DIY Doctor

Building Decking on Unused Extension Foundations

Postby bacar » Sat Jun 30, 2018 12:02 pm


The previous owner of my property was planning a rear extension and got as far as building the lower courses of blockwork but no further. This is adjacent to some existing decking.

I have no intention to build the extension and would quite like to extend the decked area to where the extension would have been. Rather than demolish the blockwork and build a wooden frame, I figured I should be able to use these existing foundations to support the new decking/joists for a lot less effort & expense.

The blockwork are double skinned, some kind of concrete blockwork outside and on the side near the garden, 3 courses of bricks on the inner skin (for some reason). The height of this brick/blockwork is pretty much the height I'd like the top of any deck joists /decking to be.

Can anyone offer any advice? In particular:
    - Is this generally a good / bad idea for any particular reason? Would a blockwork frame (rather than timber) throw up random problems? For instance, I can screw the deckboards to the joists, but around the edges it wouldn't be so simple to attach deckboards to the blockwork. How should I secure that?
    - How should I mount the joists? I figure I could remove some of the inner skin of bricks then rest the timber joists on the blockwork underneath. What should I pad it out with underneath to ensure it's the right height? Do I need to put in a damp proof layer? How do I ensure the joists stay vertical? Am I better off using hangers to attach to the inside of the blockwork instead, if so what hangers attach to blockwork?
    - The top of the brickwork is the same as the height of the top of the existing adjacent decking. This means the new area would be higher - by the thickness of one deckboard. That seems like a trip hazard (not a visible as a full size step). Since the old deckboards are badly cut/laid and a bit knackered anyway, I'm thinking of overboarding the whole lot with new boards anyway to bring it all to the same height, unless that is ill advised?

Will the whole project be actually a lot simpler if I remove a course of blockwork all the way around and build a rectangular wooden frame that rests on the blockwork underneath?
Some pics for context:

1 & 2. Extension foundations, partially covered with some scaff planks. You can see the double skin.



3. Detail of the block/brick



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Simply Build It

Postby ronan01 » Fri Jul 13, 2018 10:20 am

While When spacing your joist structure, you should keep the gap between joists (from centre to centre) no longer than 400mm. Simple spacer acts as an effective guide and a 2mm fall in the gradient of the joist in every metre is enough to see the rainwater runs away and stops any pooling, and low maintenance required later. attach the boards down safely with two fixings per joist. Screws will give more sage fixing than nails. While joining two cut deck boards together, fix with screws 15mm from the end of each board, at an angle to make sure safe fixing into the joists below.
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