A trussed roof will have very much smaller joists and rafters anyway - so
totally unsuitable for an attic room. Which is why converting a modern
house is so much more expensive than an older one - you effectively have
to replace the lot.
*I want it all and I want it delivered
Dave Plowman email@example.com London SW
replying to <me9, Laura.... help.. wrote:
I have just read yr post re roof.. i have a L shaped conversion on a valley
roof. I believe one of those walls has been removed then filled with studwork
the house is twisting like billeo and dropped. What the hell do i do now ...
I'm assuming the original posting date on the thread this is joined to has
nothing to do with the person with the new problem. Get a structural
engineer in together with a rep from the insurance company you are with and
look suitable pleading I guess.
This newsgroup posting comes to you directly from...
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.
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.
You might think so but that doesn't make it a fact.
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
Beginning to look that way. :-)
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.
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".
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 the purlins are widely spaced the rafters need to be substantial.
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
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'.
Which doesn't do anywhere near as much as a purlin would to stop the
rafters bending under load.
Actually it does *more*.
Materials in bending are never as good as in compression. The binder
across a rafter pair effectively locates the end points of the binder
precisely. However it leaves a large trapezoid structure underneath. The
maximum sag will be about halfway between the tie point and the eaves.
Which is why the nasty modern trusses use W bracing at about halfway
down the rafters.
With full triangulation any bending stresses are limited to much shorter
lengths of timber.
The only reason I can see to use structural ridges and purlins is when
you have cheap wood no machinery and expensive labour. i.e. its easier
to square off a couple of tree trunks and support them on some masonry
and add a few bits of broomstick over, than make up a few 6x3 rafters.
Today there is no excuse for that waste of wood.
How that structure behaves will depend on the stability of the footing
in the eaves. Take a triangular truss with a 'binder' half way up and
the binder will be in compression, not tension as you suggested earlier.
Take an A shape structure and the binder can be in compression, neutral,
or in tension depending on how much movement there is in the feet as a
result of the uniformly distributed roof load. Should the binder be in
tension then far from strengthening the roof structure it is weakening
it by increasing the bending moment on the rafter. It might be
convenient to view each joint as a pin joint but treating the binder
location that way gives a false impression of the stresses involved.
On the other hand a purlin will always provide support for the rafters
resting on it. (If the ends are not supported it is not a purlin).
At the point where a roof full of rafters would have had a purlin
(assuming it didn't require two).
Still a choice between waste of wood and waste of loft space.
On mine, there were three purlins on each of three sides of the hipped
roof. The front and back ones were rested on a corbelled out section of
brickwork on the party wall, and nailed to third purlin at the other end
with a compound mitre. There were then three struts - each at mid span
of each purlin that were approximately at right angles to the pitch of
the rafters, that took the purlin load down to the spine wall across the
middle of the house.
On my roof they carried a substantial proportion of the roof load.
This is the sort of thing you often see on trussed roofs - often wide
flatish timbers (4x1") are run (often at a jaunty angle) across the
rafters to add lateral restraint but not much else. I would hesitate to
call those purlins though.
I would go along with that... The purlins were about 7x4" on my roof.
Having actually done this I can tell you what happens! ;-)
On the hipped section of roof I was removing, we had de-tiled it when we
took the strut supporting the purlin out. The whole centre roof section
and purlin promptly dropped about 3 to 4" as the rafters sagged just
under their own load and that of the purlin. Had the tiles still been on
the roof, I expect the results would have been far more dramatic!
I would tend to have to agree with the dissenters - a common 1950's
terraced council house roof structure involved heavyweight purlins
running from gable wall to gable wall supporting lightweight rafters.
This is equivalent to a long span roof with a number of trusses.
The essence is that the roof weight is transferred to the trusses/
gable walls by the purlins. We only have to look at the roof
structure of old buildings to see that the purlins were of significant
size, the actual roof timbers light in comparison and the structure
supporting the purlins massive too.
That is not strictly a trussed roof at all, then.
And shockingly bad design.
I guess you could get away with it over short roof spans. Spans that are
e.g. less than the cross span from front to back.
But you still need binders to hold the rafter ends in, unless you have a
massive ridge also, and then there is a distinct possibility of the
rafters sliding down the ridge board..you are relying on nails alone to
hold them up.
At this point it really isn't a truss roof at all. you are in fact using
the purlins and ridge as the main structural elements as simple beams.
and nailing rafters over the top simply to transfer load from the tiles
to the purlins and ridge.
It seems to me to be a very wasteful way to do it.
No idea. Possibly because whoever did it didn't do the sums, and just
decided to play safe in an ignorant sort of way. Its pretty common. I
have yet to meet a builder that really understood engineering
mathematics. The guys that did my house didn;t even understand triogonmetry.
They had to lay out the rafters at the right angle (52 degrees for
thatch) on the ground and measure across the span to get them cut to the
right length. Using a large protractor,...
Or why the timber purlins half way up the roof of my chalet bungalow
have bowed inwards? ;-)
If this helps:
I'm about to put sloping-roofed dormers in - two, both facing
rearwards. This involves cutting out about 9 feet of purlin at each
end of the roof, on the same side (total length of building is about
35 feet). New purlins will go in from each end of the building,
supported by the gable wall at one end, and at the other by a post
resting on a ground floor load bearing wall. The roof at the centre of
the house will still be supported (if that's the word) by the middle
section of the old purlin (in fact two lengths overlapping in the
middle), whose ends rest on load-bearing ground floor walls, built up
within the eaves.
The span of the longest new purlin is 4300. My structural engineer
specified 152 x 152 steel, or 300 x 200 C16 timber. I was going for
timber, which I would leave exposed, until I discovered the size he
was asking for! I'm sure this is over-engineered, but these are the
calcs I have, to show to Building Control. The old purlin is 8 x 3
Hope this is useful. The answer to your question is discuss the design
with a structural engineer, and get him to calculate what you need. I
don't think you'd need a new beam at the ridge, unless perhaps the top
of your dormer was at ridge height. You would need some kind of purlin
where the top of the dormer meets the existing roof, and another beam
at the bottom of the face of the dormer.
I'm not an expert, but this is based on a loft conversion at a
previous house, and my engineer's advice on this one.
Because the rafters were insubstantial. And someone maybe replaced
lightweightshingles with heavy tiles at some point or equivalent.
Purlins will not really fix a badly designed roof.
Right. We get to the nub. The purlins have been cut. And its over a
relatively short span of 11 meters or so, and they are end supported.
Ask him why. Have you cut any binders? Or any rafters?
If you have cut rafters to make the dormers, you will have an area of
weakness for sure, and will need to transmit loads in a most unfavorable
way to adjacent ones. It sounds like that he has decided to do the
simple calc of assuming at this point that the roof truss has entirely
vanished, and that the new 'eave line' above the dormers has to carry
the whole weight of that side of the roof. In which case it ceases to be
a purlin at all in essence, and becomes a beam - an unsupported wall if
Makes the calculations easier, but is a totally unwarranted worst case
By my crude analysis,what you now have is sections of normal roof,
between the dormers, where things are more or less as they always
were,plus some foreshortened rafters that are now 'hinged' outwards to
form the dormer rooves.
The issue is to stabilize what's left, and support the new.
Supporting the new is in essence down to the vertical face of the
dormer. That takes the weight from the new dormer rafter ends and
transmits the loads down to the eaves positions. As long as the new
rafters are reasonably substantial, thats not a problem.
Stability is harder to achieve. Across the roof what is needed is
binders between one side of the roof and the other. As low as possible.
If you have decent height then these will be adequate at 'new ceiling'
If you are going full height ceilings - up to the ridge almost - you do
have a real problem. Maybe that is the issue. You have utterly destroyed
the integrity of the trussed roof at that point.
where does this steel purlin run? I can't see how it works since the
span of the roof now has dormers punched in it?
As far as lateral stability goes, actually simply taking the tiles or
slates off and nailing plywood over everything is probably enough to
stabilize the rafters laterally. Or you could do that internally. after
insulation and before plasterboarding. There will be a discontinuity at
the dormers for sure, but the stresses in that plane are not large.
You may choose to do what I did..draw up a structure and then ask the
engineers to calculate how substantial it needs to be, rather than
simply let the engineers decide what structure is appropiate.
When I did this here, there was a lot of teeth sucking, then they went
away and came back with 'well its not as bad as we thought' type remarks..
I actually do not have binders at the eaves level at all....what I have
is tinmber framed walls that are tied together at first floor level,.
and project upwards a meter or so ro where the eaves are. Then teh
rafters go from those eaves to the ridge, and have further binders at
first floor ceiling level. One section of roof has no purlins at all -
or if you like they are formed by the ceiling support elements being
bound together by a purlin type structure that runs on top of the
The other roof section is higher, and has got purlins.. which are not
that substantial. nor supported in any way. They just tie the rafters
together. It also has a couple of ridge poles in the center of the spans
but I am not convinced that those are necessary. The builders just 'put
them in' - they were not specified on the engineers plans.
Anyway, I would go back to the engineers and ask te question 'why are
these here and why are they so massive?" and "is there no other way to
It is always easy to shove in something after a quick calculation that
is massivley over-engineered and definitely wion;t fail. Its a lot more
work to design a minimalist structure that doe sthe job butr takles a
lot of matyhemetics to prove it does.
'an engineer is someone who can do for sixpence what any damned fool can
do for a quid'
Now that's interesting, because both I and the joiner who will be
building the structure had the impression that all the load was
carried at the front, in that fashion. Our original design had no new
purlin at all, but simply interleaved the new dormer rafters through
from the back to the front of the roof. However, *two* structural
engineers have told me that this is *not* the case, and that the load
will also tend to push the other side of the roof out if a new purlin
is not inserted. Hence the revised design with the new purlins.
I note the wikipedia definition of purlin, linked to above:
"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."
So perhaps we have a difference of terminology. My existing 8 x 3
purlins ("under purlins") support 3 x 2 rafters at 400 centres, iI.e.
they "support rafters over longer spans than the rafters alone could
span" The rafters span nearly 5 metres, at a 45 degree pitch. The
purlins are clearly taking a considerably load, as they have bowed.
Perhaps if you had considerably more substantial rafters, you wouldn't
need purlins in this role. As it is, my purlins support the rafters at
just below the mid-point of their span, and transmit load to the gable
end walls, and to load-bearing built-up ground floor walls in the
middle of the house. As I'm cutting through these purlins, they need
to be replaced with new purlins to take that load. They'll be higher
up, where the dormer roof joins the existing roof.
I agree that you should work out your design before taking it to an
engineer. I had four different plans, and after discussion I settled
on the one that was technically easier to achieve. I'm sure the actual
dimension of the purlins is over-specified, but the need for them in
the first place is not.
So my advice would be to get a structural engineer in - you'll need
Building Control in order to convert your loft, and if you don't come
armed with professional calculations, they're likely to insist on a
certain amount of over-engineering in any case. But your question
concerned design, and the first thing you need to know is what is
Right. All now becomes clear. The rafters are basically inadequate, and
structural longitudinal members have been used instead. Over a 10 meter
span those need center support or to be VERY substantial.
OR simply increase the size of the rafters and truss them properly. Up
to using binders across the new ceiling.
Might use a lot less space.
The overriding principle of braced structures is to avoid bending
stresses over large spans as much as possible. On a typical 45 degree
roof the half span rafters are 0.7 of the span across the roof, and if
the gables are further apart from that, it makes sense to use the
rafters as the main structural element.
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