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from The Natural Philosopher contains these words:
Both ridge and purlins only serve to stop the rafters moving sideways.
(Euler buckling etc,). They have very little structural effect beyond
that. The rafters transfer all the loads to a downwards and outwards
force at the eaves level: That's where you need serous material.
Sorry TNP but I don't buy that argument. Purlins are often very
substantial pieces of timber and with a heavy roof they need to be. Very
few roofs have a pitch greater than 45 degrees so if the load is
resolved into rightangle components only half or less is being
transmitted directly down the rafter.
So where else does it go?
Onto the purlin. The rafter still has to function as a beam even though
it is by no means horizontal. Even if it wasn't nailed to the purlin it
would still load it and for the load to be in equilibrium at that point
the forces must balance.
Unless your purlins are actually PROPPED all the weight of the roof is
transmitted via the rafter ends. There *is* no other point of support.
No clear what you mean by propped in this context. Purlins are generally
simply supported at both ends and rely on their size to carry the load
transferred to them.
Purlins are simply there to either stabilise the rafters, or if above
them, to transmit te roof weight TO the rafters.
You might think so but that doesn't make it a fact.
If all that was needed to be done was to stabilise the rafters a dinky
bit of 4" x 2" (likely the same section as the rafters) nailed flat
against the rafters would do.
Thats geberally waht is used IME.
YMMV but I have never yet seen a proper roof with the horizontal members
fixed flat rather than upright (roofing laths excepted). To do so would
waste some of the strength of the beam. If the only purpose was to
stabilise the rafters using off-cuts for noggins would be more than
adequate.
I am quite sure that if I nailed a few bits of 4 x 2 across my rafters
and then cut out the purlins most of the 20 or 30 tons of stone on my 30
degree roof would promptly join me inside the house. Rafters are only 4"
x 3" (or 4 x 2 where they have been replaced) but the clear span is
never more than about 4 feet (which is perhaps half what you might
expect for a lightweight slate roof).
Well I guess wasted a lot of time doing an engineering degree then.
Beginning to look that way. :-)
Look. purlins are only there in mast cases to spread the load from the
roof weight TO the rafters
No, that is what the roofing laths do. On traditional roofs there is
little if any load to transfer from one rafter to the next except where
a rafter has been cut and the gap bridged by a trimmer, and in that case
it is the trimmer that does the transfer.
http://en.wikipedia.org/wiki/Purlins
You unaccountably overlooked this paragraph:
"In timber roof construction prior to the introduction of trusses, under
purlins were used to support rafters over longer spans than the rafters
alone could span. Under purlins were typically propped off internal
walls. For example, an 8"x 4" under purlin would support the center of a
row of 6"x 2" rafters that in turn would support 3"x 2" roof purlins to
which the roof cladding was fixed.
Not the sort of construction you would expect in a domestic environment
in the UK and and the wording seems alien to me but if you dispensed
with the sheet roof cladding and substituted tiles or slates then the
'roof purlins' would also go and you would be essentially left with the
meat of the above paragraph which is that "under purlins were used to
support rafters over longer spans than the rafters alone could span".
Definitions of Purlins on the Web:
* A horizontal structural member spanning between beams or trusses
to *support a roof deck*. In slope glazing, purlins are the horizontal
framing members.
http://www.inspectorwebhosting.com/c...erms-m-q-2.htm
* The horizontal members of the roof structure used to *support and
attach the roof panels.*
www.ameren.com/Environment/adc_ev_glossary.asp
* Timber members spanning over trusses to *support cladding or
between trusses to support loose timbers.*
www.wolf-system.co.uk/glossary.asp
* are the horizontal roofing members, which connect bents.
www.trilliumdell.com/vocabulary/
* Horizontal beam along the length of a roof, *resting on
principals and supporting the common rafters or boards.(
http://www.bbc.co.uk/homes/property/...glossary.shtml
* In architecture or structural engineering, a purlin (or purline)
is a longitudinal structural member in a roof. Purlins support the loads
from the roof deck or sheathing and are supported by the principal
rafters and/or the building walls. ...
en.wikipedia.org/wiki/Purlins
In every case it is clearly specified that these rely ON the principal
rafters and the joists (binders) to support the weight. They act merely
to transmit the weight *to* them.
They certainly don't all say that. (See the quote from Wikipedia above
for one instance). I haven't bothered to follow up all of them but those
that don't say the purlins support the rafters seem to be referring to
sheet clad commercial roofs which require a horizontal beam much more
substantial than a roofing lath for the initial fixing.
Principal rafters would seem to be substitutes for walls. :-)
"a diagonal member of a roof principal, usually forming part of a truss
and supporting the purlins on which the common rafters rest."
If your rafters are VERY widely spaced, they need to be substantial.
If the purlins are widely spaced the rafters need to be substantial.
But apart from a 'starved horse' looking roof,removing them wont cause
the roof to fall down generally.
I repeat, in a conventional trussed roof, any longitudinal members are
there to stabilise the rafters in the correct (laterla) position, and
act as local beams to transmit localised stresses to them, in the case
of the ridge and exterior elements - like the tile battens.
That is not repeating what you said before if you mean a modern truss
roof but you are now moving the goalposts to apply only to modern
trussed roofs but you are still wrong. A roof is close to being a
uniformly distributed load. I would remind you what you said up thread:
*******
"Both ridge and purlins only serve to stop the rafters moving sideways.
(Euler buckling etc,). They have very little structural effect beyond
that. The rafters transfer all the loads to a downwards and outwards
force at the eaves level: That's where you need serous material.
Unless you go for warren braced (W braced) trusses, or some other method
of connecting the joist to the rafter in the roof truss, all the
strength is actually IN the rafter and the joist itself. Any other
bracing is for stability only and carries very little load, including
the ridge."
******
All the weight of the roof transfers to compression in the rafters and
tension ion the binders, the triangulation so defined forming a net
downward load on the binder ends at the eaves.
There is some net bending force on the rafters as well, but a long
purlin does very little to affect this unless its supported at the
ends,. If less than substantial rafters are used, or widely spaced one,
the better approach is W braves from the off binder centers to midway up
the rafters, or vertical posts between the rafter centers and the binders.
Of course you can build a roof with substantial enough rafters not to
need purlins but the volume of timber used is considerable. The modern
trussed roof is the space age way round this problem but personally I
think you are pushing the definition of purlin well past its breaking
point to even call the the stabilising strips that stop roof trusses
falling over 'purlins'.
Or more relevantly when using the roof space as a room, you can add
another set of binders at 'loft ceiling' level bolted to the rafters.
Which doesn't do anywhere near as much as a purlin would to stop the
rafters bending under load.
--
Roger Chapman