Protecting a new decking base?

27 Jun 2010
Reaction score
United Kingdom
I'm building a deck which is 7740mm x 2400mm outside the house. It will be freestanding (not touching) and I've made the base out of 200 x 50mm joists(all treated and Endseled where the cuts were made) sitting on 10 caithness stone pads(only 200mmsq each) bedded on concrete pads. It's effectively 4 bays 1935x2400mm.

2 bays have been made to this size but now I'm wondering whether to reduce the next bays by 9mm (1926mm) and put in some plastic 6mm spacers between the frames so that they are not hard bolted to each other, allowing ventilation. Or should I just keep to the same sizes and bolt them together, and cover any doubled up joists with some dpc/thin roofing felt prior to putting the deck on?

Where the joists sit also concerns me- I could put down felt but isn't there any form of spacer on the market that could be put below the joists to keep them off the pads enough to ventilate more effectively around them?
Would those plastic packers (1-6mm) joiners use to neaten up the second fix be strong enough to support the weight?
I've spaced the pads accurately enough so that I could overhang them with the frame just a few mm, as this would allow me to put the fascia deck boards down to almost ground level- which would look far better and prevent gravel disappearing under the deck over the years.

There seems to be a lot of development in composite boarding, hardwood boards etc but what's the point when everyone has to use good old bog standard softwood joists- these will always be the weakest link, so what if the deck itself will last a lifetime when the frame it's on will not.... :eek:

Can anyone advise a bombproof way of maximising the life of the joists?
Sponsored Links
It appears that there is no bomb proof way, judging by the lack of replies !

I wouldn't think there is really, it's wood after all, so all you can do is treat it as much as possible before you put the bards on, but leaving a gap in the boards as you do will allow air flow.

If you've used tanalised timber, traeted the cut edges and then you treat it again before you start to install the top boards, I think you've done all that you can.

I have a shed that is now 16 years old, I installed it on trated Fence posts as bearers, traeted them with cuprinol before I put the shed on them and they're fine after all these years.

You could leave gaps between the frames, but that might cause more of a moisture trap, perhaps better to bolt them togther tightly and put plenty of wood preserver onto the joint.

I certainly hope that those plastic spacers take the weight, because I've used them on 2 decks to help with levelling !

Hope this helps.

:D :D
Ta for the reply Arthur- the frame is down and I just opted to bolt the thing tightly. I've made it to the dimensions required for multiples of 149mm (52 x (143mm + 6mm spacer)-6mm) and it's bang on. Sure as sh*t the boards won't be bang on so some tweaking will be needed. Where the doubled up joists are I plan to put down some plastic DPC and staple it so the water(if a joint falls on it) will run off clearly. God knows how UV will affect this stuff though if it is open to sunlight.....(for 3.8365 days a year)

Hopefully it will be done in time for winter so I can sit outside admiring it without midges, clusterflys, houseflies, wasps, bees, cleggs, birchfly and ticks in the freezing cold, nursing a freezing cold beer wondering why I didn't spend the money on a motorbike instead!!! :mad:
Sponsored Links
Got it done finally...the spacer things were invaluable in the end, both for getting the levels perfect but also as spacers between the boards. The boards were pretty good width wise, but after putting down 2/3rds of them at a regular 6mm spacing I measured up and there was an 8mm difference in the remaining length at each side. The spacers can be mixed and taped together which is what I did and by using 7mm at one side every second board(with a 6mm alternating) and a 7mm on the other side at every spacing I could bring the boards perfectly to the edge of the frame...

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local