Trying to think outside the box. I have plenty of headroom in the loft. What sized timber I beam would I need to use instead of the typical RSJ to support the ridge ? Loft size 5.5m x 7.5m, normal cut roof with purlins at 1/3 and 2/3 heights, V onto spine wall at purlin mid point, clay tiles etc. Thanks, Simon.
How do you know you need a ridge beam? Rafters are self-supporting and there may be little or no load at the ridge
dg
Sorry, I didn't say - there will be a dormer on one side. I do wonder about lateral forces in this situation - maybe a thin timber I beam would not be suitable. Simon.
It's more usual to install steel purlins - that's where the load is.
Purlins need to be minimized (stairs running up parallel to the roof slope), thus a ridge beam would be required to take some of the load (why not, it's in dead space !), and the purlins only need to stop the rafter sag. Roof pitch is quite steep at 40 degrees. In the main section hopefully dwarf wall at 1/3 height and a ridge beam will be enough. Purlins at 2/3 height are the awkward ones. Simon.
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.
Howerver if the rafter to joist integrity is poor,so the rafters can slide outwards at the base - a fairly common thing where upstairs conversions under the eaves have been done and tie beams cut through,leading to wall spreading - then a steel ridge running gable-to-gable can act as a primary support. At this point teh roof ceases to be a triangulated truss structure ands simply becomes a bunch of poles leaned against a structural ride pole.The wight of the roof instead of being carried as a downward an outward force on the rafter bases, is transferred to a downward force on the ridge and a downward and INWARD force on the rafter ends.
If you also add in *structural* steel purlins and carry those downwards via posts to the joists, you can end up with a completely different loading again.
That adds load to the joists, which are no longer in pure tension, but also being subjected to a bending force. You might use such vertical ties if the original ceiling was not strong enough to support rooms above - you trasnfer loads to the roof timbers that way, which are generally massive enough to take tiles or slates..they can cope.
In all cases, however, *unless* the purlins or the ridge is/are *very* strong, *and* supported at the gable ends or by a central chimney etc, they do not carry any major loads at all. They are there for stability. Increasing their size is a waste of time.
Indeed. So ... I have plenty of headroom in the loft. What sized timber I beam would I need to use instead of the typical RSJ to support the ridge ? Cheers, Simon.
What makes you think you need to?
NOT a rhetorical question, but one that goes right to the heart of the matter.
I haven't got it straight as to what is going on.
If the structure has been messed around a lot, get in some structural engineers, who have liability insurance and who do these calcs every day.
If it hasn't, why are you doing it at all?
Faced with a sound standard loft with a steepish pitched roof, all I would do would be to bolt cross pieces at head height plus to the rafters to make a ceiling, and take some of the lateral stress out of the eaves. Whilst not ideal you could nail strips on top of them at the edges to stabilize the thing, purlin wise.
Or better still, put insulation between rafters and then line with structural ply, before boarding over. That does all the job that purlins do and its only 19mm thick, plus 12mm plasterboard etc.
If the floor is weak, add bigger joists alongside or on top of existing, and plate out with chip/ply or real boards..and reduce the need for ridge support by BOLTING them HARD to the rafters at the ends.
I'm designing a dormer loft conversion. I omitted that off my original post I think. The room is to be open plan from front to back, i.e. to cantilever support as in John Rumms conversion. Simon.
So what have you removed to necessitate beefing up the structure?
Even punching a dormer out hardly weakens anything.
and surely you don't mean a cantilever..I have yet to see a cantilever structure in any house..well a few Elizabethan ones where the top storey overhangs the lower maybe.
The message from The Natural Philosopher contains these words:
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.
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.
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).
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? ;-)
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 inches.
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
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.
Rob
So where else does it go?
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.
Purlins are simply there to either stabilise the rafters, or if above them, to transmit te roof weight TO the rafters.
Thats geberally waht is used IME.
Well I guess wasted a lot of time doing an engineering degree then.
Look. purlins are only there in mast cases to spread the load from the roof weight TO the rafters
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.
If your rafters are VERY widely spaced, they 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.
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.
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.
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,...
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 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'
Mmm.
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.
Think you need to look at applied mechanics...
I'd love you to say that face to face with the architect and structural engineer involved. ;-) I've got reams of calculations proving the stresses involved.
You're quite simply wrong.
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