geraldthehamster wrote:
On 14 Dec, 17:44, "Dave Plowman (News)" wrote:

Perhaps you'd explain why my loft conversion involved the fitting of steel
purlins if they have so little work to do?

Or why the timber purlins half way up the roof of my chalet bungalow
have bowed inwards? ;-)


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.




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.

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.


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
inches.

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
you like.

Makes the calculations easier, but is a totally unwarranted worst case
assumption.

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'
level.

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
ceiling joists.

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
achieve that?"

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.

But then

'an engineer is someone who can do for sixpence what any damned fool can
do for a quid'

























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.

Regards
Richard

On 15 Dec, 10:54, The Natural Philosopher wrote:

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.

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
feasible.

Regards
Richard

geraldthehamster wrote:
On 15 Dec, 10:54, The Natural Philosopher wrote:

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.

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.


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.


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.


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.




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
feasible.


Indeed.
Regards
Richard