11 FLAT ROOF CONSTRUCTION

Transcription

1 CARPENTRY & JOINERY FCA CARPENTRY & JOINERY THEORY 11 FLAT ROOF

2 Flat Roofs The definition of a flat roof is any roof which has a slope of less than 10 degrees or a fall ratio of 1:6. This type of roof is associated with garages and small house extensions. The waterproof covering is usually bitumen felt although the new innovation is a glass fibre covering, but it is much more expensive than traditional bitumen felt. Roof falls The direction of the slope of the roof is called the fall. There are three common methods of producing the fall and these are shown in the sketches below

3 Roof Falls The fall of the roof will depend upon: The type of roof construction and its covering. The location of the building. The position of any surface water drain. The amount of fall should be sufficient to clear away any water and direct it to any surface water outlet pipe. If the fall is insufficient to accomplish this, the water may form pools which can increase the weight of the roof or lead to water penetration. Construction Methods Used To Obtain The Roof Fall There are three basic methods of obtaining the fall of a flat roof. The chosen method will depend upon the type and use of the building and the internal ceiling fi nish required. Sloping Roof Joists In this type of construction, the slope or fall is obtained by sloping the roof joists. This type of construction is used where there is no level ceiling required. Tapered Firring Pieces In this type of construction the slope or fall is obtained by attaching tapered pieces of timber to the roof joists. This type of construction will produce a level ceiling. Deepened Joists (Parrallel Fittings) This type of construction involves parrallel firring pieces, the firings are diminished section and are nailed at right angles to the fall of the roof

4 Roof Joists All the roof joists used in the construction of flat roofs must comply with the requirements of current building regulations. These regulations govern: The sectional sizes and grades of timber. Dead loads imposed upon sectional areas of roof. Maximum clear span for imposed loadings. Spacing of joists in relation to covering and imposed loads. NOTE: The spacing of roof joists is always shown as a distance relating to the centre of one joist to the centre of another. When determining the centres of the joists, the ceiling material such as plasterboard must also be taken into consideration. It is good practice to ensure that all unsupported edges of the roof covering, as well as the edges of the ceiling material, are supported by means of noggings nailed to the inside of the joists. Sometimes solid noggings are used. These have the added advantage of stiffening the joists and preventing twisting

5 Supporting Joists Depending upon the slope of the roof, joist ends must be supported at: The eaves. or The verge. or Where the joists abut to another building. To determine the size of the joists, there are certain dimensions required. The drawing above shows the dimensions the building regulations relate to determine the size, grade and section of the joists to carry certain imposed dead loads. Two of the most important dimensions are the clear span of the joist and the centres at which they are fixed. Methods Of Supporting Joists There are various methods of supporting joists.the method adopted usually depends on the position of the building and the fall of the roof. The methods that can be used are: Joists built into walls. Supported by steel beams. Supported by joist hangers built into walls. Supported by timber bearers and a supporting fi llet. Supported by timber bearer and framing anchor. As part of a frame held together by framing anchors and joist hangers

6 Methods of Supporting Joists Built In Ends of joist built into wall. The ends must be treated with a preservative to prevent rot. Attached To Steel Beam This occurs where there is an opening in the wall and a steel lintel has been inserted. Joist Hanger Joist hanger built into wall to accommodate the joist. Wall Bearer And Fillet A timber wall piece is fixed to the wall, and a fillet is fixed to the bearer. Joist is notched over the fillet. Framing Anchor 11.05

7 The drawing below shows the construction of the flat roof to a garage. All the details shown in the following sheets pertain to this roof. The roof is a flat roof abutted to the gable end of the bungalow. The front wall of the garage has a parapet, which is capped with a brick on edge finish. The joists span the shortest distance and are built into the gable wall and extend over the eaves and rear verge to form an overhang. The decking is of cement bonded chipboard and covered with layers of bitumen felt with a final coating of bitumen bedded chippings

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9 Box Gutter An alternative to an external gutter construction is a box gutter. A box gutter is a gutter formed within the overhang of the eaves. It is an integral part of the roof construction and gives a neat appearance to the fascia. However it is much more costly and time consuming to form this type of gutter. The rainwater downpipe includes a cover to prevent any debris flushing down the pipe and blocking it

10 Types of Roof Decking The sheet material used to cover a flat roof is called decking. Decking must conform to the requirements of current building regulations and must be able to withstand imposed loads such as: Dead loads the imposed loads associated with the weight of the roofing materials as well as snow and ice. Live loads the imposed loads of people working and walking on the structure. Wind loads the imposed loads of strong winds. There are numerous types of decking materials that can be used. The chosen material will depend upon the type of construction and the thermal insulation qualities required. The most common types of decking are: Tongued and groove boarding. Plywood sheets. Chipboard sheets. Strand board (OSB). Preformed metal sheeting. Composite boards

11 Roof Insulation The main function of any thermal insulation is to prevent heat from leaving or entering a building. All good insulating materials have a cellular structure that traps air within it and air is an excellent thermal insulator. There are many good insulating materials available in the form of a rigid sheet or as a semi-rigid slab or quilt. The more common types are expanded polyurethane or polystyrene slabs, mineral wool, or glass wool quilt. The position and way the insulation material is incorporated within the roof structure will determine whether the roof is classified as either a warm roof or warm deck construction or a cold roof or cold deck construction. Cold Deck Construction Warm Deck Construction 11.10