The form of pitched roofs

The simplest form of pitched roof is the lean-to or monopitch roof that is commonly found above many rear extensions of Victorian houses. It can also be found as the roof form on a great number of housing estates built since 1960. Originally such roofs were constructed on site from individual timbers but the modern version is normally formed with trusses and often has an extremely low pitch (17.5 degrees or less).

A lean-to roof can only span a short distance but a simple double pitch or couple roof can span a wider length. Incorporating a collar to act as a tie at about one third of the height enables even larger areas to be successfully roofed over, as it reduces the tendency for the roof to spread.

Lean to or monopitch roof

A simple lean-to roof has a maximum span of about 2.4 metres.

Lean to or monopitch roof

A simple lean-to roof has a maximum span of about 2.4 metres.

Lean Roof Maximum Span

This span can be increased to about 3.8 metres if ceiling joists and intermediate support to the rafters are provided.

In practice there are lots of variations. The roof on the left has a series of struts, fulfilling the same function as the purlin The roof on the right has a principal rafter supporting the purlin mid span. This may be necessary on long roofs

This span can be increased to about 3.8 metres if ceiling joists and intermediate support to the rafters are provided.

Victorian Ceilings Joists SpanMidspan Ceiling Joist

A high collar is not effective

Collar roof

A good joint between the collar and rafter is vital to ensure the roof does not spread.

A high collar is not effective

Collar roof

A good joint between the collar and rafter is vital to ensure the roof does not spread.

4.5 metres max.

A raised collar enables greater internal head height and Is commonly found in Victorian housing.

Collars bolted to rafters

Collars bolted to rafters

Short Rafter Construction

A collar roof can still only span a relatively short distance but is at its most efficient when the collar is positioned at the feet of the rafter. It is then known as a closed couple roof. Even greater spans are achieved by the inclusion of purlins, struts and braces. Such construction is typical of roofs formed during the last 150 years.

Closed couple roof

Closed couple roof

Hanger & binder to support ceiling joist

Pfettendachstuhl

A 19th century purlin roof

Hanger is in tension and prevents ceiling from sagging. Struts are in compression and prevent purlin from sagging.

Hanger & binder to support ceiling joist

A 19th century purlin roof

Hanger is in tension and prevents ceiling from sagging. Struts are in compression and prevent purlin from sagging.

The purlins span laterally between party or internal structural walls that support the loads involved. Where no such wall was available, one or more roof trusses, of the type known as king post trusses, would be constructed at intervals. These were formed of large timbers which acted as intermediate supports to the purlins and hence to the rest of the roof structure.

Kina Post Truss

Kina Post Truss

Green Roof Post Joist
Some of the best examples of trusses can be found In parish churches

Often, the roof space would be required for accommodation or storage purposes. This could be achieved by the use of an open truss known as a queen post truss or, for smaller spans, a single mansard roof.

Mansard Roof with King\Queen Post Truss

Mansard Roof with King\Queen Post Truss

Pitched Roof Defects

Historically, the quality of roof design, workmanship and materials was variable. The higher the social status (and the greater the income) of the occupant then, usually, the better the quality of the building. Speculative building was undertaken for all social levels and this often resulted in poorer quality construction, notwithstanding the eventual occupant's social status. However, it is generally recognised that traditional construction, in particular of the roof, achieved its highest quality during the years between 1900 and 1914.

Shortages of materials and skilled labour, combined with post-war economic problems, resulted in there being little domestic construction in the 1920s. This was followed by a period of greater activity until 1939 in both the public and private sectors. Pitched roofs were still of traditional 'cut' construction but the quality, although generally good, was usually lower than the peak reached just before the First World War.

The Second World War and the years immediately after was a period of little building activity except for some types of system building (refer to Chapter 15). Between 1945 and 1954 timber was subject to rationing and this led to the extending or stretching of the centres of rafters and trusses in order to save materials.

Domestic construction commenced again in earnest in the 1950s. A great number of the roofs of the period between 1950 and 1970 were of 'cut' construction but this period also saw the introduction of modern trussed roof construction, so that by 1970 almost all roofs of low-rise domestic housing were formed with trussed rafters. The advantages of modern trusses are that they are relatively cheap to manufacture, easy to install and do not require skilled site labour. They are also lightweight and are capable of spanning wide distances. It is now uncommon to use anything other than trussed roof construction for new housing.

Connectors can be bolts and washers or 'Gang Nail' plates.

There are many different types/forms of truss but all work on similar principles

Connectors can be bolts and washers or 'Gang Nail' plates.

There are many different types/forms of truss but all work on similar principles

Bolted 'Gang Nail' plates

Variable span - much greater than a traditional roof.

Bolted 'Gang Nail' plates

Variable span - much greater than a traditional roof.

A truss of this design should span 12 metres or so without any intermediate supporting walls.

Roof Construction Defects

The majority of low-rise domestic pitched roofs are of timber construction. However, since the Second World War some use has been made of pre-cast reinforced concrete members and steel members although these are often found to be installed as supports to what is essentially a timber roof (eg reinforced concrete trusses and purlins supporting timber rafters).

PITCHED ROOFS - THE DEFECTS

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Responses

  • justin
    What is coupled roof,collar roof,coupled close roof?
    3 years ago
  • rosaria
    How to build a mansard roof?
    2 years ago
  • delfino
    How to pitch a lean to roof?
    2 years ago
  • Aman
    What is post pitched roof?
    2 years ago

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