That is how I see it Mike.

Ken, as I flip through a English Historic Carpentry, Hewett, I see a variety of roof systems, most prominent, principal rafter with purlins and rather minor common rafters. Are these common rafters really directing thrust to the walls? Or are they heavy strapping?

In same book I see almost no supporting ridge beams, Mike.

Hi Mike,

I feel that a Rumsfeldian moment is about to descend upon me. Either "I know what I don't know", or worse still, "I don't know what I don't know".

I have scrolled through all the screen show photos again to find that you are correct in stating that this roof does not have common rafters and instead relies on common purlins to bring back all the roof loads to the cross frames [bents]. That said, joint design is not all about handling static loads one also has to take into account dynamic (wind) loads and this particular frame (appears) to be equipped with two devices to counter transverse wind loads - the tying joint and the cross braces. When under transverse wind action the braces on the leeward side of the building will tend to try to lever up the tie beam - this is countered by the teazle tenon peg. The end of the tiebeam is also put into tension by brace action and this is supposed to be countered by the dovetail tenon in the wall plate but due to cross grain shrinkage these do not work very well in this respect. That leaves the two post tenons to counter the tie beam tension by resisting same in vertical shear. There is also bending moment generated at the wall post and this is resisted by the tying joint which acts as a moment connection. "Usually" the post tenon is mounted outboard and therefore one can rely upon almost the full depth of the post and jowl in this respect. In this particular case the effective post depth between the two tenons has been greatly reduced by bringing the wall plate tenon inboard and thus the post is now more liable to split down its length ouboard of the post tenon. There do not appear to be any intermediate posts or studs in the cross frames underneath the tie beam to take up the load should a failure be experienced at the tying joint and hence it appears (to me) that this frame is potentially more vulnerable to disproportionate collapse than if the post tenon had been mounted outboard in the more traditional fashion.

Historically it is rare to find a ridge in a common rafter roof with 2 major exceptions - king post and cruck. Purlins are not generally provided to duct rafter loads back to the cross frames (bents) but are present in conjunction with windbraces to stop racking of the roof. Later rafter butt purlin roofs where the rafters are interrupted by the purlin will duct part of the roof load back to the cross frames via the purlins. Common rafters send nearly all of their load to the wall plate. Really early roofs have neither ridge, purlins or wind braces and rely instead upon a mass of lath and the interlocking effect of tiles and / or thatch to stiffen the roof. There are always exceptions to every rule.

Given that this is a Guild sanctioned project that received big billing on the home page one would hope that someone involved with this project might be able to explain to us the design rationale that lay behind this fairly significant joinery design decision.

Regards

Ken Hume P.Eng.

Hi Tim & Mike,

I dug out an old photo of Cecil Hewett that I took of him in the Barley barn at Cressing Temple, Essex back in 1993.

The Master at Work - 1993

You will see that an aisle post has split down its length due to a tying joint trying to resist the out thrust of the common rafters.

The folder that Cecil is holding contains the orginal illustrations included in English Historic Carpentry and a few moments after taking the photo he dropped the folder and the drawings blew away in this rather drafty barn. A mad scamble ensued by all present to help recover same. When examining Cecil's books and drawings have you seen any illustrations that feature failures ?

Cecil worked mainly in Essex or in the wider area of East Anglia and London and thus most of his illustrations tend to reflect this region. Many of the buildings that he illustrated could not really be described as run of the mill buildings and therefore it would be easy to forgive a person remote from this insight to believe that what one sees in this book is widespread in practice, for example, Cecil does not include any cruck frames so does that mean that these do not exist ?

Regards

Ken Hume

Ken, if I am not mistaken the dovetail on the bottom of the tie beam ends is not a dovetail but a cog, allowing it now to work as you suggested it should.

It can be seen right here, http://picasaweb.google.com/109129016758...744278862278274

So, where is the book with all the failures?

I think you meant to say: