Page:Cassells' Carpentry and Joinery.djvu/112

94 wood-block floor, the ground should be taken out from 8 in. to 11 in. (according to the thickness of the blocks) below the intended floor-line; the surface should be roughly levelled and rammed solid; 1-ft. 6-in. stakes are then driven into the bottom about 6 ft. apart, and levelled off to 6 in. above the ground; the site is then filled in with concrete to the depth of the stakes, and the surface beaten smooth. A blue lias lime, or Portland cement, should be used for the concrete, in the proportion of 1 cement to 6 aggregate. The concrete bed should be allowed to settle and dry before proceeding with the next step, which is the floating of the top with a ¾-in. layer of Portland cement and sand, 5 to 1; preparatory to

Fig. 391.—Herringbone Pattern of Wood-Block Floor with 18-in. Blocks.

this screeds of cement about 3 in. wide should be run around the margins, and across the room every 6 ft. or 8 ft.; these should be accurately levelled and struck straight with a long float, and when set will become levelling points from which to strike off the surface of the cement; before the latter has become hard it should be brushed over with a birch broom to score the surface; it must then be allowed time to become perfectly dry, as any trace of moisture will be fatal to the adhesion of the bitumen coat next to be laid. From seven to fourteen days, according to the state of the atmosphere, will be required for this purpose; and as an additional precaution just before laying the bitumen, or matrix, as it is termed, dust a little fresh lime or some fine dry ashes over the surface; these must, however, be swept thoroughly off before running on the mastic. The bitumen is sometimes laid in two coats, the first being allowed to set before proceeding with the second; the purpose of this is to ensure a substantial layer of bitumen between the blocks and the cement, but this is only necessary on very damp sites. The objects of the three different layers under the floor are: The concrete is to form a substantial and unyielding foundation, and also to prevent the ground-air arising; the cement layer is to form a hard and regular surface to which the matrix can adhere; and the matrix is a damp-proof layer that will effectually prevent any moisture that may pass through the cement from reaching the blocks, and also, being strongly adhesive, it keeps the blocks attached to the cement. Various mixtures are used for matrices, the best having mineral bitumen as a base; but frequently a simple mixture of Stockholm tar and pitch, in the proportions of 2 of tar to 1 of pitch, is used (note, gas tar is unsuitable). When this is laid, in a single coat, screeds of wood about ¼ in. or ⅜ in. thick are nailed lightly to the cement to form bays about 4 ft. or 5 ft. square; two of these should be filled in with the melted matrix, which should be boiling hot, and the first filled in will be ready for laying the blocks by the time the second is filled. The best consistency of the matrix for laying is when it is thick enough to receive the weight of the block without allowing it to sink in, and yet warm enough to amalgamate properly with the mixture adhering to the blocks. Scaffold boards should be laid across the bays, resting on the screeds, for the men to kneel on whilst laying.

Laying the Wood Blocks.—The mastic, as the fumes are suffocating, should be heated in a large iron cauldron in the open air, and brought into the building in iron pails. The blocks should be stacked in the room near the doorway, each cut to its proper size and each series stacked by itself. To do the work properly two men at least will be required to lay, working into each other's hands, and one to deliver the blocks as required. The order of delivery and of laying will depend on the design, as will be mentioned presently. The blocks are dipped to