Roof Spread: What is it, How to Avoid it and How to Correct itAdmin
Of all structural problems that can affect a house, roof spread is one of the most visible, particularly with traditional-style buildings. Many roofs of historic buildings have spread at some stage, and can often be left alone and monitored, with no remedial work required unless progressive movement is observed.
However in some cases, for example in the case of careless renovation, roof spread can be a serious concern. Roof spread occurs when the downward pressure from the weight of the roof thrusts the roof rafters outwards, sliding the roof down or pushing the walls outwards. In this article, we discuss what roof spread is, what causes it, and what to do about it.
What is Roof Spread?
A typical roof is made up of, very simply, two rafters and a ceiling over two wall plates. This forms a wide triangle shape on the top of a house, with a ceiling on the bottom and the two rafters forming the roof. Battens rest on top and perpendicular to the rafters, to which you fit a roof covering. In a ‘couple roof’, the most common and most straightforward roof design on older homes, two beams lean against each other and are tied at the top. Modern beam design uses roof trusses to provide a structural framework to support most structures.
Roof spread occurs when the downward pressure of the weight of the roof starts pushing against the rafters enough that the structure itself begins to move: either the roof starts moving downwards, or the walls start pushing outwards. This is a prolonged process but can cause severe damage to the integrity of a home.
How Common is Roof Spread?
Roof spread is especially prevalent in Victorian Gothic style houses. This is because, when they were built, they commonly used a raised tie around halfway up the total height of the roof, rather than a traditional ceiling at the bottom of the rafters. The tie holds the base of the triangle together, and as it is raised upwards more force is imposed into the rafters, and the rafter size needs to be increased to account for this.
Victorians could get away with their higher tie design because roofs were often made of lightweight slate, which meant the roof could still resist spread. However, generations later, many original slate roofs reached the end of their performance life and were replaced with much cheaper, but also much more cumbersome, tile roofs. Particularly, if a home does not have heavy stone walls, roof spread is very likely in this instance.
What are the Signs of Roof Spread?
Roof Spread Cracks and Pushing Walls Out
Roof spread occurs when the principal rafters of the roof are not adequately supported, causing them to open in a scissor-like pattern. Therefore, the first signs of roof spread are often cracking, bulging, or leaning at the head of the masonry, causing bowed walls.
Something else to look for in regards to roof spread is sag. The line of the ridge in the roof should be horizontal. If in the middle of the roof it is dipping, then the rafters have almost certainly started to sag or bow, pulling down the ridge with them. You can most easily see signs of bowing in the fascia and gutter of a property, where there might be substantial gaps between the roof and the rest of the building.
Roof Spread is More Likely to Effect Older Properties
Roof spread in most post-war houses is relatively rare. This is because partial truss designs were common after 1950, and modern truss rafter roofs are virtually universal by the 1970s, both of which are not prone to spread. By contrast, Victorian and especially Tudor houses commonly experience roof spread.
What to Do about Roof Spread:
As discussed above, roof spread can occur for a multitude of reasons; due to sections of the roof structure having been removed, additional loads having been placed on the roof from replacement finishes, or because of a defect to the structure caused by water ingress and rot.
Use the Services of a Structural Engineer
The cause of the roof spread will need to be investigated by a structural engineer, following which the specification for remedial works can be put together. In some cases this can include installing additional beams and struts to strengthen the roof, or simply looking at the existing beam design and upgrading the connections between the existing timber members.
Access to the roof space in order to complete the remedial works needs to be considered within any design, and if your property is Listed you may need to consider whether an application needs to be made for consent, before the work can be undertaken. You may also need consent before undergoing any work in this case – read our article on listed building consent for more information.
As structural engineers in Truro and the West Country, if you have any concerns with the structural integrity of your property, get in touch with us, and we would be happy to provide you with our expertise. We have extensive experience in beam design, roof design, and have worked on numerous homes where roof spread was a concern. Just give us a call on 01579 345777 or get in touch through our contact page for more information.