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Posted: Mon May 20, 2013 10:54 pm
by joydivision
I know this isnt a roofing forum, but thought id try here first as there always seems to be someone who knows something!
Anyway, on this quick sketch of a plan view of a roof im unsure about point A. This is a traditional roof with ring purlins running round in a 1930's property.
Question is, what is stopping point A from collapsing/sagging?
Point A is not propped of a wall and the hip does not continue down to the wall plate.
Any advice?
http://s802.photobucket.com/user....4442862
Cheers
JD
Posted: Mon May 20, 2013 11:37 pm
by lutonlagerlout
generally there would be a king or queen post coming down from a purlin,but not always
the way the roof is constructed the loads get spread
you wouldnt be allowed to do it now of course
have a look here for more info
LLL
Posted: Tue May 21, 2013 5:47 am
by cookiewales
lutonlagerlout wrote:generally there would be a king or queen post coming down from a purlin,but not always
the way the roof is constructed the loads get spread
you wouldnt be allowed to do it now of course
have a look here for more info
LLL
that flemish garden wall bond is what the germans use on there face work but more random you are not allowed more than 7 perps going up you have to reverse then it stops cracks following perps .loving the timber drawings 
Posted: Tue May 21, 2013 7:36 am
by joydivision
Cheers LLL, indeed some interesting drawings on that site.
The reason I'm questioning that detail is this is my own house. I'm putting up an extension and that roof detail at point A is copied on the extension roof. The architect has drew it up exactly the same detail but there will be nothing to put a a post on to for support. I think I will have to run the hip down to the wall plate for structural reasons but then conceal it beneath the tiles.
Cheers
JD
Posted: Tue May 21, 2013 8:43 am
by henpecked
As LLL said there will be uprights supporting the purlin insde the roof space. I have the same gig in mine.
Did have an idea to replace with steels and go off that, but it just encourages the kids to stay here till they're 40
Posted: Tue May 21, 2013 9:17 am
by joydivision
Pretty sure there isn't uprights supporting the purlins, there isn't anything beneath to support the purlins off. I only have 1 solid wall right in the middle if the house and nothing is sat on that.
The purlins run from the party wall, fixed to the hip, round to the valley, fixed to that, round to the second hip, fixed into that then round to the back hip then back across the back of the house and into the party wall.
It must all work as LLL says with spreading the loads though, just can't get my head on it and again as LLL says it won't be allowed now which is what's on the extension plan.
I am getting on to the architect, arnt they wonderful!
Cheers
JD
Posted: Tue May 21, 2013 10:27 am
by lutonlagerlout
cranked steel work here we come!
LLL
Posted: Tue May 21, 2013 7:39 pm
by digerjones
is the hip rafter a big 1. is it the hip rafter holding up the end of the purlin?
Posted: Tue May 21, 2013 8:00 pm
by joydivision
yes 12x2. And yes, purlin either side nailed to bottom of that hip.
Cheers
JD
Posted: Tue May 21, 2013 8:14 pm
by gonchy
as lll said not all purlins get supported we only used to support them on long runs
they only used them back then as roof timbers were only 75mm-100mm to stop sag
the flying hip is no weakness either we still do them
also pulins are coming back as roofs are so over egineered these days they need to hold there own weight let alone tiles and snow load and they are trying to steer away from steels as this has been proven to cause collapse in fire as a timber roof can survive a certain amount where as steel buckle in no time
where does the extension cut on to the roof
Posted: Tue May 21, 2013 9:01 pm
by joydivision
Cheers Gonchy,
Iv uploaded anpther sketch, im no Tony Hart but it will have to do! unfortunately I only have a hard copy of the plan.
The extension roof is pretty much a carbon copy of the house roof without the purlins. The rafters on the extension are 6x2 instead.
Point B is the new detail on the extension.
I can see that the valley rafter is holiding the bottom of the flying hip in place to the front, but how does it get support from the back? Is it simply the rafters?
http://s802.photobucket.com/user....0272423
Cheers JD
Posted: Tue May 21, 2013 9:29 pm
by gonchy
yes your common rafters,jacks from both sides will support it but if you are worried you can run hip down to plate then run ridge and vally against the hip to form the smaler hip end would be alot easier if that makes sense
Posted: Tue May 21, 2013 9:36 pm
by joydivision
I know exactly what your saying, thats what my joiner has suggested also.
Cheers for the advice.
JD
Posted: Tue May 21, 2013 9:57 pm
by lutonlagerlout
TBH we would normally leave the existing hip in situ and work the cripple rafters on to it once the commons are in place
maybe just take out the longest existing rafter for access for insulation electrics etc.
the reason for this is that most old roofs use 3by 2 or 4 by 2 rafters ,
most modern drawings call for 5 by 2
cheers LLL
