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I wonder if anyone can help with calculating the size of roof supporting joists I’ll need for the Victorian Lean-to greenhouse I shall be making.
Attached is a plan view and a poor Sketchup image.

I shall be using 50mm cedar, but am not sure of what width I should be ripping it to support the weight over the span.
It is asymmetric as the floor plan is asymmetric.

I found an American website with some info. but it seems a bit too complex for me. ... e/spancalc

The width is 3900mm the large span approx 3300mm and the smaller span approx 2330mm

The centres of the joists will be at approx 550mm

I shall be using 4mm toughened glass (weight approx. 10kg per square metre).

It will be at a slope of approx. 35 degrees.

roof plan (Copy).jpg
Last edited by NOTTNICK on 20 Feb 2017, 16:58, edited 1 time in total.
By RobinBHM
Im not sure if the span measurement, is the greenhouse depth on plan or the actual rafter length.

If rafter length then as a guide on a flat roof a 6 x 2, C24 will have a span max of 3.1m at 400centres and load of 0.25Kn. I realise the calcs dont apply for a pitched roof, but are still relavent

If your span is plan view then the longest rafter will be about 3.8m long, and you would need a bigger joist.

Id be tempted to get a length of softwood joist 5 x 2 and 6 x 2 and see what deflection you get when you stand on the middle of it.

If the 1st rafter is about 3.2m long then maybe 6 x 2 would be fine, maybe down to 4 x 2 at other end. But thats for C24, iIm not sure WRC would be ok, its a pretty soft timber, often pretty wet. Maybe consider doug fir, siberian larch or iroko for the rafters?

If the glass rests on rafter gasket they should be quite well protected from condensation internally. Condensation will be the only cause of rotting assuming the roof does not keak, so you wont need extremely durable timber.
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Hi Robin
I see how things are ambiguous, my mistake.
It's not a plan view - it is the actual roof dimensions. so longest rafter length is actually 3100 (as the longest - edge - span will be supported).

Plan view is 2700 1900 3900

I suppose it doesn't need to be cedar, I had been thinking of larch for the whole construction, so maybe I'll check that out.
I haven't used larch before.
By RobinBHM
Siberian larch is a pretty durable dense timber, although not easily available in the UK in joinery sections and unsorted, you may find lower grades with some splits and knots.

Im not sure if European larch is either durable or stronh.

Thinking about it, in a strong timber, you may be ok with a smaller section, maybe 5 x 2 or 4 x 2, but there maybe some deflection under its own weight.

The pitch is quite steep, so probably snowloading not too much of an issue.
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By Sawdust=manglitter
Hi Nottnick, I've only just seen this post. Did you get something sorted? If not I'm a structural engineer so will do a couple of quick calcs on my lunchbreak today to tell you exactly what you can get away with :D
By RobinBHM
Sawdust=manglitter wrote:Hi Nottnick, I've only just seen this post. Did you get something sorted? If not I'm a structural engineer so will do a couple of quick calcs on my lunchbreak today to tell you exactly what you can get away with :D

I would be interested to know :D

My guess is that calculations for a greenhouse or conservatory could allow for a larger deflection than the 1/360 (which I think has something to do with plasterboard?), I guess its a trade off between a slim structure with bowed rafters or big sections that are dead straight.

One problem I have is obtaining any structural calculations for specific timbers. It seems there are tables for softwood in C16 or C24 but not for hardwoods like oak and iroko
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By Sawdust=manglitter
I've done some calcs (took longer than anticipated), but you can get away with the following...

Western Red Cedar (Structural Grade GS) - 175x50mm at 550mm centres
British Larch (Structural Grade GS) - 150x50mm at 550mm centres
Iroko (Structural Grade HS) - 140x50mm at 550mm centres
Oak (Structural Grade TH1 & TH2) - 140x50mm at 550mm centres
Standard off the shelf C24 grade timber - 140x50mm at 550mm centres

RobinBHM wrote:My guess is that calculations for a greenhouse or conservatory could allow for a larger deflection than the 1/360 (which I think has something to do with plasterboard?)

The span/deflection ratio should be 330 for all brittle finishes, so like you said for plasterboard, but glass is also a brittle finish so should still aim for the 330 span/deflection ratio.

RobinBHM wrote:One problem I have is obtaining any structural calculations for specific timbers. It seems there are tables for softwood in C16 or C24 but not for hardwoods like oak and iroko

There are properties available for various timbers in the British Standards. We can do calculations for all of the timbers listed. I'll post table extracts from the BS shortly to show what timbers we can do calculations for. There are no 'span tables' available, and it is pretty complicated to do calcs on different timbers, but it can be done!

If Building Control are involved then they will require a copy of the actual calculations though, but that would have to be done properly with headed paper etc though the company i work for. Apart from that i hope the above helps :D
By RobinBHM

Thank you very much for spending the time to work out and post the above structural information, it is very interesting to me -I shall print out the tables you have posted and keep on file for future reference. (for guide purposes not as a basis to calculate loads to building regs! -I realise that requires the proper calculation and paperwork)

I guess your section sizes calculated for the OP's greenhouse question are for 3.1M long rafters?

If toughened glass is used then the calculations could be perhaps be based on a less stiff material as tough glass is very flexible -certainly more than timber could ever be. I have seen a bow of about 6" on a long double glazed roof unit.
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By Sawdust=manglitter
No worries Robin.

I based my calcs on approx 3.3m max length, so a horizontal span of 2.7m @ 35degrees. The snow load varies depending on the angle of pitch. For reference, the max snow load to BS is 0.75kN/m2, but for 35degree pitch it works out to be 0.6667kN/m2.

In terms of the glass deflection, the only guidance i have is the following...


And seeing as the glass is on a 35degree pitch then i stuck to span/330 rather than go for the OTT span/360. I fully appreciate that things in practice can be very different, but thats due to all of the safety factors that you need to consider in the theoretical design.

Sent from my iPhone using Tapatalk
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Hi Robin
I feel really guilty about not getting back to you, really sorry.
The project went on hold as I had to get planning permission. I didn't check back in the forum after I looked at the original replies.
I really should have done as it is such a great resource. Please accept my apologies for this.
Your earlier advice / calcs really appreciated.

Anyway, I now have permission so am getting organised again and I'd really appreciate a bit of advice.

Looking at figures and suggestions etc. I am thinking of going for Douglas Fir for the rafters rather than cedar (they won't be showing on the outside and will be stained anyway).
It should give me a bit more rigidity. Not as heavy as larch.

A final bit of help with the calcs would really be appreciated.
The whole roof will be pitched at 30 degrees.
The longest rafter is 3 metres the shortest about 2.4 metres.
They'll be at 550mm centres.

I was wondering though, if I braced the longer rafters against the supporting wall. I should have more strength. (see attached diagram). It'd

I am wondering if you think I'll get away with 125mm X 50mm douglas fir if I do that?

brace (Copy).jpg
By RobinBHM
Hi Nick,

Thanks for the update, Im pleased to hear its still going ahead.

Douglas fir is a very good timber for external joinery, its durable, quite stable, much harder than western red cedar.

It does have its faults, it can have resin pockets, that seem to run diagonally through boards so can appear on a face and edge.

For rafters you will need boards that have good straight grain that is parallel to the edges and no knots -at least if you want to achieve strong rafters.

If you want to undersize the rafters to 125mm, which probably fine, you will find the longest rafters may bow under self weight -see the advise from sawdust=manglitter's information, he is a structural engineer so far more qualified than me to base sections sizes.

Toughened glass is very flexible and will cope with a significant bow in a rafter without breaking.

Douglas fir is a bit less dense around 530KG/m3 compared to Siberian larch 570-650 kg/m3 although that depends on tightness of grain
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By Sawdust=manglitter
Hi Nick,

Glad to hear that you're had the relevant permissions etc. Particularly as you've had to go through planning I'm assuming you will need to involve Building Control? If that's the case then you will need to get a 'proper' set of design calcs from a qualified structural engineer to satisfy the B.C. officer.

In terms of putting in a 'brace', are you thinking of a steel threaded rod type thing, or another timber member, forming a sort of truss? A steel rod type bracing is only for tension forces, whereas the member you've shown on your sketch will be in compression. But in terms of checking all that, it's more involved and will take longer than an hour over a lunch break.

I think that 125x50mm Douglas Fir rafters at 550mm centres with your 'brace' a timber member the same size cross section as the rafters will likely work, but again a structural engineer would need to design this to confirm. My advice would be to see if there are any small structural practices local to you to check the design of the rafters. Checking the racking walls on the other hand (which make sure that a gust of wind wont 'flop' the building over) is a totally different kettle of fish. Just to give you an idea, for a full set of timber frame calcs on a 3 bed detached timber frame house we would charge around £400 (+VAT), and the calcs would be around 100 pages long. But for full set of timber frame calcs for this structure, particularly with materials differing from the usual C16 or C24 timber, then an engineer may charge £200+. (people dont tend to realise how much work is involved)

Sorry I cant me more help that this, but one guy in the office got the sack last month for doing jobs on the side using work's software during his lunch hours (licences cost thousands/yr)! :?