The under part of the Flooring, and the _Plat-Fonds_, must be made also with great Care. To make the _Plat-Fonds_ or _Flat-roofs_, in the Form of a Vault, you must nail to the Joists of the Boards, or to the Rafters of the Roof, from two Foot to two Foot pieces of crooked Timber, and Choice must be made of Timber that is not apt to rot; such as, _viz._ _Cypress_, _Box_, _Juniper_, and _Olive_; no _Oke_ must be made use of, because it will warp and crack the Work. The Joists being fastened to the _Summers_, you must fix to them _Spanish-Broom_ with _Greek-Reeds_ well beaten. These Reeds are in stead of Laths, which at present are made use of to make the Eaves of Houses; over these Reeds must be laid a Plaster of Mortar, made of Sand, to hinder the Drops of Water which may fall from above from endamaging these _Plat-Fonds_. After which, the under part must be Plaster'd pretty thick, making all Places equal with Mortar made of Lime and Sand, that it may be afterwards Polished with Mortar made with Lime and Marble. [Sidenote: _Lib. 5. Chap. 10._] The Ancients sometimes made double Vaults, when they were afraid that the Humidity which is engender'd, by the Vapours which mount up might rot the Wood which is upon the Vaults. This Method they principally made use of in their Baths. The _Corniches_ which are made use of under the _Plat-Fonds_, ought to be little, lest their great Jetting out, or Projecture should make them heavy, and apt to fall. For this Reason they ought to be made of pure Stuck of Marble, without any Plaster, that all the Work drying at the same time, may be less apt to break. ART. VI. _Of Plastering_. To make Plaster that it may continue a long time, and not crack; you must take Care to lay it on Walls that are very Dry; for if the Walls be Moist, the Plastering being expos'd to the Air, and drying faster than the Walls, will crack. To do this Methodically, it must be laid, Bed after Bed, or Lay after Lay, having a great Care not to lay one Bed till the other be almost dry. The Ancients put six Lays, three of Mortar made of Lime and Sand, and three of Stuck. The first Lays or Beds were always thicker than the last, and they were very careful to make use of no Mortar made either of Sand or Stuck in their Plastring, that had not been a long time beaten and mix'd together; especially the Stuck, which must be beaten and mix'd till it will not stick to the Trowel. They took likewise a great deal of Pains to run several times over and beat the Plaster, which gave it a Hardness, a Whiteness, and Polish'd it so well, that it shin'd like a Mirror. These Plasterings so made, serve to Paint in _Fresco_ upon; for the Colours being laid upon the Mortar before it was dry, pierced it, and Embodied with it; so that the Painting could not be defaced though it were wash'd; which would easily be wash'd off if the Mortar were dry.