Roof framing - rafter spacings options

**Blocklayer** · 30th July 2008, 12:25 PM

Currently on the roof framing calculator on blocklayer.com, you enter rafter spacings (centres) you want and depending on the building (roof) length, it calculates the spacings to fit evenly in the length. So, if necessary, it alters your spacing input to get even spacings. A couple of blokes from the US have emailed saying they use sheathing directly on the rafter and need exact spacings. So, to keep the spacings exact, if the total length (of the ridge) doesn't suit, either 1, or both end spacings will be smaller. I have altered (a test version of) the calculator to see how it will work.
You select either 'Adjust Spacing' or 'Exact Spacing'. If its exact, it calculates BOTH end spacings down. So the commons at each end of the ridge have smaller spacings than the rest of the commons and creepers.
So, does that make sense ?

The test version is here http://www.blocklayer.com/roof/default2.aspx
To test, enter 10600 for length - select 'Exact Spacing' and it should show end spacings of 500mm

Thanks for any guidance here, and sorry for the longwinded explanation and post

**Bloss** · 30th July 2008, 05:02 PM

Seems to work fine and I used a few of my own sizes too. US is mainly still using their feet & inches although trying to get used to metric I know.

They generally use ply over all external surfaces on walls & roof so would want standard sheets to join over the centre of rafters (or studs) - so your exact spacings should give them 18" centres (~450mm) or 36" (~900) or 4' (~1200mm) etc whereas modules of 300mm often work better in metric.

If it were me doing the building I would prefer the spacings to be exact from one end (either would do) to suit full sheets then a single odd spacing - just because I am lazy and would prefer just one cut than two. And might be a bit fussy to deal with if the small space is very small - eg: say under 200mm. Not sure if that's possible, but maybe that is an option too?

BTW - your site is just brilliant - so useful!

**Blocklayer** · 31st July 2008, 07:15 PM

Thanks for taking a look Bloss

I now have the metric and imperial versions set up with options to select 'Adjust Spacing' or 'Exact Spacing'.

For Exact spacings, the drawing shows both end spacings adjusted, but you can easily just work from 1 end and adjust the other. If I add another option to select 1, or both ends, it will just get to complicated?

.

**Bloss** · 31st July 2008, 09:52 PM

I reckon it will be OK just with those options.

I suppose you could add a note somewhere indicating that adjustments can be made at either end, but most people who would understand what the calculators are doing and know the terminology should be smart & experienced enough to work it out themselves.

Getting angles etc worked out for you sure beats using my old steel roofing square, clamps and rafter tables!

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