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Last edited 05 Sep 2019
A roof structure has basic functional requirements that have to be fulfilled, these can be broken down as follows:
The main basic requirement is to keep the weather out and the warmth in, enabling it to maintain a comfortable environment for its inhabitants to live in and carry out the social activities that the building was designed for. A roof should provide adequate insulation as it is the main area of a building where heat loss may occur.
 Strength and stability
A roof has to have the ability to carry the self-weight of the roof covering and structure and be able to resist forces from winds and the applied load of snow. The structure should be built as light as current technologies allow to keep imposed loads on the supporting walls to a minimum and finding the most economic means of carrying the roof structure and its load over spans of varying degrees.
The type of roof used can alter the appearance of a building. There are many types of coverings with different colours and textures which may add to the appeal of the finished building. The slope of the roof also has a major impact on the aesthetics.
The mono pitch roof was commonly used to form extensions in Victorian times and is still used in a similar fashion today. It 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 were fitted to form level ceilings, and could be raised to give more height. They were commonly supported by struts, which stopped the rafters from sagging. Victorians often built the timber joists into the wall, creating a risk of timber rot and cold bridging. This could potentially compromise the stability of the wall. This is no longer considered good practice and joist hangers are now commonly used which give the same support, while not affecting the wall.
The rafters sit on a wall plate - a length of timber usually 100 x 75 mm bedded on mortar on top of the wall. The wall plate provides a fixing point for the feet of the rafters, and is an efficient means of spreading the load exerted by the roof structure down through the walls without creating pressure points where each rafter meets the wall. As the mortar does not bond the wall plate to the wall, steel straps are used to ensure that the roof structure remains secure.
The couple roof has a very limited span - approximately 3.5 m. Historically the problem with this type of structure was that the weight of the roof created natural deflection in the supporting walls by pushing them outwards at the top. Walls could be reinforced but this would require extra brickwork, adding unnecessary expense. In order to support their load, rafters needed to be thick to prevent twisting under pressure, making them difficult to handle on site and expensive.
By adding ceiling joists, a length of timber running horizontally in-between the rafter feet, typically 75 x 50 mm to the couple roof form, the structure became much more secure. The joist acted as a tie preventing the outward deflection of the wall and increased 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. Ridge boards provide restraint for the top of the rafters, preventing lateral movement. The timber used for ceiling joists was commonly thinner than the rafters, so they required support.
This was often provided by internal load bearing walls, although in their absence 'hangers' and 'binders' were used. Often built into the gable wall for extra support, binders were nailed to the centres of each ceiling joist parallel to the ridge board. Problems associated with this method were damp penetration, timber rot and also the risk of compromising the supporting wall.
By raising the height of the ceiling joists the collar roof allowed any upper rooms to be constructed partly in the roof space, leading to some economies by slightly reducing the height of the external walls and therefore the amount of brickwork needed.
The problem with this method was that lifting the ceiling joist reduced its restraining force, therefore increasing the instability of the supporting walls and decreasing the span to approximately 4 m. In order to maintain the required stability, the maximum height the ceiling joist could be lifted to is 1/3 of the height of the roof.
In order to increase potential roof spans without compromising wall stability, increasing rafter sizes or attracting extra costs, purlins were introduced. By installing a purlin into the roof structure, rafters were given extra support and no longer needed to be as thick and heavy, allowing a potential span of 8 m.
To ensure as small and economic purlins as possible they were sometimes supported by struts, bedded on wall plates on load bearing internal walls. Internal load-bearing walls allowed smaller ceiling joists to be used and were positioned to cover the shortest distance across a room as possible. Hangers and binders were also used to stop the ceiling joists sagging.
Purlin roofs were constructed on site, and included many sections of timber, which needed to be assembled to form the final structure. This was obviously very labour intensive and skilled craftsmen were needed in order to install the roofs. The benefit of this form of roof was that by using struts, much of the roof space could be utilised for storage and ultimately allowed the space to be used for accommodation if needed via a loft conversion.
A problem with purlin roofs is that the purlins themselves need support at intervals along their length. Although this could be achieved by an internal load bearing wall, this was expensive and also disrupted the clear open space within the building. A method of providing this support was developed using large sections of timber creating a truss.
Commonly used up until the late 1920s, trusses enabled wider spans to be achieved than previous methods. It was common to have two trusses in a typical dwelling, however more could be used, depending on the load.
Frames comprised a beam, which spanned external wall to external wall, upon which the rest of the truss was based. Trusses supported the purlins, spanning truss to truss. The ridge board sat on top of the truss, to which the intermediate rafters were attached. The intermediate rafters were attached to the wall plate and supported at their centres by the purlins.
The most common form of truss was the king post, named after the vertical member in the centre. Other forms were available, and were adaptable to the type of building required. The queen post truss allowed the roof space to be utilised leaving clear space for access.
The Timber Development Association (now known as TRADA) was formed in 1934 with the task of addressing the problem of materials shortages. They identified roofs as a source of wastefulness, and sought more efficient construction techniques in the form of trussed rafters. The TDA produced various designs first published in the 1950s, which gradually lowered the pitch, while increasing the span.
In modern house construction the most common form of trussed rafter is known as the fink or 'w' truss. This consists of a rafter incorporating tension and compression members in the shape of a W. This trussed rafter is capable of spans up to 12 m and can be designed to accommodate many different pitch angles.
The timbers are butt jointed and fixed using punched metal connector plates also known as 'gang nails'. These were introduced around 1967 and saw a vast improvement over previous methods of fixing increasing the strength of the rafter but also reducing timber usage.
A timber wall plate is still used for a fixing point for the rafters; this is placed on top of the internal leaf of the cavity wall on a mortar bed. The rafters are lifted into position by crane and fixed to the wall plate using nails or truss clips.
Another important part of trussed rafter formation is the addition of diagonal bracing from eaves to ridge on the underside of the rafters. The purpose of the bracing is to bind the whole structure into one unit rather than a series of individual trusses, providing protection from possible collapse due to wind forces.
For further protection from wind forces, especially wind uplift, straps are also used to hold the structure down. It should be noted that pre-1970s, trussed rafter structures did not incorporate this method of strapping so wind collapse is quite a common problem on some trussed rafter roofs.
The process of laying coverings onto the roof structure has remained unchanged for hundreds of years. A series of timbers called battens is laid parallel to the ridge across the roof, they support the covering and in modern roofs keep the under felt in place. Their main function is to provide a fixing point between the covering and the structure.
Under felts is an impervious barrier that is placed beneath the main waterproof covering to the roof. It should not however be considered as the main barrier and is there to prevent wind blown moisture and dust entering the roof-space. Recently micro porous or breathable felts have been developed which reduce the need for the roof-space to be ventilated.
Plain tiles have been used for hundreds of years with the early examples of these being hand-made. Traditionally they were made from clay, although concrete has been used since the 1950s. They are fairly small in size, commonly 265 x 165 mm, and have to be laid double lapped. This means that there has to be two layers of tile throughout the roof and at the end laps of the tiles, the top tile must overlap the tile two below it. Due to the double lapping and the small size of the tiles, these are expensive; slow to lay and very heavy.
As the name suggests, these tiles overlap and interlock at their sides and so can be laid single lap.
Traditional examples were clay pantiles and roman tiles although since the 1970s the majority are manufactured in concrete. They are commonly around 420 x 300 mm and can be laid to angles as low as 15°. Due to their large size and single lap, they are relatively cheap, very quick to lay and will weigh around 50-60% of a plain tile covering.
Slate has been used for hundreds of years as a roof covering but only after the industrial revolution and the construction of the railway network did they become common outside of the slate mining areas. It is an excellent material to use for roofs as it is impermeable and will last hundreds of years.
There are many different sizes of slate available although a common large size is 600 x 300 mm. They can be laid on roof slopes with a pitch as low as 30° but this varies dependent upon the size of the slate. Due to the double lapping and requirement to nail every slate, they are an expensive, slow to lay and heavy roof covering.
Artificial slates are a cheaper alternative to natural slate but their longevity is only around 40 years or so. They come in similar sizes to the larger natural slates but are considerably lighter in weight. They are normally fixed with 2 nails per slate, centre nailed, and a further fixing is used to secure the bottom edge of each slate to prevent it lifting upwards in high winds or due to thermal expansion.
Technically a flat roof is any roof with a slope less than 10°, however in practise they tend to be much shallower, commonly being expressed as a gradient and can be anywhere from 1:40 to 1:80. They have traditionally been used for small domestic extensions although there were periods of more common use for entire house roofs in the post second world war period and then also in the 1960s.
Traditional coverings have been bitumen felt, this normally being called 'built-up roofing' as it was applied in either two or three separate layers, or asphalt. A more modern material is single ply membrane, this essentially being a thick polymer sheet.
This article is a shortened version of a paper available from Open Resources in Built Environment Education. OBREE offers a collection of free, high-quality teaching and learning materials. The original paper was written by Glenn Steel.
 Related articles on Designing Buildings Wiki
- A-frame house.
- Barrel vault.
- Cool roofs.
- Conical roof slating.
- Crinkly tin.
- Dormer window.
- Flat roof.
- Flat roof defects.
- Gambrel roof.
- Green roofs.
- Hyperbolic paraboloid.
- Long span roof.
- Mansard roof.
- Metal roofing.
- Pitched roof.
- Rainwater downpipe.
- Roof slates.
- Roof tiles.
- Roofing defects.
- Shell roof.
- Shingle roofing.
- The history of fabric structures.
- Timber roof.
- Thatch roofing.
- Types of roof.
- Wall plate.
 External references
- Marshall, D. & Worthing, D. (2000) The Construction of Houses. 3rd Edition. Estates Gazette London.
- Riley, M. & Howard, C. (2002) Construction Technology 1 - House Construction. Palgrave Macmillan. Basingstoke.
- University of the West of England (2005). Construction of Houses - The Video Project. Version 5. Bristol.
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