How to build large deck on sloping ground?

https://www.workshop.bunnings.com.au/discussion/comment/59873#Comment_59873

Thanks Mitchell,

I will get some excavation done to that side to get the clearance i need.

How does it look for the rest - enough (or too many) stumps?

is 140x45 bearers enough for rigidity?

Thanks again

EricL

Hello @Minyarra

To help with the protection of your bearers from heavy moisture, I suggest painting the bottom section of the bearer and a bit of the side with Gripset Betta 1L Waterproofing Membrane Bitumen Rubber. I recommend covering the top of the frame with CAgroup 50mm x 20m Bearer Protector G-Tape. This will protect it from sitting water on the surface. I'm a bit of a conservative when it comes to post spacing, I propose pulling the posts back to 1400 spacing and adding another row on both sides of the decking frame. The 140 x 50 should be strong enough as bearers.

Let me tag our experienced members @Nailbag, @Dave-1 and @AlanM52 for their recommendations.

If you need further assistance, please let us know.

Eric

https://www.workshop.bunnings.com.au/discussion/comment/92389#Comment_92389

Definitely looks over-engineered @Minyarra Check out this link to span tables which is just one of many similar resources.

Nailbag

AlanM52

Hi @Minyarra,

Maybe Concrete Foundation Blocks.

Concrete Foundation Block.jpg

Which should reduce the amount of excavation.

DEKO Block2.jpg

If you bury that @ 75mm maybe you can fit in the 140 x 45 bearer.

Buy a few and have a fiddle with them.

For that space in that area I am a bit concerned you are using post stirrups.

Do you intend to put a roof on that and if that's the case ignore the above.

If not, I recommend set those posts in concrete.

Cheers

https://www.workshop.bunnings.com.au/discussion/comment/59901#Comment_59901

thanks for that suggestion @AlanM52 I have used these elsewhere where the ground was stable and without the 'stormi-ness' of this current build. I was going to dig down 600mm for concrete stumps and either use stirrups or else, cypress post straight into the concrete (with bitumen paint seal)

There won't be a roof on the pergola

Is there a problem using stirrups?

https://www.workshop.bunnings.com.au/discussion/comment/59898#Comment_59898

thanks @EricL

I am going to get the ground excavated so that we have 250mm below the boards (minimum). This will give the 140mm bearers better ventilation and ensure that water runs through easily.

@Nailbag - I will stick with @EricL 'conservative' approach given that we get huge winds here and lots of temperature variation (730m elevation) and I dont want to here this squeek or rattle.

I really appreciate the help that each of you are giving me.

https://www.workshop.bunnings.com.au/discussion/comment/92394#Comment_92394

Good morning @Minyarra

I would go for excavating out the low end as well, Having a decent airflow that will keep the timbers drier would be my aim 🙂 I also would go with @EricL's suggestion about spacing, I have walked on too many decks that feel mmmm spongy? so something that will be less likely to bounce.

Cant really add to @MitchellMc and @EricL's suggested course of action 🙂

I think it will be a beautiful deck when you are finished and would love to see some pics in a project.

Dave

https://www.workshop.bunnings.com.au/discussion/comment/59873#Comment_59873

thanks @MitchellMc

That is super advice - will take it on board and hiring an excavator this weekend.

https://www.workshop.bunnings.com.au/discussion/comment/60052#Comment_60052

hello @EricL @MitchellMc @Dave-1 @AlanM52 @Nailbag

I learnt a valuable lesson on the weekend - do the excavation work before you build the pergola - the good news - the operator only bumped it once and we got the ground down a further 200mm to improve spacing under the bearers.

My question for y'all, (the experts that will prevent me from making a total mess-up)

Which is better:

1. 100x100 cypress pine stumps connected to a concrete foundation (600mm deep) using stirrups, OR

2. 100x100 cypress pine sunk into the concrete foundations by 500mm (using a bitumen paint for additional protection)

Your help is greatly appreciated

excavator under pergola 2.jpg excavator under pergola 1.jpg

EricL

Hello @Minyarra

I prefer traditional methods, and your use of bitumen paint is right up there on my list of things to apply when using a timber post. I'm putting my vote on number two as it is straightforward and not difficult to level or trim when it needs to be adjusted. It would be interesting to see what the others will vote on to use for your deck.

Eric

https://www.workshop.bunnings.com.au/discussion/comment/60052#Comment_60052

Option 2 @Minyarra

Nailbag.

https://www.workshop.bunnings.com.au/discussion/comment/92488#Comment_92488

Thanks @EricL @Nailbag ,

I will go with option 2

Is there any time when stirrups are actually better?

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/112347#Comment_112347

They are the better option when you're using timber that can't be concreted in the ground, like H3-treated Pine, @Minyarra. Concreting a post into the ground is really the most secure solution.

Mitchell

https://www.workshop.bunnings.com.au/discussion/comment/123689#Comment_123689

Morning @Minyarra

I will go against the grain... 🙂

I prefer option 1, Stirrups in concrete mean I will have way less chance of the post rotting and I like knowing that the post is connected to a solid base.

🙂

Dave

https://www.workshop.bunnings.com.au/discussion/comment/112347#Comment_112347

Hi @Minyarra

It just comes down to the most suitable application and material being used for the job. I have replaced countless more posts in stirrups than direct in the ground.

Nailbag

hello @Nailbag @Dave-1 @MitchellMc @EricL @AlanM52

next question;

what is the best way to fix bearers to posts? and then, what is the best way to fix the joist to the bearer

I am using 100x100 cypress posts buried in the concrete (sorry @Dave-1 ) with 140x45 bearers and 90x45 joists.

option 1 - Is it satisfactory to 'hang' the bearer off the post (this is much easier) or

option 2 - cut the bearer into the post

Then, what is the best way to fix the joist to the bearers?

a) pryda joist straps (one each side) fixed by nail or by screws

b) 150mm bugle batten screw from the top

c) both

d) something else?

Thanks again

bearer joist setup.png

https://www.workshop.bunnings.com.au/discussion/comment/60108#Comment_60108

Good Afternoon @Minyarra

lol 🙂 No worries at all.

I have used and prefer option 2, but I only cut into the post by a max 1/3rd so it gives a base for the bearers to sit on taking some of the load without weaking the connection point too much.

Just went and checke dmy deck, I actually went half way through 😲 mmm I still prefer 30% of the posts width tho.
Pool deck stage 1 is my project, there is a part 2 and later on an update if you troll through my bookmarks 🙂

Dave

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/60108#Comment_60108

A rebated joint is the most secure method as the weight of the bearer sits directly on the post, whereas with the side attach method, the only connection is through the fixings @Minyarra.

For connecting the joists to the bearers, I'd go with a Pryda Joist Strap 162mm on each side, fixed with Pryda clouts.

Mitchell

https://www.workshop.bunnings.com.au/discussion/comment/60108#Comment_60108

Hi @Minyarra

I agree with exactly @MitchellMc suggestions here. Pretty much standard practice build.

Nailbag

https://www.workshop.bunnings.com.au/discussion/comment/92523#Comment_92523

Thanks @MitchellMc @Nailbag @Dave-1

super helpful. I thought the rebate would be best but wanted to reduce workload and complexity 😂😅

https://www.workshop.bunnings.com.au/discussion/comment/112374#Comment_112374

🙂 @Minyarra

I know that want, I try to ballance it with "a little bit extra is worth a factor of 3 to 4 down the track. You "know" you have done the hard yards and somehow it just feels better. 🙂

Dave

https://www.workshop.bunnings.com.au/discussion/comment/123706#Comment_123706

absolutely @Dave-1

not a job I want to come back to because i compromised

Thanks again y'all

@EricL @MitchellMc @Nailbag @AlanM52 @Dave-1

I will upload photos with commentary as I go.

Currently I am digging out the holes, of which there are 44# (I think) all of which must be dug out by hand because of the ground, which is rock-laden dry clay - literally using a straight bar and gloved hands - Auger wouldn't work because the rocks send it off-course (one rock was 65kg, but most are less than 10kg) and the post hole pincer shovel couldn't pick up the dry clay dust.

Hello @EricL @MitchellMc @Nailbag @Dave-1 @AlanM52

Im racing to the line to get at least half the deck 6mx5m finished by Christmas Day, and I need a little bit more help.

When joining the boards to the joist, I am concerned that, when there is a join, the screws of both boards have very little timber to screw into and in addition, are quite close to the end of the decking board (about 10-12mm). Is it better, when there is a join in the decking boards, to add an additional block to the joist so that each board has more to fix into (obviously the block must be well secured to the joist (or be a complete additional joist).

If you can assist with some options that would be appreciated.

I will send some photos through soon showing the progress

Thanks again connecting boards to joist.png

https://www.workshop.bunnings.com.au/discussion/comment/60712#Comment_60712

Hi @Minyarra

It's normal practice to drive the nails into the joists slightly skewed so, essentially option 1.

Assuming you're using a standard gauge decking screw, ensure the pilot hole is between 10mm-15mm back from the edge and driven in on a slight angle to get the bite you need into the 45mm joist. To be safe you could use the next 1/2mm up so there is no stress on the board. Depending on the screwed type of the screws your using will determine if you need to also countersink. I use a seperate countersink bit for ends over combo ones for end boards, but I drive that straight. And lastly avoid over-driving the screws here.

I would experiment with a few offcuts to work out best distance from the end, angle and countersink method.

Nailbag

Screenshot 2025-12-20 at 7.30.49 am.png

JacobZ

https://www.workshop.bunnings.com.au/discussion/comment/60712#Comment_60712

Hi @Minyarra,

As @Nailbag has already mentioned, it is normal for your screws to be driven in on a slight angle. With a 45mm wide joist, as long as the join is centred over the joist, you can set them back from the join by 15-20mm and drive them in at a slight angle, which will give them plenty of purchase in the joist.

Let me know if you have any further questions.

Jacob

https://www.workshop.bunnings.com.au/discussion/comment/92814#Comment_92814

Good Evening @Minyarra

I did my screws around 10mm -12mm ish from the ends of teh boards and I went straight, not at an angle (had never done a deck before) It does make sense what @Nailbag and @JacobZ have said about screwing at an angle but straight still works 🙂

Dave

https://www.workshop.bunnings.com.au/discussion/comment/92813#Comment_92813

@Nailbag

thanks for the help and the photo. It is really good advice. on your photo, it looks like the ends of the boards have been cut with a slight angle also; is that correct? if so, what angle are you running?

https://www.workshop.bunnings.com.au/discussion/comment/92814#Comment_92814

thanks @JacobZ Jacob; very helpful