bearers and joist for deck.

[mickfromperth]

Hey All,

A while ago I posted a Q about building a low lying deck, talking about what options there are in terms of structure.. Here is is:

http://www.woodworkforums.ubeaut.com.au/showthread.php?t=34208

I've decided to go with treated pine bearers and use joists, using joist hangers to reduce spance. I started shopping around for prices, but then realised I really didn't have a clue about dimensioning the blasted thing..

My local salvage yard currently has these two dimensions of treated pine:

140x45 (6m lengths) (@$6.5/m) and
90 x 45 (4.8m lengths) (@$4.6/m

If I need the 140's I'm going to have to drop a few inches out fromunder the alfresco, so I'd rather not but maybe I have to...

I have this book here "Decks and Pergolas" by alan staines, which has some tables about 'continuous span' and 'single span' bearers. What are they reffering to with this 'span stuff'? There explanation is 'continuos span bearers have intermediate support'. Can someone explain this to my in monkey terms? ;-)

Also, my sizes (the ones above) aren't listed in his table, can someone advise if these are usable and if so what dimension of the support structure is required?

I will be using Batu 19ml decking, so I know my joists need to be about 450 appart, but there the only 2 things I'm sure of!! hehe.

Mick (from perth..)

[Bob Willson]

One beam across two posts is a single span.
One beam across three (or more) posts is a continuous span

[journeyman Mick]

Take the next closest size down for your timber dimensions ie if the tables list 150 or 125 and you've got 140, then treat them as for 125. With spans take the next size up ie, if the span tables go from 2.4 then 3.0 M and you want to span 2.7 treat it as 3.0. You'll also need to know the strength grade of the timber this can be as low as F4 and go to F27 or higher. Obviously the stronger the timber the smaller dimensions you ca use for the same carrying capacity. I'm guessing your treated pine wouldn't go any better than F7 and is probably only F4.

Mick

[Bluegum]

yep what mick has said. Alan Staine's book is a good one and easy to follow. I found it a good read as it was able to answer most of my questions.:)

[mickfromperth]

Thanks Guys,

Treated Pine over hear is 'normally' H4, F7. But I'm make sure what it is before I make an order.

I don't have the tables on me (at work..) but from memory it's something like, I would need 2 x 120 x 45 for some minimal continual span (around 1800) for F7.

Given that I'm basically resting the bearers on the ground, or am able to, it seems like overkill from a downward force support persepective. I'm not being blase' about structural requirements, I just think that most of these specifications were written with the intention of safety when mounting off the ground. I *think* there should be some different spec's when lying on teh ground, but not sure.. Please comment..

Mick

[journeyman Mick]

So are you actually laying your bearers on the ground? Highly unadvisable. Even if you get the ground compacted and dead level you've still got moisture to worry about, even with treated timbers, plus, your batu deacking probablt isn't treated and it will be less then 600 off the ground. It will suffer from fungal attack. If your bearers aren't on the ground then they'll be supported by your stumps, in which case it matters not whether they are 50mm or 5M off the ground. Do it right, do it once. Do it dodgy and throw your money away.

Mick

[echnidna]

So are you actually laying your bearers on the ground? Highly unadvisable. Even if you get the ground compacted and dead level you've still got moisture to worry about, even with treated timbers, plus, your batu deacking probablt isn't treated and it will be less then 600 off the ground. It will suffer from fungal attack. If your bearers aren't on the ground then they'll be supported by your stumps, in which case it matters not whether they are 50mm or 5M off the ground. Do it right, do it once. Do it dodgy and throw your money away.

Mick

Exactly :cool:

[toddles]

Mick,
We've just finished something similar at my mother in laws place - 6.5 x 4.5m deck, 90x45 treated pine joists with 19mm batu decking. We built it about 200mm high over a sloping concrete pavement so didn't have moisture issues. You will have dramatically fewer moisture issues in Perth with our dry climate than those easterners would but if your laying it over lawn/garden area you would want to use a moisture barrier like tough black plastic and ensure there is no where for water to pool and that there is decent cross ventilation under the deck ie: 150mm or so. I've lived in NSW and spent a lot of time in QLD and you would be amazed at how mouldy everything gets - I couldn't get used to it. We just don't have the same sort of problems in dry ol' Perth - even when it buckets down humidity remains low.
Go to Collies in Malaga if you can and speak to a guy called Mike. He was a great help to my MIL and helped her plan the deck and did a very good job of it.
Be aware though that if you use 90x45 that each joist will need to be supported every 1.5 to 2m either by a beam or post to keep the structure rigid and that you'll need to allow for 15-20% wastage when buying boards. I'll post some pics as soon as i can (they're on MIL's camera).
Cheers,
Toddles

[mickfromperth]

Hi All,

Mick, not so much 'lie them' on the ground, but I have to excavate the to install the deck. So it will be pretty easy to have support every meter or so, as a substitute for having stronger wood. I was thinking of doing the formwork for concrete, pooring the concrete supports, then digging out around the concrete. And the just lying the bearers across the concrte supports. In this manner, it won't make much difference in terms of effort to have one support every 2 m's than one support every 1m.

Also, I've got the tables here now. I understand this a little better now. I need to first do my joist dimentions, then do my bearers right? So, I have F7 90x45 joists. My tables say for F7 joists, 1300/1700 single/continuous span. As I'm using joist hangers, it's all signle splan right? WHich means every 1300 I need a bearer available.

Now I look at my bearers, I have 140x45. I look at my tables, I have to know my load width, and my span, then it gives me the wood sizes to choose from.

Load width is basicalky the distance between joists right? If so, my laod width is 1300, but my tables minimum load width is 1800. At 1800, it says the minumum dimentions to use is 2/190x45 at 1200 span. This has to be a typo, as the figures go down on the next size up span, 1800 = 2/140x45.

Can someone check for me in the TRADAC manual what F7 dimension options there are for 1300 span joists?

Yay. I feel like I just had a brakethough. I think single 140x45's bearers is going to be achievable..

Mick (from perth..)

[Elmcroft]

Mick
The questions you are asking are exactly the same I have been wrestling with... danm span tables...
This one seems to suggest that you can do quite a bit more than 1300 if you are doing 400 or 450 centres for the joists...
http://www.designpine.com/images/pdfs/spantable_F7_floorjoist_new.pdf

I am still trying to get my head around the bearer figures....what I don't get is why does the joist length have any impact on bearer size... surely the span of the joist and the span/size of the bearer are the critical measurements here...

http://www.designpine.com/images/pdfs/spantable_deckbearer_F7_new.pdf

My deck is going to run as a 5000 *10000 rectangle with the long side supported by a plate on the brick wall of the house. Joists will be 10 m long i.e. as long as the deck (duh) and I will space them at 450 centres out to the 5M width i.e. decking will be at 90 degrees to the house. Why does the joist length as mentioned in the link make any difference at all!!!

I am confused...perhaps because it is a kiwi website...they are a little strange!!

[journeyman Mick]

...............................I am still trying to get my head around the bearer figures....what I don't get is why does the joist length have any impact on bearer size... surely the span of the joist and the span/size of the bearer are the critical measurements here....................... Why does the joist length as mentioned in the link make any difference at all!!!..............

Longer joist = more m2 of decking that bearer needs to support. I've no idea what a M2 of hardwood decking plus associated joists and fasteners weighs but just to give you an idea, a M2 of 16mm particleboard weighs more than 10kg. Plus more M2 of decking = more available space to pack people onto = up to 100kg per M2 for a really popular party (at a guess).

Mick

[mickfromperth]

I've been staring at the table for bearers that you linked to. I don't get it either. It seems to indicate that further the joist span, the closer the bearer 'support' span (ei, the more supports along the bearer). But then you look at say the most extreme example in the table. 6 meter spans on a joist?? What The?

Then I remembered. They need to be considered in "parallel". You pick your joists, then work down. So, if you have 90x42 (as per your links) joists, spaced at 450, then your max span between bearers is 1550.

Then you go to your bearer page. We just picked 1550, so as per Mick's comments above, go one up, 1.8. So, if you've got 138x42 (as per your tables..) you'll need to support your bearers (ie posts) every 1700.

OK great. So using your links I can build my deck ;-). But, these don't seem to be australian standards :-(. So I'd really apreciate is if someone could answer my earlier question's (#9 in the list).

[Elmcroft]

Mick
I agree, not Australian...which is disappointing...but I am stuffed if I can find Aussie ones on the web (well... for free at any rate) these guys look like they will eventually... good pricing too which is very handy for those of us who don't live near them...but whose local bunnies feels an overwhelming desire to match and better the price : )

http://www.thelogfactory.com.au/timber_span_details.aspx

cheers,
Elm.

[JDub]

This might help :)

http://www.hyne.com.au/downloads/dih_downloads/timberTDS/04_decks.pdf

[bpj1968]

Spans take into account 2 things, the weight supported and the "bounce" of the structure. It will be very springy long before it will fall. As an example put a plank flat between two supports few metres apart and stand in middle and lightly bounce. Now move the supports closer and do the bounce again. It will be more stable. This is only in 1 dimension.

Think of the supports as the posts and the plank as the bearer. Now imagine another plank and supports paralell to the first one. (another bearer) using a 3rd plank (joist) at 90 degrees between the two bearers and supported midway between the supports, like a "H" pattern. now if you bounce in the middle of the joist, it has teh flex of the joist and the flex of the bearers. That is why the load width (joist span) is taken into account when selecting bearer sizes. This is now 2 dimensional

If you go at the absolute maximum spans allowed then you will still have more bounce than if it is over-engineered.

Also consider using a weatherproof strip on top of your joists and bearers to keep the water of the top of them. Bunnies sell one, but I can't think of the name.

I have teh same book and I would agree that the 2/190x45 is a typo and should be 2/90x45.

[ausdesign]

90*45 f7 seasoned joists @ 450 centres span 1400 single
2/90*45 f7 seasoned bearers (single span) with a floor load width of 1200 can span 1700, with a flw of 1800 can span 1400 - you can interpolate between these for your 1300 joist span.
2/90*45 bearers (continuous span) can span 2100 with flw of 1200 & 1700 with a flw of 1800.

You may be having trouble with the tables as bearers are nominated within a height/width ratio. For a single deep member, tables for 'lintels over openings' may better suit.

These figures are all for decks less than 1 met. above ground level.

Hope this helps

[toddles]

This book might help (it's Austn) "Roof Cuts and Rafter" Lengths (10th ed) - $19.95AUD; Author: Hancock; Publ: McGILLS; ISBN: 0731647033<table width="100%"><tbody><tr><td colspan="2">
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