Thread: Timber size required vs span

People would need more design criteria before advising, what shape do you want the span eg peaked or flat, what covering too use shade cloth or tin/lazerlite or slats and what sort of buget.
4.8x4.2m isnt a that bigger span off 4 posts.
Go to a place that makes patio's and veranda's, check out what they use for around your size area.

A general rule of thumb told to me by an old master builder was ( in feet 'n inches), if you multiply the thickness of the wood by the width of the wood, then call the answer in feet, then that's how far you safely span.

E.G - 6 inches by 2 inches = 12 foot span max.

8 inches by 3 inches = 24 foot span

So. if your doing 4.8 metres, that's about 16 feet, you'd need at least 8" by 2" or 6" by 3" (near 'nuff). 8" by 2" would be better.

As said above, a lot depends on what timber you use and what's on top of it. The above calculation was intended for use with Jarrah or Karri. Pine would require bigger dimensions and I'd add about 30% margin when using pine.

Bilbo

Hope your professional indemnity policy is up to date bilbo!!

Firstly we don't know where craig is, he could be in the most windless place in Central US (not likely because he's talking metric - maybe he lives in France!). On the other hand if he lives near Cliff or Mick he'll have a bit of uplift to contend with.

Next: Jarrah isn't all that readily available round the country and we don't know the stress grade of the timber he's looking at using.

In Qld the old-time rule for HARDWOOD was half the span in feet + 1 = depth of joist in inches...ie a 12 ft span would need a 7 x 2 or if it was a bearer or roof beam it would be 3" wide. Of course that timber would have been about F35 or something stupid had F grades been in then.

Now with new fangled engineering requirements, and moments of deflection divided by the modulus of elasticity, you need a whole heap of calculations to come up with exactly the same answer!

Craig, go to your local library and have a look at a copy of the light timber framing code, (its and Australian Standard but I don't know which one off the top of my head), you'll need to know a few more things about what you are going to do though, like the roof material etc.

You are going to have to produce a drawing to get a building permit anyway, why not ask your local building surveyor?

Cheers,

P (ducking the question with good reason! )

Hello,

If you are sheeting the roof area don't overlook LVLs, you can span long lengths on sizes much smaller than you would use in hard or soft sawn timber. They are value for money for what they do. Load & span tables are available from the resellers. Cheers, Bill

the easiest way to sort it out craig is to get the Allan Staines book on Australian decking and pergolas. He has all the tables you need, as well as a description on how to do it all. I think Bunnings sell them, otherwise most bookstores will.

The key consideration is whether you are going to roof it or not. Normally you are better off assuming that you will roof it at some point, even if you don't want to now.

It also depends on the look - do you want hardwood or is painted treated pine OK. TP is light, cheap and easy to work with (but toxic).

Good luck

Trav

Midge,

I said it was a rule of thumb. I ain't a builder. I'm a mechanic - I fix builders trucks

I'm a recent addition to this forum. I think I'll stick to "mechanics' forums" - far less nitpicking on those.

B

Yoh bilbo, don't spit the dummy & go, stick around, you'll get tougher.

The point was, it was TOO general.

Craig didn't say where he was & you didn't state your quals up front or in your profile.

When I needed to know the same answer, I went down to the local council planning department & told them what I had in mind. The Inspecter photocopied the book of rules that he was playing with & gave me the page. He also told me of a supplier that had a heap of tables that they gave away for free.

[QUOTE=bilbo]Midge,

I said it was a rule of thumb. I ain't a builder. I'm a mechanic - I fix builders trucks

I'm a recent addition to this forum. I think I'll stick to "mechanics' forums" - far less nitpicking on those.[/QUOTE
Hey Bilbo, read my signature.

Remember when you give someone advice they might just go and build it.... if you don't know all the criteria, you simply can't design a beam.

In mechanics terms if the question had been "how much oil should I put in my engine", and I had replied "as a rule of thumb I put in four litres" I am sure your response would have been in a similar vein, that you really can't answer the question without knowing the size and type of engine!

As Cliff said, stick around.... we'll all learn something. (If there's less nitpicking on those mechanic's forums they must be BORING AS BATPOOP!!)

Do you have any good pancake topping recipes?

P

thanks for all the replies so far.
i was only after some rough ideas as to wether the whole project would be viable, hence the lack of detail as i hadn't put a lot of thought into roofing style, materials etc.

After reading the advice so far, my thoughts are a flat roof maybe using slats on beams and covering with shade cloth or maybe laserlight in clear and opaque to allow some shade and sun.

One other thought would be 4 posts 4 beams and stringing a shade sail between as i have some shade sail in the shed. the one dilemma i would have is the shade sail is 3.6m square and the frame would be something like 4.8m x 4.2m and my concern is the length od cable/shackles etc to make it fit.

to answer one of the questions my location is SW Sydney

any ideas and suggestions are welcome

thanks

Craigv

Hi CraigV,

The standard you need is 1684.4 - residential timber framed construction, part 4 is simplified for non-cyclonic areas such as Sydney.

The 1684 series replaced the NSW Timber Framing Manual written by State Forests of NSW in 1999.

It gives span tables using the "F" stress garding system and MGP (machine graded pine) for seasoned softwoods such as treated pine.

Spans of 4.2 and 4.8 metres are not excessive.

All timber is the following info is for MGP10.

For rafters for a sheet roof:
900mm spacing: 190x45mm for a 4.2 span
240x35mm for a 4.8 span
1200mm spacing: 240x35mm for a 4.2 span
240x45mm for a 4.8 span

For your beams, 4.8m is a big span, it would be better to place an intermediate post with a continous 2.4 span or use a structural steel member. Spans of 4.8m in wood are not covered in the standard.
For a 2.4 span a beam of 190x45 is enough.

You also need to think of the size of your posts. For posts spaced at 2.4m 70x70mm is enough for a verandah up to 2400mm high,

If you want to go higher speak to you council or get a civil engineer.

You can try to get hold of the standard though a library or online through the standards website.

Lastly, remember the bracing. Good luck

Cheers
Pulse

Some great software is available from Tilling Timber...they manufacture laminated beams, I beams and the like...anyway its called SMART FRAME.

Say you are ripping out a structural wall, enlarging a window etc etc, key in all of the information ie wind speed, roof type, heights, spans of other members type of ceiling etc etc etc and it will calculate what type of beam is required depending on the loading you want on the beam. Cool little program.

The only draw back is it calculates only Tilling products, although it covers oregon.

Enjoy

http://www.tilling.com.au/smartframe...tSoftDload.htm