Jack arch roof is composed of arches of either brick or lime concrete, supported on lower flange of R.S.J.s. The joists are spaced 1.0 to 1.5 m centre to centre, and are supported at their ends either on walls or on longitudinal girders. The rise of the arch is kept 1/12th of the span. The minimum depth at the crown is kept at 150 mm.

Due to superimposed load on the arch, the tension develops on the supporting member, specially on the walls at the end span. Tie rods are provided at the end spans to counteract the tension. The tie rods of diameter 20 to 25 mm at 1.8 to 2.4 m centre to centre are anchored into the wall. The bottom of the floor is not plain which is a disadvantage.

Repairs to Jack Arch Roof:

Jack arch roof, due to ageing, undergo decay and requires repair for restoration to original functional condition.

Repairing of Jack arch roof will depend upon the extent of damage undergone with respect to:

i. Infilling materials — concrete,

ii. Arch, and

iii. Joists.

Thorough inspection and investigation need be carried out to ascertain the damage.

The loose infilling materials are removed first and if the arch and the joists are found in good condition, they need not be disturbed. After total removal of the infilling materials — which is generally 150 mm over the crown — a cast in situ R.C.C. slab of thinner section may be laid.

This would reduce the dead load and also would increase the room height. The R.S.J.s. will have to carry lighter dead load would be relieved to some extent.

But, in cases of damaged arches, if the damage is not heavy and can be restored, the arch may be repaired by wedging from the bottom. When the arched are restored, a thin cast in situ R.C.C. slab may be laid as before. But if the arches are damaged beyond repair, it is better to demolish those and remove the whole thing keeping the R.S.J.s only in position, provided of course the joists are in good condition.

After removal of the filling materials and the arches, cast in situ R.C.C. slab may be laid over the R.S.J.s. The section of the slab would be thinner as the spacing of the R.S.J.s are 1.0 to 1.5m. The R.S.J.s, If found partially corroded or for better protection and aesthetic view, may be encased and integrated with the slab over them.

This will provide better functional efficiency. The R.S.J.s relieved of dead load of filling and arch, with only thinner slab as dead load will function better and longer. In case the R.S.J.s are encased, these should not be painted, otherwise, when left exposed, should be scrapped, cleaned and painted.