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Author Topic: Purlins
Ken Hume
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Posts: 431
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Post Purlins
on: October 26, 2015, 19:20
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Each purlin for the cruck frame has been hewn from two second logs (a log taken from immediately above the prime quality butt log).

Second logs generally have more large live knots than the butt log which is cleaner with either no knots or a number of smaller dead knots where branches have been suppressed, died back or dropped off.

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The diameter of the second log is not as great as the butt log and so a hewn second log will not have crisp arises (corners) and instead will have waney (round) edges over the full length of the log with protruding knots. The knots are removed with the adze (or axe) and then the whole waney edge is cleaned using a draw knife.

Image

Because the second log is taken from higher up the tree these logs will contain a larger proportion of (white) sap wood after hewing which contrasts with the underlying (red) heart wood, however the second log is still predominately heart wood and so joints cut into the wood can be made with heartwood to heartwood faces that will not degrade. Since the purlins are entirely weather protected (under the roof) durability of the timber is not a major design and timber selection factor.

To make a purlin that extends over the full length of the cruck building it will be nesessary to scarf together (join) the purlins shown above to effectively make one long timber.

Ken Hume OWG

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