FBE 03 Building Construction Science Lecture 5 Pitched

FBE 03: Building Construction & Science Lecture 5 Pitched roof constructions

Introduction • Pitched roofs have their surfaces sloping • Types of sloping roof: ü one side only (monopitch), ü on two sides (dual pitch) or ü on three or four sides (hipped) ü lower portion of the roof is pitched at a steeper angle than the upper portion (mansard)

Pitched roofs • formation of pitched roofs: ü couple roof ; ü close couple roof; ü collar roof; ü double roof; ü triple roof; ü trussed rafter roof; ü hipped roof; ü valley roof and ü lean to roof

couple roof • simplest form of pitched roof construction and is suitable for short span roofs • approximately 3 m span • rafters bear onto 100 x 75 mm wall plates • wall plates are fixed to the top of the supporting walls with the aid of galvanised mild steel straps at a maximum of 1. 2 m centres • 175 x 32 mm ridge board place at 400, 450 or 600 mm centres to a 175 x 32 mm • foot of the rafter is cut with a bird’s mouth joint to fit over the wall plate and nailed to the wall plate

couple roof

galvanised mild steel straps

bird’s mouth joint

close couple roof • spans exceed 3 m the dead load • superimposed loading on the rafters could cause them to ‘spread’ at their base • imparting an inclined thrust to the supporting walls • the paired rafters on each side of the roof may be joined together at their base by a horizontal timber called a ceiling joist • ceiling joists perform another useful function in that they can be used to support the ceiling finish on their underside • raised up the roof to form a collar and to provide an increased ceiling height

close couple roof.

collar roof

double roof • roof spans exceed 6 m the rafters will require support in order to prevent them deflecting excessively • support is provided by the purlin, positioned normal to the rafters at mid- span, creating a double roof • The purlins span between gable end walls on dual pitched roofs • support may be provided by inclined struts

Double roof

triple roof • comprises a truss consisting of rafters, struts, ties, hangers and ceiling joist • prefabricated into a self-supporting framework • whole roof is then tied together at the ridge, purlins and binders

triple roof

Hipped roof • roofs that are pitched on three or four sides • comprising common rafters, hip rafters at the external intersection of pitched surfaces and short jack rafters, spanning between the wall plates • hip rafters need to provide a fixing for the jack rafters, they need to be deeper than common rafters

Valley roof • internal intersections of roofs valleys are constructed • valley rafters need to be deeper than common rafters due to their need to support the end of the valley jack rafters

hipped roof

Pitched roof coverings • Coverings to pitched roofs are normally in the form of small tile units, fixed to battens, which in turn are fixed to the rafters • Weatherproofing of the covering • prevent wind-driven rain from entering the roof under the tiles, a secondary weatherproof barrier is provided using sarking felt (bituminous felt)

• Roof tiles materials: ü Slate ü Clay ü Concrete

Slate • • • Slates are derived from metamorphic rocks tiles 3– 11 mm in thickness slates are blue–black durable covering because derived from rocks expensive due to limited supplies Slates are either head or centre nailed to the battens using copper 2 nails to the batten

Slate covering to a pitched roof

Bonding of slates in successive courses

Clay tiles • hand-made or machine-pressed clay and kiln fired in a similar manner to bricks • Many choices of colours • smooth or a sand-faced finish • tiles incorporate nibs on their undersides, which enable them to be hooked over the tile batten so that they do not need to be fixed at every course • Nailing is therefore undertaken at every fourth course of tiles

Clay tiles • clay tiles absorb water they must be laid to steeper pitches than other tiles, to allow water to run off the roof more quickly

Types of clay tile

Clay tile

Clay tile • Roman tiles ü 420 x 345 mm in size ü single lapped with opposite corners chamfered ü interlocking side joints ü consists of grooves on one side edge of the tile and a corresponding nib on the other side edge ü smooth, sanded or glazed finish

Concrete tiles • from a fine mixture of cement and well-graded sand • machine made by placing the mixture into steel moulds and curing for 1– 2 days • Many colours choices • sand-faced finish although smooth finishes are available • more durable than clay

Special tiles • for features such as ridges, hips and valleys • manufactured from either clay or concrete

Flashings • Flashings needed when roof meets projecting external walls or where a chimney stack • Flashing is an impermeable material • minimum of 150 mm up • To prevent rain water ingress into the roof through the interface

Flashing

flashing