Joining old and new foundations

Hi all,

I’m taking down an old conservatory and extending it out to create a larger single storey extension. I’ve got some 25mm x 600mm steel bar to tie the new footings into the existing foundations and had a few questions. What size drill bit do I need? Is 26mm okay or do I need it to be slightly bigger (say 28mm) for the epoxy resin. I don’t want to buy an expensive resin gun to use it once either and I know you can buy some epoxy resins in a syringe that pre-mixes it. Any idea on which one’s best? Thanks

The ties are not required. Neither is resin, as any potential movement will be vertical not horizontal

Hi Woody,
Thanks for taking the time to respond. I did wonder this but saw that someone else had posted something similar a few months back on a different forum. The person responding to that question had said that you shouldn't just butt the old and new foundations together as the older foundations will have fully settled and the new ones will still move (I would hope not much!) and that they should be tied in. Wouldn't the horizontal ties reduce reduce movement? Thanks again.

Souljim said:

Hi Woody,
Thanks for taking the time to respond. I did wonder this but saw that someone else had posted something similar a few months back on a different forum. The person responding to that question had said that you shouldn't just butt the old and new foundations together as the older foundations will have fully settled and the new ones will still move (I would hope not much!) and that they should be tied in. Wouldn't the horizontal ties reduce reduce movement? Thanks again.

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A metal tie is going to do nothing again several tonnes of downforce. Although foundations shouldn't settle anyway

Souljim said:

Thanks for taking the time to respond. I did wonder this but saw that someone else had posted something similar a few months back on a different forum. The person responding to that question had said that you shouldn't just butt the old and new foundations together as the older foundations will have fully settled and the new ones will still move

Click to expand...

If that was the case, then every single new extension would be tied into the existing house foundations ... and yet they are not, and there is no need that they are. There may be some limited times and specific situations when foundations may warrant being tied together - underpinning bays come to mind.

It's a case from that person, and others who promote this idea, that they don't actually know about building or foundation design/concepts.

I'd be more concerned as to whether your existing conservatory foundations are suitable for an extension!