The structural elements of the roof support must be sufficiently strong to deal with weight of the roof coverings, wind loads and also snow loads.  These loads are transferred via the structural elements such as rafters, purlins and bracing to the load bearing walls or columns.  If the structural elements fail it can lead to serious repercussions for the rest of the building. 

 

The majority of roofs over residential properties are pitched roofs.  These are defined as roofs which have a slope of more than 1.5 degrees or a fall of 1 in 40.  The pitch to a traditional roof over a dwelling is usually 22 to 25 degrees.  Roofs with a slope of less than 1.5 degrees are defined as flat roofs.  

 

The majority of domestic pitched roofs are constructed of timber and are generally of two forms of construction.  The traditional cut roof consisting of rafters, purlins, struts and collar ties or a modern prefabricated truss roof. 

 

Failure of the structure can occur for a multiple number of reasons. 

 

Poor design and specification can be an issue in older properties or properties that have not been designed/constructed subject to Building Regulations.  Where the structural timbers are undersized and/or the bracing and lateral restraint have been omitted or poorly fixed the structure will be unable to deal with the imposed loads.  As a result the roof structure can spread due to the loads, this can often force the external walls outwards compromising the structure.  Cracking to the brickwork at wall plate level will often occur as a result.

 

Alterations to the structure such as the removal of purlins or truss webs can have a serious impact on the capability of the structure.  Works to convert attics into habitable rooms will often involve removing structural timbers to create additional space.  If additional structural elements are not put in place to compensate for the removal of the timbers the roof structure may not be capable of dealing with the imposed loads. 

 

Replacing original roof coverings in older properties can often cause issues to the structure if correct procedures are not followed.  Traditionally roofs were covered in natural slates.  When they reach the end of their serviceable life the slates are often replaced with concrete tiles as these are cheaper than natural slate.   Concrete tiles are approx. 4 times heavier than slate which the rafters were designed to carry.  When the load is increased to such an extent the rafters gradually deflect in a phenomenon known as ‘creep’ and a noticeable dishing across the roof can occur.  Additional timbers can alleviate the issue, however it is important to ensure these are fixed back to load bearing walls and not merely fixed to ceiling joists. 

 

Roofing timbers like any other joinery are vulnerable to rot and insect infestation.  Timbers that are damp are at an increased risk of attack.  Roofing timbers can become damp for a wide range of reasons such as defective roof coverings, defective and overflowing rainwater goods and poorly ventilated roof voids.  Rot and insect attacks can seriously affect the capability of the timbers and can result in extensive remedial works.