Roof
Also called: roofing, roof truss, trussed roof, king post roof, queen post roof
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A roof is the structure that covers and closes the top of a building, keeping the weather out. Which shape it takes is decided by the building's size, shape, location, use and looks, taken together.
Roof shapes and their names
The gable, or pitch, roof, two flat slopes meeting at a ridge, is a widespread and effective design, common on farms and small houses. A lean-to roof takes its name from usually resting against the wall of a larger main building, and this form is common where a shed or annex is added onto a bigger one. A sawtooth roof grew out of the lean-to form: it amounts to a run of lean-to roofs set side by side over one building. Where two roof slopes meet, the line they form is named by which way it points. A hip is the roofline running from ridge to eaves where two slopes meet in an outward-facing angle. A valley is the roofline running from ridge to eaves where two slopes meet in an inward-facing angle.
Framing the roof
Where the joists follow the fall of the roof, short outrigger joists, timbers that project out past the end walls, extend the roof beyond them; those same outriggers also give a place to fix both the fascia, the board that closes off the rafter ends, and the decking material above. A marked pivot point acts as the reference datum for laying out the bevels cut on roof timbers, including common rafters and hip, valley and jack rafters, the shorter rafters that run between a hip or valley and the ridge or eaves. An attic roof can also be framed with horizontal curved rafters instead of the usual vertical ones, each rafter following the outline of a window's front elevation and blending into the main roof's slope higher up. Setting a dormer window vertically, rather than in line with the plane of the roof, tends to give a better-looking roof and often better light, though not always. A dormer's own framing, and the layout of its sashes, is shaped by the type of roof, the width the window needs to be, and the architectural style of the building.
Curved and jointed roof members
Sound, straight timber alone makes flowing curved roof shapes very hard to form; shaping a roof's curved ends instead depends on the natural bendability of heartwood, the dense timber from the centre of the tree. Fitting together the members of a complex roof truss can test a carpenter's skill, calling for very elaborate tsugite and shiguchi, the interlocking scarf and mortise joints used in Japanese timber framing. One such roof looked as though it would need welded connections at its joints, yet clever flying rafters, rafters carried out to the eaves without a post beneath them, carried the eaves without any.
Covering the roof
Canvas roofing sees heavy use as a waterproof deck covering on excursion steamboats. In roofing installation, once cement has been applied, the lower edge of the first sheet is lifted again and more cement added the same way as before; nailing itself starts at the roof's centre and works outward toward each end. In American platform-frame construction, roof sheathing is typically made from the same choices of material as wall sheathing and the subfloor. Thick cement roof tiles leave almost no room for error, so their layout dimensions have to be worked out precisely. The depth of a keraba, or soba, eave, the projecting edge of a Japanese tiled roof, is set by two things: the roof's total width and the size of the clay tile, called kawara, used to cover it; that depth also varies again with whichever roofing material is chosen, whether Japanese clay shingles, cement roofing, or standing-seam metal, roofing made from long metal panels joined at raised, interlocking seams.
Fire safety and roof glazing
Solid blocking fitted between the rafters at a roof's overhang can help cut down ember buildup there, a measure aimed at wildfire protection. USDA (2021) reported that research from the Building Research Institute has linked angle complexity within a roof assembly to how flammable the roof can be. In one American system of post-and-beam timber-frame building, attractive exposed rafter work tends to push the space under the roofline toward living space rather than storage, and roof windows offer a cheap way to add light and airflow into it. Keeping each pane of roof glass short is one way to hold down the total glazed area; even so, one builder using this system fitted roof glass on only about a dozen of the more than two hundred houses built.