This section is from the book "A Practical Treatise On The Joints Made And Used By Builders", by Wyvill J. Christy. Also available from Amazon: Practical treatise on the joints made and used by builders.
This is formed when the edges of grounds are bevelled off to enable the plasterer upon laying on his first coat of rough stuff to key it well against and behind them. Similarly it is made whenever stucco or plaster is worked to an angular edge by trowelling against cores, or brick cornices, or bracketings, or any other projections on the exterior or interior of a building either when covering it in the one case with ornamental features, or in the other when decorating the walls with dados and pilasters, etc, or groining or panelling the ceiling, or adorning it with coved or other elaborate cornice.
When lathing is properly executed the butt joints of the laths are well broken by snatching.
Butt Joint occurs in lathing when the laths are nailed butting end to end and not overlapping, which latter arrangement only prevails in common work.
This is formed as a recessed moulding in stucco to imitate rustications. The plaster soffit also between the fascia and wall plate in half-timbered houses, etc, is keyed into grooves run in these pieces.
Grounds by being grooved or else mitred or splayed, or bevelled, afford a good key to secure the edge of the rendering. Laths are nailed to joists, quarters, battens, bracketings, etc, at intervals apart of about £ in. so that the plaster may pass between or become insinuated, and by curling and widening out at the back, form a key or clench to preserve the junction. The joints of walls are left purposely rough or else raked out with the same object. Rendering and pricking up coats are crossed or scored with a scratcher to give a key to the next coat, and in plastering under fireproof floors, etc, various devices such as dovetail grooves, indentations, and bedded fillets are adopted to yield the necessary hold.
This can scarcely be called a joint since the parts of a plastic substance enter into a somewhat similar union to that effected by fusion. There is some reason, however, in regarding it as one, considering that in running mouldings they are always broken off before reaching the mitre. In rendering walls the plasterer occasionally mitres with the grounds when bevelled to an angle of 45°. In this artificer's work both inside and outside mitres occur at every angular turn or change of direction of skirtings, cornices, mouldings to pedestals, pilaster caps, bases, panels, etc, and it is essential that they should be accurately formed, showing well-defined angles and arrises. They are usually finished off by hand after the longitudinal parts have been worked by running the mould. Where enrichments are used to interior cornices the plaster castings have likewise to be neatly mitred at the angles, bedded and jointed in plaster, and screwed on where necessary.
Tongued Joint is formed when trowelling or working up to grooved grounds.
The joint between plastering and laths is often only a toothed one, because when the surface covered is woodwork, between which and the plaster there is no room for the latter to curl round, the plastering can only adhere by the tooth afforded by the roughness of the wood. In superior work such a contingency is obviated by brandering or counterlathing. When the laths also are nailed with insufficient interval between them, as is by no means unfrequently the case, the plaster mainly sticks by the tooth of the laths. A vertical surface, however, scarcely requires the key or clench between the laths so essential to ceilings and soffits for their security.