Building new Deck

25 Jun 2007
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United Kingdom
I want to build a deck over an existing patio and then extending further into the garden. It will be a simple 4m x 4m deck that is raised less than 50 cm from the ground. The question I have is that I don't want to pull up the existing patio but there is limited space between the existing patio and where the deck boards will be - 10cm at most. It is possible to lay the joists on the patio and then the deck boards on top but I am afraid that the joists will then start to rot because they are in contact with the patio and therefore water will pool against them. I was therefore thinking on putting a spacer between the joist and the patio which will allow water to run off.
Does this sound feasible?
What is the minimum height that the space should be?
Any good thoughts on what to use as a spacer?
Does it matter that the joist height will be greatly reduced? I am assuming that they are usually a certain height to support the weight of the deck. With the joists now supported on the patio is it ok that the height is less?
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The joists should be a minimum of 100mm deep but if laying directly on concrete/patio then they will be supported their full length anyway, so you can use 2x2. So you could use 2" thick paving slabs atop your existing patio under where the 2x2 bearers are.

A much easier idea is to use 100m joists on existing patio, pulling up and removing any slabs which aren't in contact with the joists, these areas you then cover with a weed membrane and gravel over, this becomes your drainage. So you won't have to pull up all the patio.
In any case sit the bearers on a layer of damp proof course (DPC) and (obvious) use pressure treated timber.

If the patio was laid properly it should have a 'fall' so that water runs away from the house, as long as any timber on ground runs in the same direction as the fall then you shouldn't have a problem.

Or. Just sit 10mm joists on some roofing tiles ;)
anything less than 4 x 2 and you will struggle to stop the deck bouncing. The problem with laying the entire length of timber over the patio is that there may be some undulation in both the timber or the patio. We normally build a framework off of the walls, using rawlbolts and then use small blocks of 4 x 2 as feet to lift the frame just off of the the floor. Basically a very very short post! the best way to do it is to raise the deck just a samll amount before you secure the posts in position,that way the deck will force them down onto the concrete and keep it all firm.

Whatever we build a deck over we always use a black memebrane over the frame. Firtsly it gives a good appearance to the deck and stops anything falling through, such as rings or scraps of food, which can encourage vermin. Secondly weeds such as bindweed etc can still find their way around and over membrane laid onto the floor and then up through the deck.

As for drainage if you build it as above, and your concious of where the fall on the patio is then there wont be a problem.
Whatever we build a deck over we always use a black memebrane over the frame. Firtsly it gives a good appearance to the deck and stops anything falling through, such as rings or scraps of food, which can encourage vermin

Interesting, Not heard this approach. Doe this cause any problems with drainage ? (I know membranes allow water through but they are not a sieve!)

Thermo - a question - what span do you allow on your 100x47 mm joists between post supported joists ?
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no doesnt cause any problems whatso ever. Seen so many decks where you can see the framework or weeds growing up through it that you can do nothing about

as for span that very much depends on the frame and how its supported. We normally allow about 1.5m at the most, but it does depend on how its constructed etc. We always do a bounce test over all the framework before decking it to make sure nothing moves
This might be a slight hijack but just after some advice on building a frame.

The deck I will be building is a thinnish area out the side of the house. At it's widest it's less than 4m, narrowing to less than 2m, over an 8m length.

I want to lay the deckboards across the gap - I know it's a lot more cuts but it should look better and also give an illusion of it being a bit wider.

What's the best plan to build a frame for this sort of shape (like a blunt triangle!)? Make it in many sections? The problem I forsee is that only about 5 or 6 of the beams ill go the full length and then they will have to get progressively shorter.

Make up the shape from rectangular frames where poss, then create the odd shapes separately, and bolt them to the main frame. Plenty of support will be needed.
secure any timbers to the walls that you can with rawlbolts. if you have walls each side youre laughing! It doesnt matter what shape the deck is the pricipal for the frame is the same. Build the ouitside perimeter of it to the shape you want then span across it in which ever direction with the joists. Youll need a cross meber half way as you wont get a length of timber to go the full 8m. just cut the ones that will meet the outside frame at an angle, on the mitre. That will give you a bigger fixing surface as well.

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