roof covering materials and the construction of common roofing systems in the country, which can be consulted.
All roofing members and materials must be partially or totally replaced after their service lives have passed when there are signs of distress or deterioration in any of them. Causes of leakages in roofs must be found and immediately remedied to avoid any deterioration in the roofing members. All accidental damages must be repaired immediately they are detected.
There is an urgent need to train and re-train all artisans who are directly involved in the functional roof realization especially those in the rural communities to at least know the basic roof construction techniques so as to reduce the common occurrence of blown off roofs in the country.
There is also an urgent need to accelerate the development of alternative roofing systems which will make use of our locally available building materials, thereby reducing the cost of roofs and increase our construction capacity in the housing industry.
Finally, it should be emphasized that, it is better to have a roofing system with a high initial cost with reduced maintenance and trouble-free operation that to spend less on the construction of a roof with the risk of unsatisfactory job and possible high future maintenance cost.
Table 1: Identified Bad Roof Construction Practices and their Corresponding Procedures
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1 Addai S., Amoah-Mensah K., "Monograph on Common Roofs and Roofing Systems in Ghana for Improving Roof Construction Technologies in Settlements in Ghana", Shelter Delivery Research, Building and Road Research Institute (C.S.I.R.), Kumasi, July 1995.
2 Schreckenbach H., Abankwa J.G.K., Construction Technology for Developing Countries, W. Eschborn, GTZ, 1982.
3 Arnoah-Mensah K., "Monograph on Cost Saving House Building Techniques for Improving Housing Delivery Performance in Ghana", Building and Road Research Institute (CSIR), Kumasi, February 1988.
4 Fullerton R.L., Building Construction in Warm Countries, Parts 1, 2, Oxford University Press, 1978-1979.
5 Belevich V.B., Roofing Works, High School Publisher, Moscow, 1987.
6 "Construction Technology", Edited by S. Ataev, Mir Publisher, Moscow, 1985.
My sincere thanks are due to Mr. K. Amoah-Mensah, a Principal Researcher, the Head of Construction Division of the B.R.R.I and the Project Leader of the Optimum Shelter design and Construction research group for his indispensable help for guiding me through the entire research and also reading through the final manuscripts and making very important suggestions and other helpful comments. I am also thankful to the participants of the B.R.R.I. National Seminar on the perennial roofing problems for their valuable contributions.
I wish to finally thank Miss Esther Achiaa and Mr. Owusu-Mensah for the excellent typing of the final report and Mr. Bempah and his crew for the enormous work done in documenting the rip-offs on video cassettes during the roof design and verification survey.
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