Concrete roofs are cast similar to other suspended floors. They are finished to fall towards the gutters to shed the water. They should be cast to overhang the walls by about one foot. This is to ensure water from the roof slab does not flow down the wall facade. This creates water marks on the walls which when dry spoils the external finishes. The surfaces become discoloration. It is therefore important to treat the edge of the concrete slab to avoid this problem.
The edge of concrete slab can be treated using different methods. This allows the shedding of water away from the building fabric. The edge overhang can be finished a throating and nibs. This are fabricated below the concrete slab overhang. The principle is that as water flows on the edge of the slab, it drops off. It will not be able to have a back flow below the slab. The reason is that the throating is recessed into the concrete slab and the nib projects away from the slab.
The finish on the concrete overhang commences by first casting the roof slab. The nib is formed by using wire nails and mesh wire. The nails are fixed one inch of the concrete edge below. They are then tied up with mesh wire. This is for holding the plaster. The plaster is done using strong mortar mix. The mix is one part cement and two parts sand. It is done in two layers. The first is done and then the next is applied after one hour. A wood float is used to finish the nib square.
When finishing the concrete edge with a throating, a round bar is used. The slab is hacked about one and a half inches deep. A mortar mix of one part cement and two parts sand is done. The round bar is about three quarter inch in diameter. The mortar is applied first into the recess. The round bar is then pushed into the mortar to form a smooth half round finish on the recess. After completion, both the nib and the throating should be cured for at least four days.