The amount of light received in a dwelling-house is largely determined by the =width of the street= and the distance between the backs of the houses in adjacent streets. The model Bye-laws of the Local Government Board insist that no new street shall be less than 36 feet wide if it exceeds 100 yards in length or is intended to be a carriage road, not less than 24 feet in any case. Furthermore, a new house must have in the rear an open space exclusively belonging to the house, at least 150 square feet in area, and free from any erection above the ground level. This must extend along the entire width of the house, and must never measure less than 10 feet from every part of the back wall of the house; the distance must be at least 15 feet, if the house is 15 feet high; 20 feet if 25 feet high, and 25 feet if 35 feet high or more. [Illustration: FIG. 37. DIAGRAM ILLUSTRATING THE NECESSARY REQUIREMENTS AS TO OPEN SPACE IN FRONT OF AND AT THE REAR OF DWELLING-HOUSES.] Streets should never be less in width than the height of the houses in them; and a line drawn from the ridge of the roof to the foot of the wall of the opposite houses (Fig. 37) or in the rear to the foot of the wall or fence dividing the back yards of contiguous houses, should not make an angle of more than 45º with the ground (Fig. 37). This is the angle required for new buildings in the residential parts of Liverpool, and was proposed for London, but unfortunately not made obligatory. The size of windows is discussed on page 216. The light received in a given house is often diminished at corners of streets by contiguous houses. The =Soil= has an important influence on the healthiness of a site. The relative merits of the different kinds of soil are discussed on page 219. Undrained soils of whatever kind are bad, and made-soils are always to be regarded with profound distrust. The =planning= of a house should be carefully considered. The principle is that the sun should enter every living room at some time of the day. The relative positions of fire-place, window, and door in each room are important. With the sole window of a room in the same wall as the fire-place the area ventilated is the least, with it situated on the opposite wall the area ventilated is the greatest. The door should be as remote from the window as possible, in order to secure occasional perflation of air; the two being preferably on opposite walls. Staircase windows are indispensable to secure through ventilation of a dwelling. Houses constructed “back to back” cannot be properly ventilated as no through current of air is possible. Hence the necessity for open yards at the area, as well as air-space in front of the house. (Fig. 37). In the =construction= of a house, apart from access of light and air, the main problems are to secure dryness and equability of temperature. We shall consider in this connection the materials used in the construction of walls, floors, and roofs. CHAPTER XXXI.