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Roof Structure
This is a huge subject to try to cover. There are so many types of roofing styles, methods, materials etc. In this “Hints, Tips & Ideas” section, I’ll just be giving you the basic of the main types of roof construction. – Self Build Simplified will cover the subject in more depth.
The main types of roof structure fall into 2 categories:
1) Pitched Roofs – Made up of “Standard Trusses”, “Attic Trusses” or “Hand Cut roofs”
2) Flats Roofs.
Starting with Pitched, Standard Trussed roofs:
trussed roof on a self build projectThis is probably the most common type of roof used in the UK. – It’s the quickest, easiest and cheapest option. – Its fairly rare if you find a Commercial Housing Developer who does not use a trussed roof construction. – They are very keen on keeping “Unit Costs” down, and trussed roofs are the best way to do just that.
A standard roof truss can be made to any length to suit the design of the building. – There are limits to the spans these trusses can be designed for. – If the design of the house gives a particularly long span, then a support wall somewhere near the middle will allow the truss to be designed at a longer length without the tendency to “dip” mid span. – The main restriction to the maximum length of these types of trusses is the need to be able to transport them from the manufacturer to site, via possibly windy, sometimes country roads where the width and length of the load may be restricted at some point on the journey.
Standard roof trusses can be made to any “pitch” (angle) and are normally manufactured from something like 4” x 1.5” pressure treated timber, with all their joints being held together with metal plate connectors (those silvery coloured rectangular metallic plates we are used to seeing). – These connectors give exceptionally strong joints without the need for cutting standard timber joints in all the trusses, - and therefore help to keep the cost down.
The trusses usually sit on a “Wall plate” (usually made of 4” x 2” timber bedded on top of the top course of the internal skin of the external wall, then held down with “Wallplate straps” (long strips of galvanized metal bar about 1 ¼” x ¼”, which are bent at one end to give a 4” flat section which can be nailed to the wallplate itself. The straps are then also nailed into the wall (on timber frame buildings, the truss sits on the top of the inside skin wall panel of the frame itself, which is designed to take the load, and the straps are usually nailed onto the vertical members of the wall panel.
roof trussesTrusses are set normally at 60mm (2ft) apart, with the first and last truss being set about 1.5 inches inside the gable wall. – If there are going to be such things as roof lights or stair openings into the roof space, and the spacing of the trusses needs to be greater than 600mm, then, either side of where the opening is to be formed, the trusses are usually doubled, or even trebled up (depending on the width of the opening), in order to make up for the fact that the support structure of the truss which should be “mid opening” will not now be there.
Because these types of trusses are not designed to take a great deal of load on the horizontal member, they are not recommended if you are thinking about using the attic as rooms, either when you build, or in the future. They can be boarded and take some “imposed” weight, but basically just your normal storage for your old boxes and suitcases etc.
Once all the trusses are in position they have to be “braced” to give the whole roof “rigidity”. – The Building Inspector and Structural Warranty provider will want to check the roof before you cover it with felt, batten and tiles.
For speed and cost effectiveness, this type of roof will be your best bet. But, if you are thinking about converting the roof at any time in the future, - or if you want to give future owners the same option. - Or, if you are thinking about using the space for more than just lightweight storage, you should think about installing:
 Attic Trusses
attic roof structureAttic Trusses are basically the same as standard trusses, but are designed to give you the option of opening up the enclosed space as living space.
The main difference is that the main structural members are more substantial than for a standard roof truss, and the bottom member is basically a floor joist.
You will see from this photo that using these trusses opens up the whole of the space as basically a long corridor which you can then split down into rooms. – This “attic” contained 3 good sized bedrooms and 2 bathrooms, with a standard set of stairs coming from the ground floor.
The structure still has to be braced for rigidity, but rather than the insulation going between the horizontal members, it goes between the vertical short member which you can see, and then up the remaining part of the slope of the roof, thus creating an insulated cell within the structure.
You have various choices for types of insulation. Some are expensive,
some are fairly cheap, timber frame interiorbut take longer to install. – Check with your
designer which will suit your project and your budget best.The cost of attic trusses is possibly 3 -4 times that of standard trusses. This may sound a lot, but if a standard truss costs £35 and an attic truss costs £120, then for a 10m long building, the total extra cost for the trusses will be about £1500.
When you think that they take the place of floor joists and turn a space that was only usable for storage into living space which could add tens of thousands of pounds to the value of your property, I reckon they are a bargain! – And so do the Commercial Developers who are building more 3 storey houses than ever before!!
Hand Cut Roofs:
Hand Cut Roofs are the most expensive, slowest and most difficult type of domestic roof to construct. They are not widely used these days where speed and cost are “prime movers” in both the commercial and private housing industries.
They take a lot more work to design and it can cost you quite a lot to get your designer to design a full hand cut roof (standard trussed roofs are usually designed free of charge by the manufacturer). Calculations then need to be done and passed for Building Regulations, and then you have to find joiners who are going to be capable of building a good quality roof, getting all the angles and intersection correct first time.
It’s actually quite impressive to watch a good joiner putting up a hand cut roof and working out how to cut all the intersecting timbers to an accuracy of within less than 1mm sometimes!! – BUT, someone has to pay for all this painstaking work!
eyebrow roofIn appearance, there is no real difference to the external appearance of the house whichever type of roof your use, but internally a hand cut roof allows you to include all sorts of Architectural features, such as vaulted ceilings, post and beam construction, “eyebrow” roof features etc.
So, that’s a brief run through the main types of roofing. – If you just want a quick cheap roof, then standard roof trusses are probably your best bet.
If you want to have the option of converting later or using the loft space straight away, then attic trusses are the probably going to be the most cost effective option.
If you want an all singing, all dancing architecturally designed feature roof, then the Hand Cut Roof may be the right choice. – Your designer should be able to guide you as you start the designs.