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Property Surveying Blog
April 16, 2018

An In-Depth look at Roofs

Written by Karim El Shenawi-Ali

Throughout the many site visits I have undertaken over the past year, I have come to familiarise myself with a number of different roof structures. One question that will always fall off the tongue of an owner is what roof shall I have for my new extension. Through this blog post, I aim to provide a fundamental understanding of roof structures as a whole and give a form of guidance to what roofs you should choose when constructing a house.

Brief History on Timber Roof Structures.

Since the early 1900’s, roof materials, technology and techniques have evolved significantly, allowing more recent roof structures to become more efficient, safer and more reliable over the course of 30-50 years.

Prior to World War 2, the majority of houses were constructed with purlin’s or in common terms, a cut roof, which I will delve into a bit later in this blog.

Due to the extreme shortage of timber post World War 2, due to the unfortunate disasters that occurred and the demand in rebuilding timber structures, house builders had to come up with a way to utilise the amount of timber available, so that timber was not used in unnecessary ways.

Eureka!

The truss roof structure was then introduced in the United Kingdom. Roof trusses were formally used in the United States, many years before it was introduced in the UK. The implementation of roof trusses, allowed roofs to be constructed in a timely and efficient manner and covering the required amount of area with less timber being used, which during the post war period, was simply a revelation.

Brief History on Roof Coverings

Now that we have an understanding of the history of timber roof structures, now would be a good time to understand the history behind the roof coverings that are applied on top of these roofs to protect if from weathering.

Lets take a look back a bit further than the world war. The great fire of London, one of the most significant disasters in the history of the United Kingdom.

Pre 12th Century roof structures were commonly covered by thatched roofing. Thatched roofing is typically made up of straw. This was an extremely flammable material, it caused leaks during periods of rain and pretty much provided next to nothing in terms of protection of a roof structure.  So as you can imagine, such roofs would become an accelerant during the great fire of London. The government then had to implement immediate changes to the way roofs were constructed. Thatched roofing became illegal and flat clay tiles became the norm in terms of roof covering. Clay tiles were a lot more fire resistant which during the 12th century was needed!

Clay tiles were to become the norm over the coming century, until, the post war period when the demand for houses was at its highest, the availability of clay tiles was reducing and the manufactures of clay tiles could not keep up with the sheer demand of houses. Another changed was then implemented and concrete tiles were introduced during the period of 1920’s-1930’s.

Concrete tiles were a lot more efficient than clay tiles, they provided the adequate sizes for roof covering, and builders found that it was easier to install concrete tiles on to timber roof structures.

However, over the on coming years, clay tiles started to be used again and it was simply a matter of preference for house builders and different tiles would then be adopted based on design preferences.

Now that you understand the historic fundamentals of timber roof structures and the roof covering applied on top, it’s now time to understand a bit more about the structures at this present time and what thy bring to each home.

Timber Roof Structures

Cut Roof

A cut roof is a typical roof structure that is used within pitched roofs. The cut roof consists of timber members that are cut on site to form different aspects of the roof. These are: rafters, ridge boards, purlins, hangers, and joists. Each timber member has a purpose but, collectively, they provide a sound and integral roof structure.

Below I have detailed the fundamental purpose of each aspect of a truss roof. In addition, I have also attached a sketch of a typical cut roof structure.

Rafters

Rafters are found running from the roof peak down the slope of the roof connecting to the eaves. Rafters support the roof sheathing that is applied on the roof covering. In addition, they carry the dead load of the sheathing, roof covering i.e roof tiles and any remaining load applied on the roof.

When undertaking a property inspection of a loft, one of the first things I see and assess is the rafters. The condition of the rafters gives a real indication of the condition and life span of the roof structure.

Ridge Board

A ridge board is found at the top of the roof pitch in between where the rafters seen on either slope meet the top of the roof pitch. The ridge board is essentially considered as a prop for the neighbouring rafters to rest and sit against. Please note, a ridge board is not a structural entity in the roof, as it is only used as a bracing system for in incoming rafters to sit against and they provide no support for the rafters.

Purlins

Purlins are a form of structural member that spans horizontally from gable ends in a timber roof structure. Purlins are designed to provide sound support for rafters. In addition, if you are considering constructing a wider roof, it is deemed more adequate to use a cut roof structure as the purlins provide mid-span support and provide more efficient support to the overall weight of the roof decking.

Hangers

Hangers are a timber member that is placed between the ceiling rafters and floor joists. The purpose of hangers is to ultimately hold the ceiling up and prevent it from significantly moving or detaching, along with providing support to the floor joists. Hangers are placed on every fourth rafter at ceiling level but this is dependent on the overall structure of the rafters and joists. When considering the design of your construction, it is important to note that the larger the floor joists, the less likely hangers will be needed. If you view this from a logical perspective, floor joists are a major form of support for the roof structure, therefore the larger the joists, the more support is give to the roof structure, therefore resulting in the lack of need for hangers.

Joists

Joists are another form of timber member. They are spanned horizontality across from opposite walls at floor level. At roof level, the joists carry the remaining load of the roof structure above and pretty much anything else in the roof such as surveyors! Therefore joists need to be design, cut and applied correctly in order to provide support to anything resting on top of them. The science behind why joists provide so much support is quite simple, the top part of the joists are compressed by the sheer weight of what is resting on top of the joists and the bottom part of the joists goes into tension an soaks up the weight above.  So I stress again, joists need to be designed and applied correctly in order to provide such support, otherwise they would just sag and ultimately fail.

Why choose a Cut roof?

Cut roofs by nature are essentially cut on site and are crafted to suit the needs of a particularly designed roof or building. As the roof structure is individually placed together, this provides the ability for the roof structure to be altered quite easily. In addition, as a house owner, you will always want the ability to convert your roof space in to living space with minimal complications. As cut roofs provide a larger timber structure and a more adaptable structure, this allows you to convert your roof structure quite easily.

Why wouldn’t you choose a Cut roof?

As a builder or a homeowner, you want efficiency, speed, cost effectiveness and a stress free construction process. When construction or preparing the cut roof, this is deemed a considerably slow process, as it can be a timely procedure to cut each aspect of the roof on site to a point where each member is efficiently cut without any faults. In addition, a cut roof can be a more expensive route to construction a roof structure. One of the fundamental reasons why it is considered to be more expensive is because you are paying more for builders on site to cut the timber member, there’s a lot more disposed timber when the timber is cut. In simple terms, the more time spent on site, the more you pay and the more timber used, the more you pay.

How long will it take you to construct a Cut roof?

Cut roofs are relatively simple to design and manufacture on site, however, complications can arise, and this may impact time spent on the construction. This is something that you as the homeowner or builder must consider when determining project timescales. Therefore from my experience of construction works that have been undertaken on roof structures, I would suggest that a period of 7 days is given to complete the construction of the cut roof.

Prefabricated Truss Roof

A truss roof on the other hand, is a complete and whole structure that is designed and prefabricated, which is then delivered to site and ready to be placed at roof level. This is considered to be the more common form of roof structure, as the design of the trusses are inputted onto a computer, which can then reflect the structural calculations to meet building regulations.

Truss roofs are designed specifically for the structure that it will be roofing.  In comparison to cut roofs, truss roofs are designed and manufactured off site and delivered once they are ready to be installed on top of the new construction.

The most common type of truss roof that I have come across when undertaking schedule of conditions, homebuyer reports or building surveys is the prefabricated fink truss roof. A fink truss roof forms a “W” shape structure, which is commonly referred to as the web. The web is essentially made up of joists, rafters and struts, which are all combined into one single timber truss.

Why choose a Truss Roof?

Truss roofs hold many benefits that could be ideal for many house builders or homeowners when considering the design of their new structure.  Below I have listed each advantage of using a truss roof.

Although the overall structure of a truss roof is considered to be complex relative to a cut roof, that is something that you as the house builder do not need to worry about. Truss roofs are designed and manufactured off site by specialist companies and engineers. A huge amount of skill is applied when designing such roof structures and this as a homeowner can be considered a weight off your shoulders.

Due to the sheer amount of skill placed in designed truss roofs, the end results provide a very strong timber structure that is increasingly reliable as technology move forward in this day in age.

Due to the reduced reliance of workforce and onsite requirements, truss roof structures are deemed to be relatively cheap in comparison to cut roofs, for which can reflect more healthily in the overall costing of a house builder.

Why wouldn’t you choose a Truss Roof?

Truss roofs are economical, cheaper and more reliable, however, they do attract a number of complications that some owners just want to avoid. Below I have highlighted some of those complications.

Truss roof structures that are assembled off site, will need to be transported to site. This can become complex as truss roofs can become over sized and may require special vehicles to transport the structure to its new home.

The design of the truss roof can lease a significant amount of empty unused space in the roof void which in common terms is wasteful.

As truss roofs form one whole structure, this reduces the ability to alter the roof structure. Cutting any of the existing trusses can severely reduce the structural integrity of the truss roof.

As a Party Wall Surveyor, one of the most common forms of notifiable works I have seen all over England is loft conversions.

We can see that truss roofs are complex, therefore, removing some of the structure for the purpose of a loft conversion can place a huge risk on your roof failing as the fundamental support for the roof has now bee removed, especially if the roof was designed to be support by a truss roof structure.

How long will it take?

Well, considering the fact, the design of the truss roof is completed via an advanced computer system, the structure is designed off site and the completed truss roof is simple applied on top of the below structure of the building. This would reflect a slight saving on time. Having spoken to a number of builders in the past and visited construction sites where truss roofs are being applied to a building, the average timescale of the design, construction and application of the truss roof is 5-7 days dependant on the amount of builders instructed to fit the truss roof when it arrives on site.

My Overall Opinion

If I was to undertake a construction of a house, some of the fundamental things I will be seeking are time efficiency, cost effectiveness, and structural integrity of my new building.

You can obtain all of these aspects through either a cut roof structure or a truss roof. However, what stands out in a cut roof structure is the ability to alter the roof with ease and the ease of converting the roof into liveable space more efficiently than a truss roof.

This provides a homeowner with the prospect of adding value to their property in the future. Added time and cost of the initial construction of the roof then becomes a memory as the success of a loft conversion can surpass that with ease. I therefore would personally opt in the cut roof approach due to its ability to become more flexible when I look to do future alterations to my roof.