How to build large deck on sloping ground?

[Minyarra]

Hello

I am looking to build a deck at the front of our home which is a converted steel shed on concrete slab. The deck is quite large 12m x 5m overall and it sits under a pergola that has just been completed. (see attached drawings and images.)The pergola posts (150x150) are shown in pink. the dimension pointed to the posts is the height of deck from ground level. 

I am concerned mainly about the deck's height from the ground which at the lowest point would only be about 160mm and at its highest point 885mm (this is fine; I will do steps at this section).

The lowest point, being 160mm from ground to top of deck leaves only 140mm for bearers and joists, with the bearer sitting on the ground (using a 140mm bearer) and the joists flush to the top of the bearer. I expect that I need the bearer to be a minimum of 75mm from the ground.

Is that correct? 

What should I do to make this workable? 

Is a 140x45mm H3 timber bearer suitable? or should I go to a steel purlin/box section?  

Should I dig it all out to increase the space underneath? how much?

about the number of stumps 

[Nailbag]

Hi @Minyarra 

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

Nailbag

[Minyarra]

Thanks @MitchellMc @Nailbag @Dave-1 

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

[Dave-1]

 @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

[Minyarra]

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.

[Minyarra]

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 

[Nailbag]

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



 

[JacobZ]

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

[Dave-1]

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

[Minyarra]

@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?  

[Minyarra]

thanks @JacobZ Jacob; very helpful