Extension Blockwork Issues

[Gillesa]

I have had a builder in to help with the blockwork for an extension I am building, but it hasn't gone to plan. The outer leaf has been built to a height of about 2m, and I have since added the 90mm PIR insulation, ready for him to build up the inner leaf.

The cavity at DPC level is only 90mm in places instead of the intended 100mm, so there is no residual cavity for the insulation. The outer leaf actually bows in slightly too, so if you run a spirit level down from the point where the wall protrudes most, it means that the first block would overhang the existing blocks (below DPC) by about 20mm.

The builder was due to continue with the inner leaf today but turned up this morning only to collect his tools and leave without saying anything. He has left me to deal with the problem, so I am just going to finish the rest myself.

My question is, how should I deal with the slight bow in the outer leaf? I could offset the first block by 20mm and perhaps fill the void underneath with concrete, or I could build it following the curve of the insulation and correct it with plasterboard (though would this make the wall potentially liable to buckle?) Alternatively I could trim the back of the insulation down so that it sits flat and level, though I may have to try and reattach the foil, and maybe not tell building control.

Any advice would be much appreciated!

 

[Koolpc]

Knock it down. Start again and concentrate on getting it right.

 

[robinbanks]

20mm here or there isn't ideal, but I've seen a lot worse. It wouldn't appear to be the biggest of your problems though. You seem to be saying you have partial fill (are you using a product that permits 10mm gap?) but you also say you've constructed the outer leaf.. If you're partial filling you build up the inner leaf, fix insulation against using ties and clips, tape the joints and then build the outer leaf up..

 

[Gillesa]

It's Celotex thermaclass 21, which is a full fill board, but allows for a 10mm residual cavity, presumably to allow for mortar snots so that you can fit it in.

The builder was keen to build up the outer leaf before the insulation arrived, so the plan was to build the inner leaf afterwards.

I don't think the joints need taping, as the boards are tongue and groove, but I think I probably will tape the exposed side before building the inner leaf.

 

[^woody^]

The outer leaf is mainly cosmetic.

20mm is a lot for the internal leaf on a dpc, with no extra ties.

Can the inner wall below dpc be rebuilt?

 

[Ivor Windybottom]

Knock it down. Start again and concentrate on getting it right.

I'd agree. You've got two issues - that the leafs have been built in the wrong order, and that it's bent.

Did the builder own a spirit level or string line?

Big hammer, start again. Otherwise you'll still be looking at it in ten years thinking that's what you should have done.

 

[Gillesa]

I can't really rebuild below the DPC because the floor slab has been poured up to it. The floor insulation is going on top of the slab. Would it be a stupid idea to add a sliver of block or concrete next to the existing inner DPC block? It would however mean that it was sat on the 100mm floor slab instead of transferring the weight to the foundations, but it would only be supporting the 20mm overhang.

Even if I started again, the blocks up to DPC are too close together and I can't do much about it. This would all be a lot easier if the insulation were thinner, but that might cause problems with building control.

 

[Gillesa]

I'd agree. You've got two issues - that the leafs have been built in the wrong order, and that it's bent.

Did the builder own a spirit level or string line?

Big hammer, start again. Otherwise you'll still be looking at it in ten years thinking that's what you should have done.

Yes, I do get what you mean, although the outside is to be clad and it is up against the neighbouring property so I will never see it. I just want it to be structurally sound, and this all comes down to getting the inner leaf right as it'll be supporting the roof.

It's so frustrating because this is the only part of the entire build that I was planning on not doing myself and yet it has ended up being much worse than I could have done!

 

[Ivor Windybottom]

Yes, I do get what you mean, although the outside is to be clad and it is up against the neighbouring property so I will never see it. I just want it to be structurally sound, and this all comes down to getting the inner leaf right as it'll be supporting the roof.

It's so frustrating because this is the only part of the entire build that I was planning on not doing myself and yet it has ended up being much worse than I could have done!

If it's out of sight then I agree that a workaround is probably best, if it's possible.

I think I'd slim down the insulation if that's possible. Perhaps just buy the next thinnest size, if there's one that's only a bit less.

You could tackle it head-on with building control, just ask them what to do?

 

[jono_h]

The foundations should be wider than the wall, by approx. 150mm either side. If the edge of the slab rests on that projection then you’d still be transmitting the load directly into the foundation.

 

[Ivor Windybottom]

There's likely to be a layer of compacted stone between the slab and foundation. It should be pretty firm, but not immune to movement. If you extend the floor up then any settlement may cause a crack higher up the wall instead of between floor and wall, which is usually out of sight behind a skirting.

 

[Gillesa]

That's a good point from both of you, there is a pretty deep layer of hardcore on top of the projecting foundation, then 100mm of concrete. I would think this is very unlikely to move, but it would be better if I can just slim down the insulation and keep it on the existing block.

Unfortunately the Celotex I have is already the slimmest profile, so unless I use this elsewhere and buy some different, slimmer boards, then I would have to try and slice a section off the back where needed and glue the foil back on..

 

[robinbanks]

No point glueing the foil back on if it's going to be in contact with a surface

 

[Gillesa]

I understand that there will be no heat radiation to prevent if there is no cavity, but would it be worth retaining the foil for vapour barrier purposes?

 

[johnny2007]

Couldn't you simply use rockwool insulation?