How long should rafters be

The roof pitch calculator calculates slope, pitch, angle and rafter lengths, based on input of rise M and run R. Use the diagrams on this page as a reference when inputting rise and run, and while interpreting the calculated results. To determine the finished height of the ridge beam , add the Rise M and the Y height. To determine the Y height, measure from the bird's mouth up the calculated slope X in 12 to the top of the rafter.

This is very useful information for many purposes, especially for roof framing - the slope, sometimes called pitch , is calibrated on speed squares. This is useful because sometime you may need to cut a bevel with your circular saw that is not calibrated using roof pitch style - this setting on circular saws is calibrated in degrees.

Rafter Length - The rafter length results, from this calculator, represents the length of line L on the diagram s. Note that this is not the actual length of the rafter. First thing to do is to subtract the thickness of the ridge from the span of the roof so that the ridge will fit perfectly between the two common rafters.

Next, divide the remaining measurement by two. This will give us the span for each individual roof rafter:. The length of rafter per meter of run is 1. This means that for every 1m the rafter travels horizontally, the angled length of the roof will be 1. To work out the second length of the rafter so that it runs past the brickwork and can have a fascia board fixed to it, we need to work out the overhang measurement.

This will ultimately define the width of the eaves soffit board as well, so it's important to know what you are going to use for that. When building a roof, in some instances I will work out the exact overhang needed for the fascia and soffit boards and add that to the rafter lengths already calculated.

That way you can completely finish all the cutting down on the bench before taking them up and installing them. Instances like that include things like sheds, dormers, timber framed and other structures where there's little chance of getting it wrong or the walls being wavy. You can do this by using a string or chalk line to mark them. So in that instance, I work out roughly what the overhang will be at each end of the roof and make sure the rafter tails are long enough to allow for that before cutting the rafters to length.

If you are building an extension you will probably need to match it with the existing soffit size of the main building. Otherwise, if you are building new and will be using UPVC fascia and soffit you'll want to know the sizes that it's available in. For this example we'll assume the soffit will be mm deep. That measurement though is horizontal from the end of the rafter to the brickwork shown in green.

We need to hold a spirit level up vertically against the brickwork and measure from the wall plate to the brickwork as well purple. Adding these two measurements to the original span blue will arrive at the total span for the second rafter length we need. Lets assume for this example from the wall plate to the outside of the brickwork measures mm and the soffit size we want is mm. Add these to the span we calculated before;. So the second rafter length we need is 2. Cutting roof rafters can be done as simply as with just a sharp hand saw and a bevel or two to save setting them up twice if that's all you have but it is both labour intensive and time consuming.

The problem with two bevels though is they are easy to mix up and therefore make a mistake with and they can also move if knocked. It's better to make a ply jig cut a plumb cut on an off-cut of ply or timber around mm long and a birds mouth, this can be used to quickly mark rafters once measurements have been applied or use a roofing or framing square.

For speed when cutting lots of rafters use a circular saw. It is all well and good having these lengths, but how do they transfer onto the timber to create a roof rafter? A Plumb cut - this is an angle to cut that will create a vertically 'plumb' line on the rafter. A Seat cut is a horizontally level cut, so the rafter can sit flat onto the wall plate.

Height of the scaffold Rafter Calculator Instructions Enter the value for the roof width outside to outside in feet, inches, and fraction, inches and fraction, or use the inches box to enter a value in decimals if that is your preference.

If there will be no internet connection at the job site, then go ahead and open this page on a device such as a laptop or a smartphone in a location where there is an internet connection. Once this page is opened, the calculator will work without an internet connection. Take a direct measurement of the width of the roof. This must be from walls that are of equal heights. If they are not, then click here for the unlevel wall rafter calculator.

Use the number boxes to enter the value in feet, inches, and fraction, inches and fraction. Then use the drop down to select a value for the fraction. For advanced users simply use the inches box to enter a decimal value.

Use the blueprints to establish a pitch for the roof. Select a roof pitch. Determine a thickness for the ridge and use the drop down to select it. Parts of a Rafter The top of a a rafter usually joins a ridge. Laying out a Calculated Rafter Place the board on a set of sawhorses with the crown away from you. Use a framing square to mark a plumb cut as close to the end of the board as possible. I prefer using the right hand side of the board for this ridge cut.