In the construction of roofs the average spacing of rafters is 20 inches on centres. They should be doubled around all openings. The ridge is usually of a 1-inch by 10-inch piece. The size of the rafters varies with the length of span and load. They are usually 2 inches by 6 inches for short spans and light loads, and 2 inches by 8 inches or 2 inches by 10 inches for long spans and comparatively heavy loads. Valley rafters must always be deeper and heavier than the rafters and should be designed as a girder. The hip rafters do not carry any great load, but are often made deeper to fit the incline cut of the jack rafters. All floor-joists are spaced 16 inches on centres, and should be bridged. The following is the table commonly followed for good house construction, although lighter work is most often specified: SPAN TIMBER 12' and under 2" × 10" cross-bridged once. 12' to 15' 2" × 10" doubled every other one, if good stiffness is desired, and bridged twice. 15' to 20' 3" × 12" and of long-leaf yellow pine, crowned at centre ½", and bridged three times. 20' to 25' 3" × 14" of long-leaf yellow pine, crowned at the centre 1" for the 25' spans, and bridged four times. Floor-joists should be doubled around all openings larger than 3 feet, and joists should be hung from the header beam by metal straps. There are many precautions which should be taken to prevent the spread of fire in the wooden frame house, but those will be considered as a special subject. Likewise the discussion of certain defects of construction which are commonly found in the speculative house will be dealt with later. V CONSTRUCTION OF THE MASONRY AND WOOD DWELLING In one of the previous chapters it was pointed out that the type of construction next in general use to that of the wooden frame house was the dwelling of masonry and wood. This was designated as Type II, and defined as a building with exterior walls of stone, brick, concrete, or terra-cotta, and interior floors and partitions of wooden frame construction. The difference in construction between the wooden frame structure and the masonry-and-wood building is mostly in the material used for the exterior walls. The interiors of both types are constructed in practically the same way, the floors being of light wooden joists and the partitions of wooden studs. The oldest varieties of the masonry houses in America are represented by the stone and brick dwellings of Colonial days. These are so substantially built, and often so artistic in conception, that they have become common models from which to draw inspiration. The concrete house of the monolithic or block type, and that of hollow terra-cotta tile, is a modern development. _The Stone House_