I need to work out what to sit my decking on. I’ve had the timber delivered already and the deck is going to be at ground level.
It’s a 2.5m by 2.5m deck. I’ve got 150mm deep joists at 500mm centres and the decking boards are 145mm wide and 28mm thick. The ground it’s going onto is a mix of soil and grass. The soil has a lot of clay in it.

I didn’t fancy concreting in posts to support it as it’s at ground level and I’m also worried about the posts rotting in the ground.

I like to sit the deck on some simple padstones. I was thinking concrete blocks sat on MOT 1 sub base, block either mortared on or with sand under to level them up. The top of the blocks would sit slightly above ground level.

I’m not sure if that going to be good enough though and also not sure how many or what spacing of blocks to use?

Anyone have any other suggestions?

Looking into this too at the moment and had decided to go the padstone route. Only a small bit of decking in my case and one edge will overlap and sit on an existing patio, but padstones for the other three sides and maybe one or two in the middle.

Sounds pretty similar to me wobbliscott

There are some dedicated support blocks you can buy with slots for the joists. I guess these would be expensive, although I don’t need that many

We had the cross shaped padstones under ours, eventually they became the weakness as moisture was trapped between the joist and the padstone and caused them to rot, but then they were probably decades old, the whole deck was pretty rotten. they were just the first bits to actually go.

I’m going to re-use them under the new shed where they’ll be out of the rain.

On one of the earlier threads someone linked to these metal spikes that you screw into the lawn and they have a pad that you can adjust to get the level.

One thing to watch- the sister in law has recently had to remove some decking that was just a board’s width above the ground as the frame had rotted. Deck boards were all ok, just the support. As it was enclosed around the edge they think it was a lack of ventilation to the frame that meant it rarely dried out. I’m not aware how it was sited on any footings or just placed on the lawn, or even if it was treated timber or not.

We had the cross shaped padstones under ours, eventually they became the weakness as moisture was trapped between the joist and the padstone and caused them to rot

It lasting a decade would be fine!

Though, with the frame design I have the timber for the cross shaped blocks don’t really fit in that well. I’d want them to sit “in board” and not be visible outside of the deck ideally

Deckin’ Blocks

https://www.wrekinconcreteproducts.co.uk/product-category/deckin-blocks/

Creating your new decking area is easy –

– Lay out DECKIN BLOCK at 1M centres, to form a grid
– Mark around each block using line spray or lawn edger, set them aside
– Remove grass and weeds to approx 50mm depth and fill hole with dry post mix
– Re-position DECKIN BLOCKS on prepared areas, using your first block as your datum

I could just do that with normal concrete blocks

I got rid of a crappy patio, dug out about 6″ all over of the base and then relayed a matrix of slabs on sand on which the base then sits. I can’t remember how many I used, but quite a few (deck is 5x3m and I think it was 4×3 for the slabs, which being I think 60cm slabs meant the spacing is only 40-60cm of floating rafter. May be overkill but any slabs I didn’t use I had to get rid of so the more the better!)

The rafters sit on DPM on the slabs and have been there for 15 years now, the boards are starting to rot in a couple of place but the deck is solid as ever.

Contemplating lifting the decking and replacing with composite boards, hoping the raft below is still sound.

I did similar a couple of years ago – some up-cycled old paving slabs, a few old bits of tiles and slate (plus some freebie paving sets from a local building supplier which had come from split packs and were ready to be binned)….

Almost finished…

There are some dedicated support blocks you can buy with slots for the joists. I guess these would be expensive, although I don’t need that many

Saw those and considered them too, but seemed to me that you would have to get all the mounts set to exactly the right height and level which seemed too much like hard and fiddly work to me. By having separate padstones as long as they level and flat it doesn’t matter too much if they’re at slightly different heights as you can set the individual feet to the length they need to be to sit on the padstone properly and share the load evenly across all feet.

Only thing I need to suss out is a large concrete block set under the deck with something protruding up through the deck so I can lock/chain my BBQ to it.

Dig down circa 200-300mm, fine gravel bed to lay 300×300 concrete slabs on. Double/triple up the slabs as necessary. Either cement the slabs together or go down the dry route of screwing them together to make a solid block.

Don’t rest your timbers directly on the slabs, use plastic spacers to raise the timbers to slow down the rot.

johndoh… That is quite a slab and it looks like quite large spacing between the joists. Is it not springy?

Dig down circa 200-300mm, fine gravel bed to lay 300×300 concrete slabs on.

The step that precedes this is standing in B&Q and seeing that theres no sand, gravel or slabs left at the moment 🙂

I’m just in the process of putting some slabs down for a shed base – managed just to get the slabs I need there was only one size/shape/colour that had more than a handful left. Already had most of the sand required getting – getting another 12 bags required grabbing the last few bags on the shelf in different towns 🙂

johndoh… That is quite a slab and it looks like quite large spacing between the joists. Is it not springy?

Absolutely not – no bounce at all apart from a tiny bit on the lip of the step.

Don’t rest your timbers directly on the slabs, use plastic spacers to raise the timbers to slow down the rot.

I was thinking DPC membrane?

The step that precedes this is standing in B&Q and seeing that theres no sand, gravel or slabs left at the moment 🙂

Will be doing a bulk delivery as we’re still shielding for Coronavirus. Hopefully I can get the materials!

Or a chap on YouTube just dug 300x300x300 holes and filled with concrete and sat his decking on that. I was going to do that but my ground is so pebbly and rocky it’d be a right PITA to dig down so going for levelling the ground, digging down a bit and setting on padstones.

I was going for DPC to sit the feet on. Just need to find some offcuts from somewhere rather than buying a whole roll. Plenty of new housing developments kicking off near me so might politely ask a site manager or brickie or something for some waste.

The advantage of using slabs (sds drill and screw then together) over pouring concrete is that it’s quicker, cleaner and there’s no waiting around for the concrete to set.

You want the smallest contract patch possible between joist and base. If the joists are sitting in water they will rot sooner rather than later.

I’ve still not worked out how to do this and am going around in circles on it.

The deck is 2.5m by 2.5m, 500mm joist spacing so 6 joists crossways, plus one at either end lengthways. The simplest solution I can come up with is 3×3 rows of 600mm sq. slabs laying two crossways joists on each. The distance between each row of slabs would be 300mm and that would be the max. unsupported joist span. The only down side of this is the padstones are quite large and therefore the joist contact area will be large.

Another option is some 215mm x 440mm x 100mm concrete blocks halved and placed at x spacing. Not sure what the spacing would be… I’d sit them on some MOT Type 1 and mortar or just sand to level up

Slept on it, going with the 9 large spaced out slabs to create the padstones. I may look at plastic spacers to raise the joists off the slabs though. The disc type ones look good as they’re adjustable. The only issue is using these at the edges, the discs would be visible. I think some plastic wedges would be better and cheaper.