Bulletin 23: The roof structure

The modern, usual method of constructing a house roof is to use factory assembled roof trusses that have been designed using a computer aided design (CAD) modelling tool. This is generally quite economical  for simple roofs since the trusses can be fabricated from lightweight lengths of timber of small cross-section. Economy is achieved through minimising the amount of material and by achieving a commonality of truss so that the roof is implemented using a repeated  number of the same truss design. This repetition in the manufacture of trusses is ideally suited to low cost factory production techniques. The disadvantage of this method of construction is that generally a large number of cross braces are required in the truss design to achieve the required strength from the lightweight timber sections. This coupled with the frequency with which the trusses must be placed renders the enclosed roof volume fairly unusable because of the intervening trusses and their cross braces. To my mind a roof of this sort of construction resembles “knitting in match-wood”; cheap construction but the roof void becomes unusable for anything substantial.

In the case of the Ridge End roof, examination of the plans, elevation and sectional drawings in the “Architectural Style and Floor Plans” page of the blog shows that the roof is a complex structure. Firstly it comprises four main intersecting ridges that run with the perimeter of the building with a flat roof area in the centre of the building over the central staircase. Additionally there are two octagonal bay sections  to the front of the house along with an intersecting triangular gable over the central part of the front, necessitating yet another intersecting ridge.  To complicate things further, closer examination reveals that the roof ridges to the front and rear of the building are slightly lower than the ridges over its two side wings. This degree of complexity means that a large number of different truss deigns would be required and that there would consequently be few repeated trusses. If such a construction method had been adopted it would have still required skilled carpenters to assemble the roof and construct in-situ the parts of the roof not amenable to factory production. The resulting increased design cost and scope for cock-up seemed to outweigh the advantages of a factory produced truss design approach. We therefore decided to fabricate the entire roof as a “cut in -situ” traditional structure with ridge beams and rafters fashioned as necessary by skilled carpenters/joiners. The side advantage of this method of construction is that it produces a roof void space that is wide open. This openness makes the useable for installing the various mechanical and electrical items that will be the subjects of future bulletins, whilst allowing easy access for future maintenance and repair. Moreover the open roof space provides an attic space that allows a huge amount of storage should it be necessary in future.

We would have preferred to have had a slightly higher roof pitch. Unfortunately our planning permission restricted the overall height of the building to be no more than 1metre higher than the current house. With a ground floor ceiling height of 3.1metres and bedroom ceiling heights of 2.7metres this restricted the roof pitch to 28 degrees. This angle added to the complexity  of the build since a lot of trigonometric calculation was required to determine the various rafter forms. Thank goodness for CAD tools!  An unfortunate consequence of the 28 degree roof pitch is that the internal height of the roof space is quite low meaning that movement through the roof void will be at a “stooping walk”. Nevertheless a large useful space has been achieved.

 I believe that it is remarkable that just about the whole of this complex structure was accomplished by just two men; Niall, a highly skilled craftsman, aided by his apprentice, Lee. The really remarkable aspect is that Niall is just 24 years old and owing to his all-round capability has become the principal overall craftsman/carpenter on the project.

The ceiling joists are laid are placed onto the first floor wall structure and reinforcing steel beams. Here roof form over the octagonal bay window feature of the main bedroom suite emerges.

The ceiling joists are laid are placed onto the first floor wall structure and reinforcing steel beams. Here roof form over the octagonal bay window feature of the main bedroom suite emerges.

The first of the main ridge structures is constructed, building up from the horizontal first floor ceiling joists.

The first of the main ridge structures is constructed, building up from the horizontal first floor ceiling joists.

 

 

 

 

 

 

 

 

This image shows the large enclosed open space that is achieved by the decision to adopt a "cut roof" approach rather than factory produced trusses. The intersecting beams and the increased number of trusses that would have been required would render the roof space unusable.

This image shows the large enclosed open space that is achieved by the decision to adopt a “cut roof” approach rather than factory produced trusses. The intersecting beams and the increased number of trusses that would have been required would render the roof space unusable.

This is another photograph that illustrates the large loft/attic space that we have achieved.

This is another photograph that illustrates the large loft/attic space that we have achieved.

 

 

 

 

 

 

 

 

this photograph illustrates how the main ridges round the perimeter of the house intersect each other.

this photograph illustrates how the main ridges round the perimeter of the house intersect each other.

Photograph from inside the roof structure illustrating the amount of space under the roof.
Photograph from inside the roof structure illustrating the amount of space under the roof.
Another illustration from the other ridge show the roof void.

Another illustration from the other ridge show the roof void.

 

 

 

 

 

 

 

 

 

 

This photograph illustrates the complexity caused by the number of intersecting ridges and valleys of the roof design.

This photograph illustrates the complexity caused by the number of intersecting ridges and valleys of the roof design.

 

The complexity of intersecting ridges and hipped bays.

The complexity of intersecting ridges and hipped bays.

 

Aerial shot of the roof structure and how it fits on to the house.

Aerial shot of the roof structure and how it fits on to the house.

 

 

 

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