There are different ways to allow for the thickness of the ridge when calculating rafter lengths. Most books I have read will tell you to calculate the rafter length from half of the width of the building then allow for half the thickness of the ridge. This method will work for most situations where the pitch or angle of the roof is the same for the entire roof. It is much easier to only have to allow for the ridge once.

The problem comes when there is a different pitch on each side of the ridge because the actual intersection of the two roof planes is not at the center of the ridge. Likewise, when there is a different pitch at the end of ridge special care must be taken to ensure the proper allowances are made. If a support for the ridge is needed to aid in constructing the roof, allowances for the ridge must again be made. I have found that the fewer calculations you make the less chance you have to make a mistake.

For me it is much easier to allow for the ridge thickness before making any rafter calculations. In doing this, the ridge no longer has to be considered in any other calculations regardless of the pitch. All rafters will reach the proper height where they meet the ridge. This also makes it much easier because the same measurements used for calculating rafter lengths can be used for rafter placement when building hip roofs.

After the span (width of the building) is known, simply subtract the thickness of the ridge. Then calculate the actual run of the rafter for each side of the ridge. This is simply half of the total span (after allowing for the ridge) for a roof where the wall heights and pitches are the same on both sides of the ridge. This run can then be converted to any other pitch by multiplying by the pitch it is on then dividing by the pitch you wish to convert to.