# Forum Home Renovation Decking  Post Spacing Thoughts

## TazManiac

Hi Guys 
Hoping someone can help me out. 
I am planning to build a deck 6.0m wide along the side of my house and 4.5m deep.  I have a few ideas on what needs to be done but need some help with post spacing to make sure I am not building a trampoline. 
Beams will be made from 190 x 90 (2 x '190 x 45') Treated Pine
Joists will be made from 90 x 45 Treated Pine
(If these need changing then I am very open to suggestions) 
I was planning to have a Ledger Board and two beams to cover the 4.5m depth.  Is this sufficient? 
Joists will be at 400 centers. 
I need some help to know how many posts I need to support each 6m beam. 
Any help is appreciated. 
Aaron

----------


## Bloss

Search for some span tables on this site and you'll get plenty of advice. Also buy Allan Staines book _Decks & Pergolas_ at around $30 at Bunnies and other stores - will save you heaps of money and worry - also has some span tables too. You might like to download free software from here which has span calculators and other stuff.  http://www.timber.org.au/menu.asp?se....02%20Software   :2thumbsup:

----------


## oohsam

Hey there. 
Depends what you want. I have a rule of thumb I always go with. I've built a few decks to this spec and its been very solid 
Posts, 100x100 treated Cypress pine, in stirrups at no more than 1500 centers,  
Bearers (or beams as you say) -190 x 45, house them into the posts, or if its low to ground and the beams are sitting in the stirrups, then its fine, bolt them in obviously with coach bolts, galvanaised of course.  
Joists at 450 centers, 90x45. 
That's for softwood btw.  
Peace.

----------


## TazManiac

> Posts, 100x100 treated Cypress pine, in stirrups at no more than 1500 centers, 
> Bearers (or beams as you say) -190 x 45, house them into the posts, or if its low to ground and the beams are sitting in the stirrups, then its fine, bolt them in obviously with coach bolts, galvanaised of course.  
> Joists at 450 centers, 90x45.

  Sounds like I'm on the right track so far then.  Good to know.  I was thinking maybe 1500 centers for the posts. 
I may have a look at the span tables as suggested above and see how I go. 
Time to draw up a plan I think.

----------


## oohsam

Good stuff. The Alan stains book is a great resource to have. I have used it every single time I needed to build a deck. 
I recommend a copy. It also has the span tables. 
I recommend Google sketch up if you're savvy with computers for designing, you can build to scale and play around with the sub frame.  
Do post pics as you go along or hit any snags. 
Cheers.

----------


## TazManiac

Thanks.  I have been using Google Sketchup for a while now and find it really handy. 
This will be Deck - Mk 3 in sketchup. 
Will let you know how I go in the end. 
Aaron

----------


## TazManiac

I am having a bit of trouble understanding the span tables and trying to work out how many bearers I need to have and how far apart to have them. 
I think, if I am reading it right, that for a 4500 wide deck using 2/190x45 bearers I can't have more than a 1300 span between beams and a 300 cantilever.  Is this correct, and if so, does this mean that I need to have 4 beams to cover the 4500 deck?

----------


## dcwalker

You're reading the span tables incorrectly. 
With a floor load width of around 2.2m your 2/190x45 bearers can span up to 2.7m. 
Therefore, you'll need 4 100x100 posts evenly spaced along each set of bearers (1966mm on centre, to be precise!). You'll need a ledger of 70x35 bolted to the house at 600mm centres and two sets of bearers and posts. 
Use 140x35 joists as the 90x45's will be too springy and be sure to space them at 400, not 450, or your decking will also be too springy. 
Bolt the bearers to each side of the posts - don't try to fit them both to one side. Use 2xM12 220mm galvanised bolts per post. 
All these dimensions come direct from the Australian Standard. I've attached a PDF to show you how it all goes together. 
Regards 
Damien

----------


## 2x4

Got to love good informative posts. :2thumbsup:  
45mm wide joists are easier for me to join boards on tho. :Redface:

----------


## TazManiac

Damien, 
Thank you so much for your help.  I'm not sure how I was reading the tables but thank you for giving me all the information that I need without treating me like the newbie that I am. 
My only concern is that the height I have to work with is only 340mm and if I go with 190 bearers, 140 joists and 18mm decking, I run out of air to place it all in.  Is there any way to fix this?  I had previously thought that I may be able to get away with sitting joists in joist hangers on he bearers.  That is the only thing I can come up with if I go with the larger joists. 
Maybe smaller bearers and more posts? 
Any other ideas? 
Aaron

----------


## dcwalker

380mm?? Now you tell me!!  :Smilie:  
You can hang your joists inside the bearers using joist hangers, as you mentioned. This means you'll need only 190mm + 18mm for the frame and decking. The remaining 170mm or so leaves plenty of space for air circulation. 
Don't compromise on the timber sizes I've mentioned, if you do you'll definitely notice spring in the structure, especially in a miserable wet Hobart winter! Seriously though, in wet weather the timber will reabsorb heaps of water and loose a stack of load-carrying capacity. 
Hanging from inside the bearers changes something though - you have to sandwich (laminate) the bearers together so they work as one piece. Sitting your joists on top of the bearers allows them to be installed either side of the post, but using joist hangers means the bearers have to sit together. Nail them together with 2x 70+mm nails every 400mm along both faces. Each piece will almost certainly be slightly different in width so make sure you line up along one edge and use this as the top - as the decking will sit directly on top of the bearers they must be level on the top edge or your decking won't sit flat. 
By the same token, make sure you carefully line up the top edge of your joists so they're flush with the top of the bearer. Do this by sitting the joist hanger a few mm below 140mm, this will allow you to make minor adjustments to the joist height as you nail the hanger.  
Because you must not attach the laminated bearers to one side of the posts they'll now have to be sat on top of the posts. This is a little more tricky but do-able. Concrete your posts in place, carefully calculate the finished post height then snap a chalk line across them all to mark where to cut. Use a set square to mark the cutting line around all sides then carefully cut them to length. Don't stuff this bit up! 
Attach the bearers to the posts with tie-down straps. 
Regards 
Damien

----------


## TazManiac

Thanks again Damien. 
I think I can do that.  Sounds like a bit more work but should get some good results.   
About the posts.  If I am using laminated bearers is it acceptable to use the galvanised post supports (the saddle type ones)?  I think they look easier to use due to the fact that I can attach them to the bearer and cement them in after leveling them at the right height.  Do they change the amount of posts that I need if I use them? 
Thanks 
Aaron

----------


## dcwalker

Do you mean you'd like to use long post supports in place of timber posts? If so, yeah, that will work fine, if you keep a couple of things in mind...... 
Post supports will allow a lot more flex in the structure. This won't be a big problem for you as you're not building far off the ground but it's very important you brace the whole structure well so it can't rake sideways. Use the longest stirrups you can find so they're embedded as deeply as possible in the footings. 
Also, don't use rapidset. This stuff is a handy alternative to full strength concrete mix but not suitable where you'll be using stirrups in place of timber posts. It will break apart in time due to the flexing of the stirrups. Take the extra time to use regular concrete mix and you'll be fine - it's available in premixed bags if you'd rather not buy a trailer load of gravel and you can easily mix it in a wheel barrow. 
Using stirrups without timber posts is a bit of a cheat, you're cutting some corners but as long as you install them as above all will be fine and the building process will be much simpler due to being able to pour your concrete with the stirrups already attached and in place.  
Regards 
Damien

----------


## dcwalker

.....sorry, I missed your last question. 
Using post supports doesn't change the number required.

----------


## TazManiac

I like cheating.  This sounds like a little bit more comfortable way of doing it for a beginner. 
Thanks again for the help.  I will post my 3D plan when I am done.

----------


## TazManiac

OK.  I think I am all sorted for now. 
Here's the plan that I have made 3D using Google Sketch-Up.     
Any thoughts are appreciated.  Hard to see anything really so I have included the file for anyone that is interested. 
Aaron

----------


## dcwalker

Looks good.  
2 points though..... 
Save some money on timber and fittings by using a ledger board of 70x35 fixed at 600mm centres. Your joists sit on top of this and are skew nailed on. No need for the 190mm ledger and joist hangers. 70x35 may seem a lot lighter than your joist material but when attached to the house provides more than enough support - right out of the Australian Standards. 
You've drawn the joist closest to camera sitting on the outside of the bearers. If you build it this way be sure to attach the joist to the bearers with some steel right-angle brackets and M10 bolts. Screwing or nailing through the joist into the bearer end grain will only result in the joist falling off later.

----------


## TazManiac

Thanks for the input again.  I will update the design with your tips.   
I only chose the larger timber size for the ledger as I was thinking the joists would not fit on the inside but I guess there really is no reason why I can't put them on top.  Thanks. That should save some money. 
The outside joists I thought would make the deck look nicer without having to put a fascia on it too.  A lasting result is more important to me so I will get over it and fix the design here too. 
Thanks again for your invaluable assistance. 
Aaron

----------


## dcwalker

Mate, put the end joists on the outside if you like, they certainly look better that way. Just be sure to attach with right angle brackets, that's all. 
If you're laying treated pine decking be sure to leave no space between boards if the decking is new and still wet. It'll shrink in a couple of days and leave a 3-5mm gap once dry. After a bit of rain it'll swell again and close the gap. Repeat ad infinitum. 
If you have access to a nail gun use 50mm ring shank dome head galvanised nails. They won't pull out of the joists and the dome head lets water run off. If you can afford stainless nails then buy them instead. If you're using a hammer then I don't think you'll get loose ring shank nails so go for a heavy gauge twisted shank variety. 
If the situation allows, make the frame slightly shorter than 6.0m. Then you can buy decking in 6.0m metre lengths, lay them with a slight overhang on each end which you then trim off with a circular saw once finished - super neat job that way. 
Good luck. Post some pictures when you're done.

----------


## Uncle Bob

> All these dimensions come direct from the Australian Standard. I've attached a PDF to show you how it all goes together.

  Excellent drawing!

----------


## 2x4

Pretty cool drawing. 
Are you sure you need the double bearer set up on the end of the deck? 
FLW 1100 ish on end bearer.

----------


## dcwalker

Yeah, yeah. Spot the pedantic one!  :Rolleyes:  
Just trying to keep the advice simple. If you're still reading this thread TazManiac you only need a single bearer at the front, not a laminated pair. 
2x4 is referring to the bearers' Floor Load Width (FLW) which is the amount of floor each bearer is supporting. For the front bearer the FLW is half the distance from the front bearer to the middle one. For the middle bearer the FLW is half the distance to the front one, plus half the distance to the ledger therefore this bearer needs to be a laminated pair as it's carrying much more weight. You owe 2x4 a beer for saving you $80 in timber. 
Don't forget the photos. 
Regards 
Damien

----------


## TazManiac

Thanks guys.  Great spotting.  Come to Hobart and the beer is on me for both of you.

----------


## Hoff

> Do you mean you'd like to use long post supports in place of timber posts? If so, yeah, that will work fine, if you keep a couple of things in mind...... 
> Post supports will allow a lot more flex in the structure. This won't be a big problem for you as you're not building far off the ground but it's very important you brace the whole structure well so it can't rake sideways. Use the longest stirrups you can find so they're embedded as deeply as possible in the footings. 
> Also, don't use rapidset. This stuff is a handy alternative to full strength concrete mix but not suitable where you'll be using stirrups in place of timber posts. It will break apart in time due to the flexing of the stirrups. Take the extra time to use regular concrete mix and you'll be fine - it's available in premixed bags if you'd rather not buy a trailer load of gravel and you can easily mix it in a wheel barrow. 
> Using stirrups without timber posts is a bit of a cheat, you're cutting some corners but as long as you install them as above all will be fine and the building process will be much simpler due to being able to pour your concrete with the stirrups already attached and in place.  
> Regards 
> Damien

  Great thread.  I'm going through this planning process myself right now. 
Just a question on the posts.  Do you recommend the post in ground as the best way to go over using the stirrup setup? 
Thanks.

----------


## dcwalker

Hoff 
It'd be best if next time you start a new thread to ask your question so a new topic doesn't get lost in this discussion. 
Using post supports has both pros and cons. Among the benefits is the fact that your timber is clear of the ground which means it stays drier and lasts much longer before rotting. Also, if the post ever needs to be replaced it's easily unbolted. 
The downside of a post support is the amount of flex it allows in the structure. The leg of the stirrup bends quite easily, especially when at the end of a long piece of timber. Pergola posts or tall deck posts already have more flex than we'd prefer and a stirrup only introduces more so we have to engineer additional bracing to minimise rake. When your post goes straight into the ground the structure has inherently more stiffness. 
.....however, if you put your post directly into the footing you will probably need to treat the in-ground portion with additional preservative. Timber installed into the ground needs to be treated to H5 so hardwood, oregon, macrocarpa, even treated pine all need to have a product like creosote or one of the paint-on copper products added to the base of the post before going into the ground. You can buy treated pine that has been treated to H5 but mostly it's H3 so you need to specifically ask your supplier - there are also labels on treated timber that state the H level. 
So, which one to use? I prefer to go without a stirrup for the reasons listed above unless the job requires the benefits that come with their use. Not exactly a straight answer, but it's better than "it depends", right?! 
Either way, make sure you shape your wet concrete so it slopes away from the post/stirrup so water runs off rather than pooling at the base. 
Regards 
Damien

----------


## Hoff

Thanks Damien, I appreciate your advice.

----------


## TazManiac

I have re-done the 3D design for the deck and have priced my timber at a few local suppliers.  It is going to cost a lot less than when I started thanks to the help that I have gotten from here.  Thanks again. 
Heres what I have ended up with:     
I cant express how grateful I am for all the help received here.  I will keep everyone up to date with pics of the process and finished product. 
Aaron

----------

