Hi EH & Derek,

Once again I have to agree with Derek.

If the post in the picture is 8 inches wide then this is automatically reduced to 6 inches wide as a result of the housing or landing provided either side of the post to carry the interrupted wall plate. A small cog has been left in the centre of the post where the tenons nearly meet that appears to be about 1 inch wide and so this means that the effective length left for each of the two tenons is 2.5 inches. A peg driven through each of these tenons would need to be set on a centre line at least 1.25 inches back from the aris on the post so that would leave about 1.25 inches of relish on the end of the tenon to resist pull out when trying to resist the dovetail wedge action applied by the end of the tie beam as a result of building spread or brace action.

The performance of this joint arrangement could be greatly improved by eliminating the tenons on the interrupted wall plates and replacing these with a continuous thru free tenon or spliced joint where the pegs would now be driven through the interrupted wall plate some distance back from the joint face where an appropriate length of relish could be provided.

Personally, I would try and avoid employing such a joint configuration but it is not unknown even in very early buildings to have interrupted wall plates, though these are by no means common. The open hall arch frame at No 30, West St, Farnham, Surrrey - circa 1390 has what appears to be an interrupted wall plate set on edge.

Side Purlins might appear to provide support for continuous common rafters but in reality they sag and so rafters located towards the centre span of the purlin span tend to be unsupported by the purlin standing some distance away from same.

Personally I think that the main function of the purlin is to work in combination with wind braces to prevent racking. Few buildings have continuous common rafters pegged to the purlins and wall plates except that last pair before a hip or half hip where these "singles" rafters are pegged to purlin and plate to prevent rotation of the "singles" rafters. Obviously this set of rafters has to try and resist the racking forces developed by the hip carpentry. Driving a peg at the foot of the common rafter through the wall plate is not usually seen. Rafters should stand hard on the wall plate to ensure that they do not loose their footing and so pegged connections at purlin level can in due course result in the loss of this footing if the purlin is overly deep and cannot deform sufficiently to ensure this continued rafter footing on the wall plate.

Here endeth my Easter Sunday sermon.

Regards

Ken Hume P.Eng.
http://www.kfhume.freeserve.co.uk