Decking Subframe Sloping Site

21 Nov 2015
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United Kingdom
Hi all

Just starting to plan a new bit of raised decking at the bottom of my sloping garden.

The slope is such that at the bottom of the garden, the deck will be around 5ft off the ground with the top end level with the ground (not connected to the house though).

I am after some advice for post spacing and joist spacing please. The deck will be around 6.5m wide by 3.5m deep. It is the 3.5m dimension that will span the slope. I was going to have a double bearer at the bottom end to allow the joists to overhang at that end.

1. What support post sizes and spacing would you use?

2. Would you concrete these in (how deep?) or make a concrete base and surface mount?

3. What joist size and spacing would you use?

I've got some ideas from the attached pictures but any advice welcome for this planning out stage.



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I've asked for this to be moved.

Baring in mind the bottom end will be about 5ft high, which one of these will be best:

1. 100x100mm posts at 800mm centres concreted in 600mm deep. Joists then at 400mm centres.


2. As above but posts surface mounted on a 400x400x400mm concrete base using post holders.

Ive used post holders for a fence before, admittedly the rubbish brown ones from b&q, and it wobbled like a fat lass.
I'd go with option 2 but use the type of fixing where the post is held up off the ground. This way it is unlikely to rot and will be easy to replace if it does.
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You should definately concrete the posts in. It will provide much needed restraint against the whole thing racking. Ideally a couple of braces between the tall posts too, nothing fancy just 4x2 or similar. Depth will depend on the steepness and majorly on the stability of the slope, bear in mind you should measure post depth from the low edge on a slope. In good firm clay 450 would probably do but a loose sandy soil you should go at least 600mm. If the slope is not steep then 450mm will do.

Personally in this scenario i always use concrete posts for the bottom ones or timber posts bolted to a concrete godfather. At least then it will never rot.... Timber posts detailed well would be ok (as in bring the concrete up to the surface and bench it to shed water away)

I would use 6x2 joists at 450 centres. Posts at 1m centres would be fine.
Does this wood look pressure treated for external use to you?


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hard to know in a dark photo like that. Although very rarely do i see grading stamps on treated timber but that may just be a regional thing. You can get vac vac treated stuff which is treated for use in sub floors etc indoors which is a yellowish colour rather than greenish and it would be graded.
Subframe finally built.

Im after some advice regarding fitting the newel posts to the sub....

Do these really need to be notched to the subframe? What are the advantages to this rather than just fitting with a few coach bolts?

Seems a lot of work, whats the best method?
They dont have to be notched. They are usually notched so that they are brought into the desired edge line rather than having them inside the joists and therefore inside the edge of the deck.

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