Purlin Calculator - Hip Roof

I've added a purlin spacing and total length calculation to the metric hip roof calculator at http://www.blocklayer.com/Roof/

You check the 'Purlin' box, enter purlin width, spacing and top and bottom purlin set-in and check radio buttons to either keep exact spacing (adjusting last at top) or calculating even spacings.

The purlins are drawn on the lower section of the resulting roof plan, and the total lineal purlin length and (adjusted) spacings are displayed in the results.

Is it set up ok for the way you work your purlins out? Spacing's are different for iron, tiles, slate etc, and up here, the top and bottom spacing's are usually 600 and the rest are 900, so there must be plenty of possible combinations / ways to set purlins out.

One thing I thought may be confusing is the entry for top purlin set-in, which is measured from the top end of the rafter, not the centre of the ridge. Does this make sense?

Thanks for any feedback and advice.

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