Lath can be used as an alternative to plasterboard as a base for ceiling plaster. Various types are available, but the most common are ‘expanded metal lath’ and ‘K-lath’. Wood lath has largely been replaced by these. Expanded metal lath is a metal diamond shaped mesh which is fixed in position with galvanised clout nails, screws or staples. It comes in 2.7m 600mm sheets and can be cut with tin snips. When fixing to the joists, stagger the sheets to avoid long joins, and overlap each about 13mm. Wire any unsupported joins with galvanized wire at frequent intervals. K-lath consists of a mixture of metal wire and paper. It is also cut with snips and fixed with galvanized nails or staples. Wood lath is made up of strips of timber about 1.2m x25mm x6mm. The strips should be nailed about 6mm apart across the joists, and the joints should be staggered wherever possible.
Plastering on lath
Before the normal floating and setting coat, a ‘rendering’ coat has to be applied directly to the lath. If you are using a lightweight finish coat, a plaster such as Carlite Metal Lath should be used. If using this plaster, however, you must use the same material for the floating coat. Using a sanded mix is more complicated, but it does enable you to use a slower-setting finish plaster. To mix a sanded rendering coat, first prepare a lime mortar by mixing three parts of sand and one part of lime with water (use a bucket as a measure). Hair, or a special nylon fibre made for the purpose, should be added to this mix to strengthen the finish mortar. As a guide, if you have used three bucketful of sand and one of lime mix in about one handful of hair or fibre.
Allow this mix to stand for about 24 hours, then mix six parts of it to one part Portland cement. Add water, but only enough to make the mix fairly stiff. When applying the rendering coat, press it on firmly with the laying trowel so that the plaster firmly ‘keys’ with the lath. If you are plastering over wood lath always apply the plaster in the direction of the joists, so that you plaster across the ‘run’ of the lath. Once the lath has been covered to a depth of about 6mm, key the surface well and leave it to set. Now mix your floating coat and lay screeds around the edges of the ceiling with the laying trowel. Rule them in with the featheredged rule. Divide the ceiling into manageable sections with further screeds if required.
Then proceed to fill in the sections and rule them flush with the screeds. Smooth over and key the surface with the devil float, and clean out the corners with the laying trowel. When dry, skim on a thin layer of finish plaster with the steel trowel. Go around the edges first and then apply strips from one side to the other. Follow up with the wood float, using strokes in the same direction. Then put on another layer, this time crossing the previous strips at right angles. With the steel trowel, lay on a final coat and then smooth all over. Clean out the corners where the ceiling meets the walls with the angle trowel and, finally, wash down adjacent plasterwork if required.