09 Jul A Text Book Roof
Surveying a Roof Frame
The roof structure is one of the most important elements of a property and something we will spend some time examining when carrying out a Survey. The RICS suggest that 45 minutes is a reasonable time to spend in a loft as part of a Building Survey.
The roof will provide the framework for supporting the roof coverings and upper ceilings. Typically it will also strengthen the upper walls and restrain gable walls.
A standard ‘up-and-over’ roof is best constructed as a triangle with the ceiling joists normally being joined to the feet of the rafters.
If the roof is not a triangle the weight of the roof coverings and imposed load such as snow will have a tendency to push down on the roof and if the rafters splay outwards they will exert outward thrust on the walls and cause roof-spread (leaning walls). We find that this can be a problem in many older houses of Georgian or earlier construction where ceiling joists span from side to side and where the connections of any ties or collars to the rafters may be poor.
The diagram below shows a ‘text book’ Victorian roof structure.
Not only is the roof a robust triangle but the rafters have intermediate support from purlins and the purlins have strut support to the internal wall, normally present between bedrooms. There are also tie timbers to join the front and rear roof slopes together and binder timbers which both strengthen the ceiling joists and, when embedded into the party walls, they further improve the triangulation of the roof. The ties shown in the diagram below act in compression and so they would be more accurately described as a strut.
When Surveying a typical Victorian terrace it is normally found that some of the above elements are missing. It is rare to find both struts and ties together and it is common for struts to transfer load to ceilings joists which are not designed for this purpose. It is very common to find that binders have been cut to install a loft hatch or simply fitted as an afterthought or that a roofer has provided extra struts bearing upon ceiling joists or even small timbers placed across two ceiling joists. Binders are important as ceiling joists are normally not a single timber but two timbers, hopefully nailed together above the internal wall. It is also common for only a number of ceiling joists to be joined to the rafters with the others being set in between.
Typical Victorian raters are ‘3 by 2s’ or approximately 75mm x 50mm set at around 400mm centres. The maximum span of this size of timber is around 2.0m although a more substantial Victorian house or a 1930’s property will have ‘4 x 2s’ or 100mm x 50mm timbers for which a span of 2.5m is acceptable. Many roofs will have purlin support for the rafters so the overall span can be increased. Purlins vary in size with a robust purlin being around 220mm x 75mm, most are smaller however. It is worth noting that a simple strut support for a purlin will significantly shorten the span and strengthen a roof at modest cost. Purlin struts are often the same size as rafters and being slender, they are prone to bowing.
So, when carrying out a Building Survey these are some of the points we would be checking:
- What is the size of the rafters and is the size reasonable given the span.
- What direction to the ceiling joists span and are they nailed to the feet of the rafters.
- Are there purlins and what are the bearings like.
- Are there struts and if so how is the load transferred.
- Are the ceiling joists continuous and are there binders. Are the binders embedded in the party walls.
- What is the size of the ceiling joists and are they sagging.
- Is there wood-beetle damage or rot.
- Have there been any alterations to the roof frame, for example for the installation of an access hatch.
- Are the wall plates in good condition or are they rotting due to poor roof coverings.
- Have the roof coverings been changed, for example to a heavy concrete tile imposing extra dead-load on the roof and if so is there evidence that Building Regulation approval was obtained.
As part of a Building Survey we would suggest options for strengthening a roof but the exact specification should always be calculated by a Structural Engineer and Building Regulations will apply.
We would not be expected to undertake such a detailed roof inspection as part of a RICS HomeBuyer Survey and this is one of several reasons why a Building Survey is the better option for a Victorian house where the roof construction is often so much less satisfactory than a say a 1930’s house where timbers are normally larger and connection between the timbers better.