A pitched roof is a roof comprising a sloping surface or surfaces with an angle of usually over 20 degrees. It is a bi-fold, bi-planer roof with a ridge at its highest point. To be more specific – a pitched roof requires a pitch of more than 10 degrees or a roof with two slopes that meet at a central ridge.
Pitched roofs offer a more classical appearance. They remain the most common and efficient solution in wet and cold weathers to ensure maximum protection.
Parts of Pitched Roof
Followings are the major parts of pitched roofs.
Description of these parts of pitched roofs is given below.
In the roof, the lower portion of the inclined roof which is projecting beyond the outer face of the wall. The main function of eaves is to rainwater to pour from the roof directly on the exterior wall. I also give protection from sunlight. Sometimes eaves are highly decorated to increase architectural beauty.
The Triangular upper part of the wall at the end of a ridge roof.
A strip of impervious material, usually metal used to exclude water from the junction between a roof covering and another part of the structure.
Flashings are provided to prevent moisture from entering the wall and roof through joints in copings, parapet wall, and other penetrations through the roof plane.
The outer angle (more than 180 degrees) formed by the inclined ridge between two intersecting roof slopes.
The horizontal intersection at the apex of the two rising roof surfaces inclined in opposite directions.
These are horizontal members laid on principal rafters on wall-to-wall to support common rafter of a roof when the span is large. Purlins are made from wood or steel.
The top surfaces of the purlins shall be uniform and plane. They shall be painted before fixing on top.
This is wooden board fixed to the feet of the common rafter at eaves. The ends of the lower-most roof covering material rest upon it.
Rafter is a structural component of a pitched roof. Typically it starts from roof support (column or wall) to ridge or hip. Multiple rafters are used side by side with equal spacing to support the purlin. In typical home construction, wooden rafters are used. For large industrial of commercial sheds, steel rafters are used.
These are thin strips of wood which are fixed on the common rafters or on the top of ceiling boards to support the roofing materials such as tiles, sheets, etc. Batten could be also made of metal or plastic.
Types of Pitched Roof
Followings are the type of pitched roof
- Mono Pitch Roof
- Couple Roof
- Closed Couple Roof
- Collar Roof
- Purlin Roof
- Trussed Rafter
Brief descriptions of these pitched roof types are given below.
Mono Pitch Roof
- Slopes from one side or part of a building to another commonly used to form extensions.
- Comprises a series of rafters fixed to plates at the top of a wall and the rafter feet are nailed to a wall plate, which distributes the load evenly across the supporting wall.
- Joists are fitted to form level ceilings and could be raised to give more height.
- Joists are commonly supported by struts, which precludes rafters from sagging.
- The simplest form of the pitched roof, the couple roof comprises two lengths of rafters leaning against each other, tied where they meet at the top.
- A very limited span of approximately 3.5 m.
- The downside of this roof is that the weight of it creates natural deflection in the supporting walls by pushing those outwards at the top. Walls could be reinforced but this would require extra brickwork and unnecessary expense.
Closed Couple Roof
- Ceiling joists are added, a length of timber running horizontally in-between the rafter feet, making the structure much more secure. The joist acts as a tie that prevents the outward deflection of the wall and increases the potential roof-span to approximately 5 m.
- Joists are secured to the rafter feet rather than the wall Plate to negate any potential deflection. A secure connection between the rafter and ceiling joist is therefore critical.
- The benefit of this form of roof is by using struts, much of the roof space could be utilized for storage and allowed the space to be used for accommodation if needed.
- The height of the ceiling joists are raised, such that the roof allowed any upper rooms to be constructed in the roof space, thus the height of the external walls is slightly reduced.
- The drawback of this method is that lifting the ceiling joist reduces its restraining force, increasing the instability of the supporting walls and decreasing the span to approximately 4 m.
- For maintaining stability, the height of the ceiling joist could be lifted to a maximum of 1/3 of the height of the roof.
- By introducing purlins roof spans could be increased without compromising wall stability, increasing the size of rafters or attracting extra costs. Purlins allow rafters to get extra support and become lighter and thinner, allowing a potential span of 8 m.
- The benefit of this form of roof was that by using struts, much of the roof space could be utilized for storage and ultimately allowed the space to be used for accommodation
- The fink or 'w' truss is the most common form of the trussed rafter in modern house construction. This consists of a rafter comprising W shaped tension and compression members. This trussed rafter is capable of spans up to 12 m and can be designed to accommodate many different pitch angles.
- The most significant advantage is the off-site assembly of the trussed rafter which speeds up the whole construction process.
- The addition of diagonal bracing from eaves to ridge on the underside of the rafters binds the whole structure into one unit rather than a series of individual trusses, providing protection from possible collapse due to wind forces.
- The only real disadvantage of the trussed rafter is that it requires careful design where a change of direction occurs in the roof.