We give below proper procedure of applying bitumen and laying mud phuska with brick tiles on top as given by the C. P. W. D.
Preparing the Surface :- The surface shall be painted when it is thoroughly dry. The surface to be painted shall be cleaned with wire brushes and cotton or gunny cloth. All loose materials and scales shall be removed and the surface shall be further cleaned with a piece of cloth lightly soaked in kerosene oil. ( Note : We also get bitumen primers for applying on roof instead of kerosene oil.).
Painting with Bitumen:- The contractors shall bring the bitumen to site in its original packing and shall open and use it in the presence of the Engineer-in-Charge or his authorised representative. The containers shall not be removed from the site until the painting job is completed and the Engineer-in-Charge has a time to satisfy himself regarding the quantity of bitumen actually used and given his permission to remove the same.
The surface prepared and treated shall be painted uniformly with bitumen of approved quality such as residual type petroleum bitumen penetration 80/100, hot cut back bitumen or equivalent, after healing it to the required temperature as per specifications of the manufacturers. The coat of bitumen shall be continued at least 15 cm along the vertical surfaces joining the roof. In case of parapet walls, it shall be continued upto the Drip courses.
Residual type petroleum bitumen of penetration 80/100 shall be heated to a temperature of not less than 180 C and not more than 190C and shall be applied on the roof surface at not less than I80"C. Similarly, hot cut back bitumen shall be heated to a temperature of not less than 165C and not more than 170C and shall be applied on the surface at not less than 165C.
Care shall be taken to see that no blank patches are left. The quantity of bitumen to be spread per 10 square metres of roof surface shall be 17 kg, unless otherwise stipulated in the description of the item and shall be carefully regulated so that the application is uniform at the stipulated rate of kg.
Spreading Sand :- Immediately after painting, dry, clean sharp and coarse sand at the rate of dm3 per 10 sqm shall be evenly spread and levelled over the surface when the bitumen is still hot.
Measurements:- The superficial area of the surface painted shall be measured in square metres. No deduction in measurements shall be made for unpainted areas of roof slab occupied by chimney stacks, roof lights etc. of areas, each upto 40dm2 The measurements of length and breadth shall he taken correct to a cm.
Rate :-The rate shall include the cost of all materials and labour involved in all the operations described above.
Mud Phuska :- For mud phuska, selected soil which should be good quality brick earth not containing excessive clay or sand, free from stones, kankar, grass roots and such foreign matter, shall be collected and stacked at site. The soil shall not be collected from a locality infested with white ants.
Before laying on the roof, the soil shall be made damp by adding water about 12 hours earlier. It shall be turned over with phawras so as to break clods and to pulverize the same. Quantity of water to be added to the soil shall be carefully regulated so that the soil shall have optimum moisture content at the time of laying and compaction on the roof. The soil shall be laid on the roof to requisite thickness and slope, well compacted with wooden rammers and thappies, to obtain an even surface to correct slope. Average thickness of soil after compaction shall be as specified for the item.
NOTE :- A practical way of determining the moisture content of soil suitable for giving good compaction is that the soil should contain that much quantity of moisture, which, when a handful of soil is moulded with hand to the shape of a ball, it shall just retain its form. If the soil on moulding cannot retain its shape of a ball, moisture content is inadequate. On the other hand, if the ball can be plastically deformed on pressing with hand, the moisture content is on the high side.
Mud Plaster:- After laying the mud phuska, the surface shall be given a coat of mud plaster 25mm thick and the plaster shall be allowed to dry and crack.
The mud plaster shall be prepared from the same soil as for mud phuska. The dry soil shall be reduced fine powder and mixed with water in a pit, adding fibrous reinforcing materials such as chopped straw (Bhusa) in proportion of 35 kg per m3 of soil. The choppings used shall not be longer than 2 cm. The mixture shall be allowed to mature for a period of not less than 7 days. During this period, it shall be worked over at intervals with feet and spades (Phawras) so as to get pugged into a homogeneous mass free from lumps and clods. The mud mortar shall be puddled again very thoroughly just before use.
Gobri Leaping :- After the mud plaster has dried, the surface should be given a coat of gobri leaping so as to completely fill any cracks that may have formed in the mud plaster. Mortar for gobri leaping shall be prepared by mixing equal quantities of fresh gobar and finely sieved clay and adding sufficient water to form a thin paste. The quantity of gobar used in gobri leaping shall not be less than 0.03 cum per 100 sqm of plaster area.
Laying of Tile Bricks :-- After the gobri leaping has dried, flat tile bricks shall be laid using the minimum amount of plain mud moratar ( without bhusa ) as bedding so as to obtain correct slope and even surface of tile floor. Care shall be exercised to see that mud mortar does not rise into the vertical joints of the tiles more than 12mm. The tiles shall be laid close to each other, the thickness of joints shall not be less than 6mm and more than 12mm in width. After the tiles are well set and bedding mortar has dried, joints of the tiles shall be grouted with cement mortar mix of 1: 3 ( 1 cement : 3 fine sand ) such that all the joints of the tiles are completely filled with mortar. Cement shall be mixed with 5% of crude oil by weight of cement and the surface of the finished roof shall be kept wet for a period of at least 7 days.
Curing :-- As soon as cement pointing obtains initial set, the surface of the tile brick floor shall be covered with wet gunny bags, hessian cloth or wet sand to prevent quick drying. After 8 - 12 hours, the tile brick floor shall be cured by frequent sprinkling of water on the surface for a period of 7 days. After curing has been done the surface shall be swept clean.
Slopes:-- The tile surface as completed shall be even and true to slopes of 1 in 48 or as specified and should be leak proof.