Building decking on unused extension foundations

[Baris]

Hi,

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 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



Thanks!

Baris

 

[Notch7]

Looks like it will fill up with water

 

[Baris]

Not sure why that would happen. I’m not tanking it or anything! Any rainwater should just soak away into the ground like it does the rest of the garden, no? And it’s going to see a bit less rain than the rest of the garden because of the boards — but I can see that it’s worth putting some ventilation in at least.

Put it this way — I don’t think I fills with water now and it’s been like that for ages. But I admit I haven’t really looked under the scaff planks much (is currently full of junk that need clearing out).

Or look at it a third way - I don't think the void under any other normal decking needs to 'run off' to <some other place> in order for it to avoid pooling, does it? It just soaks downwards into the ground or evaporates off like it does anywhere else in a garden.

Happy to be put right if I'm talking rubbish!

 

[bobasd]

what you suggest is doable but ther are a few things to think about.
for the time being why not work with what youve got, the boards and scaff and see how you get on?
you could attach trim and/or surface the boards to clean them up.

remove all the boards and clear all rubbish and foliage from below.
lay a membrane over the ground to stop stuff growing - dont worry about pooling, it will find its own way to drain off. but lay a few bricks on the membrane.

you might get splash on the walls that butt to the deck.splash could lead to water penetration through the walls.
check insidethe house to look for any damp signs.
take care of the down spout .remove it if redundant or run it to drainage.
have access to any gulley under the deck.

 

[^woody^]

Build frame on top. If it's too high, knock a course off throw them in the hole and just build your frame. I don't understand why the long post, or what the issues are.

 

[Baris]

Yeah tbh I am increasingly inclined to go down that route - remove a course and build timber frame. I won’t need posts as it can sit on the blocks so I’m still saving myself a fair bit of bother.

What the best way to remove a course of blocks, I’ve never done this. Can I just knock them out with a club hammer or something, or do I need to hire a big angle grinder and cut the mortar?

 

[^woody^]

Knock one out, then lever the others up at the bed joint with hammer and chisel or crow bar, or tap the top unconnected corner sideways. Start at an end or corner.

Clean up the bed joint with a hammer and bolster chisel.

 

[freddiemercurystwin]

Knock one out

Clean up the bed

This is a family friendly forum you know.

 

[^woody^]