Low level deck - What spacing for posts?

[pastolz]

Hi All,

I'm looking at building a low level 5200mm x 5200mm deck that will be more than 95% under cover of a pergola. It's an existing paved area, no concrete below. With the pavers removed and without levelling much of the ground, we'll have minimum 120mm clearance at the doorway against the house leading out to edge of pergola where maybe there will be up to 160mm clearance.

I'm planning on using 120mm x 45mm bearers as joists, 400mm spacing. The bearers/joists will span 5200mm. My question is, at what measurement would I support the bearers? I'll upload some photos of the area tonight with a rough sketch (if I can) of the deck plan... I'll use 90x90 posts as supports.

Look forward to any help!

Thanks!

Community manager's note: Check out How to build a low-level deck for expert advice.

[MitchellMc]

Welcome to the Workshop community @pastolz.

Let me tag one of our very knowledgeable community members @ProjectPete to see if he can assist us with an answer.

You might also find this instructional D.I.Y. step-by-step tutorial on How to build a deck of assistance in your project.

Mitchell

[pastolz]

Thanks Mitchell, I've had a look at that tutorial prior. Just having difficulty at understanding span tables that I find on the web. Hopefully I'll get some assistance. Thanks!

[ProjectPete]

Hi @pastolz 

Few things from me for your design...

• I suggest running 4x 140x45 bearers with ~1730mm centres.
• Then pop in 120x45 joists with the 400mm centres as you suggested but even 450mm centres would be fine depending on your decking material.
• This would require posts with 1800mm spacing. It's OK for the spacing to be less, even unequal, but should not exceed 1800mm.
• You need to address your clearance though as you really need a minimum of 200mm but ideally 250mm to allow adequate airflow to prevent decay of your timber due to moisture.
• Use your house brick wall as a ledger and remember to leave some spacing between the timber and brickwork so any moisture can escape.

Will be great to see what you come up with!

[pastolz]

Hi @ProjectPete 

Thanks so much for your reply! Just have some questions:

Are the 4x 140x45 bearers you mention used as a square frame around the perimeter for the deck? 

What decking material would you have to use 400mm centres for? We'll probably use something like merbau and possibly 140mm wide, not sure yet.

Could we just trench under the joists and near the bearers to gain more clearance? I could probably easily remove another 500mm over the entire area but it would be a low point for surrounding area.

I don't think I'm willing to drill into the wall to bolt on a ledger as it looks like a gas pipe runs in the wall in the render around where I'd fit the ledger. Will that be a problem? Alternatively I could bolt the decking frame to the pergola posts on the three sides. Would that be a solution if I don't use a ledger?

I've attached a photo of the area, hopefully that helps a little.

Thanks again for your help!

[ProjectPete]

No, the bearers would run the same direction as you 5200mm blue line in the pic above and be spaced ~1730mm centres. Your joist then run perpendicular between those meaning your decking will run in the same direction as your bearers giving you a continual flow from your back door.

Tip: Paint the tops and about 30mm down each side of your bearers black so they're not seen through the gaps in your decking. If you go with timber decking you'll need to also paint your joists black as you'll be top/secret fixing your boards. Composite decking usually won't require painting of joists as their clip system covers the joist.

140mm Merbau would be fine with 450mm centres so 400mm is even better structurally but will increase the amount work materials and labour involved.

No worries re the ledger, it's not necessary, just easier and reduces the number of posts, concrete and labour required.

You could just trench as you suggested, although not ideal, especially with timber decking. Plus you'll find that your trenches between 400mm centres for your joists will join anyway. 

Tip: Use H4 200x50mm sleepers for your outer frame (this mean you'd replace 2 of the 4 bearers mentioned above with the H4) as H4 can be in the ground therefor you don't need to worry about the clearance where your grass is, the grass can grow right up to it. Hope that makes sense.

[pastolz]

Hi @ProjectPete ,

Thanks again for your reply, that all makes sense, although I was going to run the decking the other way, parallel to the house.... for some reason that seems normal to me, but is it more common to run the decking outwards from the door for continual flow? 

I was also considering painting the bearers/joists with possibly some sort of bitumen/water proofing paint because of the low clearance, is that worthwhile?

I'll try and sketch up a plan today from what you've suggested and seek some feedback. 

Again greatly appreciated your help and thanks for the reply!

[ProjectPete]

Generally, the continual flow is the way to go but it also depends on furniture arrangement etc. And given yours is a square, it's not such an issue. Just remember your decking boards are laid in the same direction as your bearers.

You can waterproof the frame to assist with your lack of clearance but do consider the extra time/effort and cost involved. This is the product.

Having to take a look at your sketch with orientation and dimensions 😀

[pastolz]

@ProjectPete  I've tried to sketch something up based on the decking boards running perpendicular to the house, but I'll probably look at running them parallel and try and sketch something up for that, although, not really too different seeing it's basically a square deck!

We'll probably pave from the edge of the deck to the lawn about 800mm so there won't be lawn growing directly up to the deck.

I wasn't too sure where to place the first joist by the lawn end. The deck should extend past the pergola posts about 100mm, so in the sketch the first joist is kind of flush with the end of the bearers, I guess the joist could go either side of the pergola posts, what would be best or does it not matter?

The posts for the bearers start a bit in from the ends of the bearers and then they're spaced about 1600mm.

Joists are spaced 450mm.

Bearers spaced apart at 1730mm.

120 x 45mm joists - I guess they span a bit less than 1730mm, is that fine?

Bearers are 140 x 45mm. 

Is the sketch roughly what you were thinking? Please let me know what changes I should consider.

Thanks again for all your assistance!

[pastolz]

@ProjectPete If you have a chance to look at the sketch below, I'd love to receive feedback. I've worked out a 'wish list' on the bunnings site for all the materials for the sketch below, but just need some feedback first if ok. 

Thanks!