Wood decking over existing concrete slab patio

23 Nov 2009
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United Kingdom
Hi, I am about to install a wooden deck over my 2 (ugly but solid) concrete slab patios. I have enough info and help for this job, except can someone please advise the best way to support / lift the decking frame above the concrete slabs to prevent the frame from rotting due to water entrapment.

My current thinking is to use some bricks I have to support the 4by2 frame joists. However these are 2.5 inches high and would make the final height of my deck almost 8" high. I would prefer something lower than this to give me a deck height of approx. 6" high.

Hope you can help,

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Hi Stephen, Many thanks for your good suggestion. However, I see a couple of problems implementing it:

1. I have a large area to deck (700 sq. ft.). My current plan has approx. 250 brick supports with 2x4 joists at 18" spacing and bricks every 30" along each joist. So that would be a lot of cutting, drilling & bolting.
2. Also, would still have wood to patio slab contact, which I am hoping to avoid.

Cheers, Andy
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Hi Richard, Thanks for the 'heads-up'. Fortunately the decking will be < 30 cm high and will be much less than < 50% of my garden area. So we don't break either of the planning reg. rules.

Cheers, Andy
Three other conditions to watch which I just noticed are not included in the Planning Portal link I posted are;

Where the deck is situated within 20 metres of a highway.

If the structure would affect the amenity value or privacy of neighbouring properties.

If the deck is attached to a listed building or situated in a conservation area or National Park.
whats the actual dimensions of the deck in metres (l x w)?
Hi Thermo, Thanks for your interest.

One deck is 8.5 m x 3.8 m and the other is 5.0 m x 3.8 m. They sit either side of a conservatory.

youll be better off using 6 x 2 for the frame if you have the room, youll need less supports. if we do a deck over concrete we simply use offcuts of 6 x 2 or 4 x 2 as supports for the deck. if you are that concerend about damp then use some dpc under each leg, although its really not an issue as they will dry out provided there is sufficent airflow. if you use bricks thye will still draw up any water from the area when it rains.
Hi Thermo, If I understand you correctly, I could use either 4x2 or 6x2 joists straight onto the concrete patio slabs, without any supports. And I could choose either 4" or 6" joists, depending on what I want the final deck height to be. Please confirm.

Is a spacing of 18" between each joist OK? I am using 5"x1.25" decking boards.


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