# Forum Home Renovation Decking  questions on bitumen, joist and bearer spacing

## SilverSniper

Hi all, 
Just building an ironbark deck at the moment, the main part of which is 5.6m wide and just over 6m long. I read that you can go to 500mm joist spacing for hardwoods. I originally planned on using 500mm, but have chickened out due to concerns about springyness. The ~6m width translates into 13 joists spaced at 465mm. These seems like a good compromise but joist widths are normally expressed as 450mm or 500mm - is there a reason is use 50mm increments? Will placing joists at 465mm mean I have to spend ages calculating my board lenghts? (as you can tell I've never laid decking boards before). 
Due to building the deck around an exiting pergola, my joist spans vary. The longest is 1610mm (continuous span). This seems ok given span tables for 90 x 45mm pine, but will this wide span impact on the joist spacings discussed above? Does it mean I should stick to 450mm just to be safe? 
Finally, I was intending to coat the top of all my TP joists and bearers with bitumen paint to increase their waterproofness. Is this a good idea? It doesn't seem to get much of a mention on these forums so I'm wondering about its effectiveness. 
thanks 
Tom

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## Bloss

Joist spacing on decks for 90x45 pine should be 400 not 450 if using 19mm decking. 450mm if 22mm decking. In any case why try to get the maximum spacing when closing the gaps costs no more than one or two extra joists so a bit more nailing and makes a much, much better deck 'feel'. To calculate your decking needs plug in the info here:   DIY Decking Calculator - Stumps, Bearers, Joists and Boards - Metric  
Top of joists you are better to use the joist protector or malthoid strips - or simply whack a couple of coast of external acrylic paint on . . Canberra is generally a moderate rainfall area and rainfall patterns allows decks to dry out.

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## SilverSniper

thanks Bloss, I think I forgot a dimension being the decking boards are 86 x 19mm ironbark. 
So you really think drop the joist spacing down to 400mm? not even 450mm?

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## ringtail

Yes you can go 500 with hardwood but its rarely done these days. Ive had to space the odd joist at 500 to get around a problem but 450 is a much better bet. The 6 mt width is 13 joists as you say but you need 1 to close so its 14 you need. Or you could go with 14  + 1 to close and space at 428 mm - this would be my option. You only want to build it once. High set or low set ? Your board lengths wont be a issue, just cut them to suit. As long as the span is fine in the span tables you will be fine although I always go up to the next size to reduce bounce and give a safety margin. Joist protection has been done to death and you have 4 options. Good old fashoined oil based primer, rubber joist protectors, ormanoid bitumen paint or malthoid strip. The last is the most cost effective and arguably the best option but all will give some degree of protection with the last 3 giving considerabely more protection than oil based primer.

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## SilverSniper

Thanks Ringtail, sounds good. It sounds like I was right to post here and check and not rely on the span tables!  
I'm not sure about your '+1 to close' measurement. The width is about 6070mm so isn't it best to just divide that evenly giving  14 joists at 433mm? Maybe I'm missing something. 
The deck is about 600mm high. 
Thanks also re waterproofing. I already had a bucket of bitumen paint from another job and started painting it on my stumps, then thought why not use this all over everything? Its time consuming and messy tho so I'll investigate the cost of the other options you've listed.

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## ringtail

You need to + 1 to include the first joist which isn't taken into account when you divide. Draw a line and pretend its 10 mt long. Divide into 10 equal spacings of 1 mt. You will have 10 marks at 1 mt , 2 mt ,3 mt etc....10 mt. These are your joists. But you need to + 1 for the joist that sits at 0 mt.

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## SilverSniper

legend! I never would have thought of that

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## ringtail

Many a deck builder ( including me) has been caught out  :Biggrin:

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## Bloss

Or just use Blocklayers tool! DIY Decking Calculator - Stumps, Bearers, Joists and Boards - Metric

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