loft conversion timber ridge beam ?

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

Reply to
geraldthehamster
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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.

Reply to
The Natural Philosopher

Er. no. Thats what my masters is in.

It seems that the 'purlins' in this case ARE propped - at the gable ends.

And the roof is not a trussed roof at all.

Reply to
The Natural Philosopher
[snip]

From what you've just spouted then yes.

[snip more twaddle]

I certainly wouldn't engage you as a structural engineer.

Reply to
<me9

Flippin' heck. I thought the standard 1930s terraced/semi cut roof with load bearing purlins supported at either end was a very well known quantity. These are usually 3x2 rafters and, say

9" purlins. Often propped in the middle to a central spine wall. I am surpised it has caused so much debate ! I will ask my questions about timber I-beams in a different way ! Thanks folks, Simon.
Reply to
sm_jamieson

The message from The Natural Philosopher contains these words:

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 adequate.

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.

"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 the ridge."

******

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.

Reply to
Roger

Of course not. In my case - a loft conversion - the whole reason was to get rid of any intruding bracing. And the way you do this in essence is to add a steel purlin from gable to gable for each roof span.

If you're talking about roof trusses then all a horizontal beam does is tie them together - but then it's not really a purlin.

Reply to
Dave Plowman (News)

Did you have W bracing?

And the way you do this in essence is to

Assuming you HAVE gables at all ;-)

Well in fact the definitions I found rather suggest that that is in fact exactly what a purlin IS. what you describe is something else entirely..

Reply to
The Natural Philosopher

The message from The Natural Philosopher contains these words:

Which was the nature of purlins in domestic house construction in this country until very recently. (Although I don't accept that the horizontal ties in truss roofs are purlins).

No idea why you should have thought it was in the first place. There is nothing in Simons original post to suggest a loft stuffed full of struts.

And nothing in your early posts to suggest you were confining your remarks to a trussed roof either.

Reply to
Roger

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.

Reply to
The Natural Philosopher

No. It's a Victorian house. The original attic room had the wall studs which formed triangulation between the joists and rafters. I wanted to remove these walls

Then you provide steel rafters at either end to take the load of the steel purlins. Don't you ever watch Grand Designs? ;-)

The name purlin is as old as the hills - long before roof trusses as we know them today. On a simple structure where you don't want individual joists tying the bottoms of rafters together - like say a small church - the purlins are tied by joists at either end as well as the gable. And the rafters mounted on a wall plate with the same arrangement.

Reply to
Dave Plowman (News)

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.

Reply to
Dave Plowman (News)

The message from The Natural Philosopher contains these words:

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.

Reply to
Roger

Its not so much weakening that is the problem. With a substantial dormer (which basically amounted to one entire side of the roof in the case of mine), you are removing several tones of weight of roof tiles. So the ridge is no longer counter balanced by roughly equal loadings from either side.

My conversion did away with the need for a ridge beam, by having a central wall dividing two rooms in the roof. The wall was not directly under the ridge however but about a meter behind it, so the flat roof joists of the dormer sailed over it to meet the slope of the remaining front roof. These were hence supporting the front roof in cantilever:

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supporting wall is just out of shot to the left of the above photo.

Reply to
John Rumm

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!

Reply to
John Rumm

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.

Reply to
John Rumm

replying to

Reply to
Laura.... help..

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. Brian

Reply to
Brian Gaff

talk to your building insurer if you have one.

NT

Reply to
tabbypurr

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