Limestone roof - purlin sizes needed

[devsigner]

Hi folks,

I'm renovating a stone cottage and have uncovered what may be a major problem.

At some point repairs have been done to one part of the roof presumably to correct sagging. I need to work out if two softwood timbers can be removed as one of them is pretty much rotted through due to a failure of the flashing.

The roof is of very simple construction:

a 3.3 metre span with one purlin per side and a ridge beam all in oak, these are very funny shapes and done "in the round" rather than sawn.

Rafters rest on the wall top and go up to the purlin, then another rafter spans from purlin to ridge beam, these are in softwood.

The roof then has battens and huge stone slabs pegged and hung from them to make the roof covering. Apparently the stone tiles will weigh 1 ton per 7 to 8 metres square.

The roof pitch is 30 degrees

Rather than try and work out the strength of the existing purlins (which are very wiggly!) I'd like to work out what size they would need to be if I was putting them in new in oak.

I've tried to work this out myself but the tables I can find all relate to softwood and have insufficient load.

Can anyone provide any guidance/links on how to work this out correctly please?

Limestone roof

• devsigner
• 23 Apr 2015
• 1

Limestone roof 2

• devsigner
• 23 Apr 2015
• 1

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[tony1851]

Unless the exiting oak purlins are rotted, why replace them?

If you wanted to replace them in oak, the calcs are relatively straightforward, except that you would probably need to find the available size first, and then check the figures 'backwards'. to see if it works. Any local SE can do that for you.

Unless you buy reclaimed material, a lot of oak is 'green', and warps and twists as it dries out. It's a tough timber, often with alarming-looking shakes,
and is not for the faint-hearted!