Decking joist size & supports

[DIYrooky]

I am installing a decking size 5.4m X 4.2m aprox. I plan to attach the decking to the side of the house by a joist attached to the house. The ground gently slopes away rom the house and so I plan to support the other side of the decking by posts. The decking planks will run parallel with the house so the support joists will run at rightangles to the house.

The problem I have is what size of joist I can use?

I am restricted by the hight above ground I can go as the house door is just 27cm above ground level and there are airbricks 14cm above fround level. I do not want to cover the airbricks and so plan to put a joist just below these but it only leaves me enough space for a 10cm hight. The joists will be 4.2m long. Can I use 10cm joists (2X4's) as the instructions I have read recomend (2X6's). If I can use 2X4's what spacing do I need along the length for support? If not what is my alternative?

Sorry this is so long but I really need help!!

Thanks

[stoneyboy]

DIYrooky,
You'll have to fit an intermediate support halfway across the decking parallel to the house wall.
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[DIYrooky]

Thanks Stoneyboy, I will include that in my plan. I assume I can use a 2X4 as this intermediate support? Will it be Ok to support this with just 3 posts, one at either end and one in the middle? That would make them 2.7m appart. Once again thanks for your help.

[stoneyboy]

DIYrooky,
I would suggest you use 5 vertical posts for the intermediate support.
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[whiterabbit93307]

Personally I wouldn't use anything less than 6x2 for a raised deck, with additional support provided by bearer timbers.

As the deck would be open underneath, I can see little problem with running the ledger (the joist attached to the wall) up to either side of the airbricks and you could just leave a gap in the ledger, as your deck boards are running parallel to the house you wont need to affix the end of a deckboard into thin air .

Obviously you would have to plan your joists so they didn't happen to need to be fixed at the point of the airbrick gap.

You also need to leave a gap between the ledger and the brickwork of 10mm which can be achieved with galvanised square washers or other packing

Only time I use 4x2 is for a deck set flush with ground level, and I only ever use 38mm deck boards.