Rafters-Your Roof's Foundations

Rafters

Rafters are the structural beams that extend from the ridge of the roof to the wall plate, and are designed to support the load on the finished roof.

Beautiful Rafters at St. Illtyd's Parish Church, Llanharry

Roof styles

There are a number of different types of roof, and each type uses a different structural frame to support the weight of the roof load. Some of the common roof styles include:
• A-Frame roofs – These are tall, symmetrical roofs which require just one type of rafter.
• Gabled roofs – This type of roof features a gable, which is an extended section of the roof, extending from one wall, perpendicular to the roof.
• Mansard roofs – These include an extra joint in each rafter, and slope in such a way that allows for a living space in the roof.
• Hipped roofs – Hipped roofs are sloped on all sides, and are slightly more complicated, requiring a number of different sized rafters to account for the slopes of the roof.

sample drawing of the position, name and arrangement of roof trusses

Trusses

These days it is fairly common for the ‘roof trusses’ (the frames of the roof) to be constructed off-site and delivered and fitted as one piece. Truss manufacturers are able to build a truss to suit almost any style of roof, but it can be more expensive than building the roof frame in the traditional method of framing the roof piece by piece. Some common styles of roof truss include:

• Fink trusses – these are the most common, and basic, style of truss. They consist of a basic rafter, joists, and jack supports. If necessary, they can also be constructed in such a way that allows for living space in the roof.
• Scissored trusses – these are similar to the basic fink truss, but are slightly recessed in the centre of the rafters to allow for a vaulted ceiling inside.
• Clerestory trusses – These are asymmetrical, with one joist extending beyond the rest of the rafter to allow for a windowed wall along the roof.

However, depending on the type of project and the type of roof to be constructed, it can be much less expensive to cut and fit the roof trusses yourself. The next section will explain how to mark and cut the rafters, and how to fit them to construct the frame of the roof.

Marking and Cutting the Rafters

The first stage of constructing your own roof frame, involves calculating the measurements of the rafters ready for cutting. It is essential that the measurements are exact so the rafters fit correctly and provide strong structural support. These can be worked out with a construction calculator for accuracy and the important measurements are:

• The run of each rafter (in feet), which is the total length of each segment of the rafter.
• The rise (in feet), which is the height of each truss from the base of the roof to the upper-most point of the roof.
• The pitch (in inches), which refers to the amount the roof slopes for every 12 inches that it extends horizontally. This is usually expressed as a fraction, e.g. 8/12.
• The length of each rafter segment (in feet), which can be determined after calculating the previous measurements. This involves calculating the length of each segment of the trusses – the lumber for the run, the diagonal, and the sloping sections.

Construction calculators are pre-programmed with the Pythagorean formula which is used to calculate the angles of right angle triangles, and they should be used to accurately calculate the measurements for each rafter, including measurements for the seat cut or ‘birds mouth’ – the cutaway that sits the rafter flush to the wall for ease of fitting.

For most basic roof designs, a truss is required every 2 foot along the wall, and on this basis you can determine how many trusses you will need to construct to frame the roof.

With the measurements made and the roof planned out, you should be able to calculate at this point roughly how much lumber will be required to construct the roof. When you’ve bought the wood, it’s time to mark out your measurements.

The lumber should be set on saw-horses and each measurement carefully marked out before you begin cutting.

• The ridge beam is the central beam that joins both sides of the truss. It’s important to account for the width of the wood when marking and cutting the diagonal chords of each rafter.
• Mark out the plumb cuts on each rafter – the angled cut that will be made to the pitch end of the rafter so that it fits securely to the ridge beam. This will depend on the rise of the roof.
• Next mark out the seat cut on each rafter. The seat cut is the cut out on the diagonal section of rafter, where it will be seated on the wall cap.
• Next mark out the bracing joints – the sections that will provide support to the truss.

Once all measured out you can begin cutting the rafters with a circular saw. Some carpenters prefer to make the same cut on each of the rafters one-by-one, while others prefer to make all of the cuts to complete each rafter before moving onto the next. It is your choice how to tackle the cutting, but bear in mind that it could take a few days until each truss is cut and constructed, and therefore you will need somewhere suitable to store the rafters until you are ready to construct the roof.

Fitting the Rafters

• Each section of the truss can now be nailed together to create the structure, and metal truss-plates can be used to brace each intersection to secure the joints.
• The ridge beam can then be fitted into place across the length of the roof, ready for the rafters to be installed.
• Cap plates should now be measured and marked for each rafter. These are small metal braces that will secure the trusses to the walls. If you have used seat cuts for the rafters, cap plates are not strictly necessary, but it is still advisable to use them.
• The rafters can now be rested upside down on the wall plates and swung up into place by attaching a rope to the pitch and pulling it up from the ground. This can be a very delicate process, and may require the help of several people to secure the rafters in place.
• Each rafter can now be set in place, and the run joists nailed into the cap plates.

Once each truss is set in place the roof frame is complete.

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