King posts, tension or compression

[RossJarvis]

Looking at a lot of king post trusses of a certain age or older, they are often "strapped round" with a blacksmith made strap which looks like it's designed to stop the post lifting/separating from the cross beam. So it's to stop tension in the king post from separating from where it's "resting" on the cross beam. Common sense and every engineer I know seems to think that the king post is in compression from the top load so the strapping isn't needed. Is it to stop the beam from sagging? Or is the post in tension, I.e. being pulled up from the top. The reason I ask is that on the ones I've made it looks like the posts are definitely being supported by the beam and not vice versa, so no strap would be required.

 

[RossJarvis]

Ah, on further investigation, I see that a king post is in tension and pulls the beam up, whereas a crown post sits on the beam and acts in compression holding the top up. Can anybody explain the difference? It seems that my King posts are actually crown posts........This may need an explanation to the client at some stage.

 

[CHJ]

I thought that the King post was hung (in tension) from the apex of the two rafters.
The beam is to stop the base of the rafters from splaying out and applying side loads to the wall and is in tension, and is strapped to the king post to prevent it sagging in the middle with its own weight or when loaded from below.

 

[RossJarvis]

I thought that the King post was hung (in tension) from the apex of the two rafters.
The beam is to stop the base of the rafters from splaying out and applying side loads to the wall and is in tension, and is strapped to the king post to prevent it sagging in the middle with its own weight or when loaded from below.

CHJ You may be one of the few people in the country who knows anything about this, and agrees with my own understanding. I bow down at your feet ccasion5: ccasion5: . For me this would explain a lot why the rafters often "chock" into the post at the top. I presume this helps thrust the post up. Almost everyone I know seems to think the "king" post is supporting from the beam.

I'm rather thinking the "king post truss" I built is actually a "crown post truss" and bearing in mind it is more aesthetic than structural is likely to stay where I b***** well put it. It also makes me more worried about another job I worked on where all the tie-beams/joists were done away with, so that the client could have an "open roof space" As the "three king posts would hold it all up" (no I wasn't doing the roof then). With any luck those king posts are actually crown posts.

 

[HOJ]

Ive seen quite a few "King" post roofs where the post is replaced with a steel tie rod so in effect being in tension from the "ridge" to support a joist (Girt).
A King post and Crown post roof structure are very different, a Crown post roof has a rafter collar tie?

 

[Mike Jordan]

I have never even heard of a crown post truss but I am familiar with the King post and queen post truss. The queen post truss being used over greater spans and giving a usable space between the two queen posts. I suggest that you get hold of a copy of Riley's Manual of carpentry and Joinery or R Bayliss Carpentry & Joinery Book 2. EIther will show you the correct method of construction together with timber and composite (Timber & metal with tie rods ) trusses. This reads like another case of taking on work without qualifications or experience to complete the job properly and safely. Both the books mentioned are out of print but obtainable from second hand sources like Amazon.

 

[RossJarvis]

I have never even heard of a crown post truss but I am familiar with the King post and queen post truss. The queen post truss being used over greater spans and giving a usable space between the two queen posts. I suggest that you get hold of a copy of Riley's Manual of carpentry and Joinery or R Bayliss Carpentry & Joinery Book 2. EIther will show you the correct method of construction together with timber and composite (Timber & metal with tie rods ) trusses. This reads like another case of taking on work without qualifications or experience to complete the job properly and safely. Both the books mentioned are out of print but obtainable from second hand sources like Amazon.

Thanks for the book recommendations, I'll try and get them. This is a bit more a case of cutting wood to architects drawings hoping they are based on the correct knowledge. I think there is a problem that oak framing is growing in popularity, but there are not that many people around who know enough about it, myself included. I'm finding it a bit tricky to find enough resources as I would like to know more about the framing business. I've never said I'm qualified to design the structure, but generally competent enough to cut and fit the wood. Around here there are some very good companies designing and building oak frame structures, but the cost is not to everyone's tastes. There are a much larger number of cowboys doing it. I'm trying to avoid being one of the cowboys.

On a previous job I got the client to call in an engineer as the "oak framers" had decided to avoid using valley rafters and moved two veluxes by cutting existing rafters out and not doubling up or replacing. They also changed the design by not fitting any joists tying the sides together by fitting two "king posts" in.

 

[Bod]

Crown post truss.
http://greatbarns.org.uk/barn_intro/page22.html

My limited understanding is,
King post, hangs from the ridge, and holds up the tie beam.
Crown post sits on the tie beam, and pushes up the collar beams under the ridge. Allowing lighter rafters, at the expense of a heavier tie beam.

Bod

 

[Mike Jordan]

Sorry if my remarks about experience sounded a bit churlish, reading now it looks less than kind! In the older text books the King post truss is fitted with metal strapping at all joints. I think this might have been a belt and braces attitude of the times.Most of the ones I have seen have either been fitted with a stirrup with gib and cotters or the King post has been drilled up from the bottom to house a wrought iron bolt about 600mm long with a big square nut in a mortise through the King post.I have always thought that the trusses were lifted into place in pieces and assembled in their final position with the melal work ensuring tight joints. In the days when they were commonly used, wooden scaffolding would have been the norm and a crane of any kind would have been a very rare beast.

 

[RossJarvis]

Sorry if my remarks about experience sounded a bit churlish, reading now it looks less than kind! In the older text books the King post truss is fitted with metal strapping at all joints. I think this might have been a belt and braces attitude of the times.Most of the ones I have seen have either been fitted with a stirrup with gib and cotters or the King post has been drilled up from the bottom to house a wrought iron bolt about 600mm long with a big square nut in a mortise through the King post.I have always thought that the trusses were lifted into place in pieces and assembled in their final position with the melal work ensuring tight joints. In the days when they were commonly used, wooden scaffolding would have been the norm and a crane of any kind would have been a very rare beast.

Thanks Mike, I can't say I wasn't a bit crestfallen , but it was fair comment, possibly slightly different wording may have helped Most of your comments have been very helpful.

I was fairly happy doing the job because the responsibility for the structure is with the architect and engineer, plus the "king post trusses" are effectively just for show and the roof is in reality a standard cut roof with ridge board. No purlins are there to be supported.

I can confirm that no cranes were used on this job either and the oak definitely went up a piece at a time to be assembled in situ.