75. Sheet metal is seldom used in the interior of buildings except for covering walls, ceilings, etc., and even then it is only used for a cheap grade of work, so as to obtain an elaboration of ornamentation at a very low cost. The principal items for consideration are the methods of securing the sheets to the building and the selection of a design from manufacturers' stock.

It is customary for the sheet-metal worker to nail furring strips on the ceiling and walls at such distances apart as will conform with the design stamped on the sheets, but in no case should they be more than 1 foot apart. In selecting the furring strips for ceilings, particular care should be taken to allow for sufficient depth for any electric-wire conduits or gas pipes. The sheets are laid on the walls to overlap one another, and the nails are driven through the ornamentation in such a manner as to apparently form a part of the decoration. The lap seams should all be laid so that the light from the windows will shine against the edges of the sheets and thus avoid shadows which would indicate the location of the seams.

All sheet-metal wainscoting, baseboards, dados and chair rails, and, in fact, all ornamental sheet-metal work which is likely to be damaged by rough treatment, should be "pugged" behind with Portland cement.

All heavy moldings and cornices must be backed by lookout brackets which are cut to the proper profile. All wood backing should be covered with asbestos sheets when it is desired to form a fireproof enclosure. Plain black-steel sheets should be dipped in iron oxide and linseed oil before they are put up. In all damp places it is advisable to use bright tin or zinc; the latter is preferable.