Cavity: To fill or not to fill (and what with)?

[oranjeboom]

Hi All,

 

I previously removed an exterior brick wall in order to extend lounge. Where my new slab extension meets the old bungalow cavity wall,  I need to decide what to do with the cavity that goes down to a depth of approx. 800mm to the footings before I lay down my new floor slab. The old bungalow floor has been dug up in order to install 300mm EPS with UFH in 100mm meshed concrete.

 

The attached pic shows the new extension to the right of the cavity with sand blinded hardcore floor of bungalow to the left. My options as I see it:

 

1)      Leave cavity as it is and just cover the cavity with 300mm EPS.

2)      Fill cavity (with gravel / concrete)

3)      Fill with EPS beads

 

I have plenty of beads at my disposal, so I am tempted to go with option 3.

 

TIA!

 

OB

 

[Mr Punter]

Why not get rid of the bricks for several courses and have the same build up as the floor on the left?

[oranjeboom]

3 minutes ago, Mr Punter said:

Why not get rid of the bricks for several courses and have the same build up as the floor on the left?

The slab to the right is a piled slab, so is higher than the footing. I need to keep the brickwork in situ (at least the outer leaf) to retain the compacted hardcore from spilling underneath the piled slab. Potentially I could remove the inner leaf brickwork (on the left) but the ground is all levelled and measured to the nearest 5mm.

[Ferdinand]

What is the problem if you do *not* fill it?

 

It will be under a lorra-lorra insulation, and I don't see much risk of eg rats moving in.

 

If worried I would do a lean mix.

F

 

 

[Ian]

@oranjeboom

won't you need a good solid support for whatever floor finish you put down over that area?

 

Edit: unless you are going to be putting carpet down you'll likely need to install a movement joint in the floor finish along the line between the 2 slabs. Ceramic floor tiles or vinyl flooring will need a movement joint.

Edited April 11, 2017 by Ian

[Ferdinand]

1 minute ago, Ian said:

@oranjeboom

won't you need a good solid support for whatever floor finish you put down over that area?

 

I think I don't see 300mm thick EPS with whatever is on top of that (if anything) being materially less supportive over a 50mm gap than it is over the rest of the floor. I think it would be difficult to find where the gap was situated in absence of an indicator.

 

But interested to be shown mistaken.

 

Ferdinand

[oranjeboom]

58 minutes ago, Ferdinand said:

 

I think I don't see 300mm thick EPS with whatever is on top of that (if anything) being materially less supportive over a 50mm gap than it is over the rest of the floor. I think it would be difficult to find where the gap was situated in absence of an indicator.

 

But interested to be shown mistaken.

 

Ferdinand

Yes, movement wise I don't think anything is going to crack. It's a 60mm gap which will be covered by 300mm EPS70 plus 100mm meshed concrete.

 

I just feel the need to fill it as there were mice in the attic previously....could potentially eat their way down the cavity insulated walls elsewhere and then travel along the footings and then up into the EPS layer. Not sure whether they have much of an apetite for EPS....

 

I may therefore fill it with EPS beads, and close it off with some steel mesh so any rodents can travel up and munch away at my EPS boards.

[Ian]

9 minutes ago, oranjeboom said:

Yes, movement wise I don't think anything is going to crack.

@oranjeboom

Re the potential for cracking along the line of the joint I was thinking more that you have 2 different floor slabs next to each other - one has underfloor heating and one doesn't(?) + the extension is founded on piles and the original building is on ground bearing strip footings(?). If laying floor finishes that are in any way vulnerable to cracking it would be useful to have a linear movement joint in the floor finish along the line dividing the 2 slabs.

Edited April 11, 2017 by Ian

[Dudda]

Have to agree with Ian and the requirement for a movement joint on this one. It’s not because of the wall but because you’ve two slabs which are completely different in age (old and new) and way their supported. 
Can’t see how rodents would get in to be honest but no harm filling it with whatever you choose.

[oranjeboom]

1 hour ago, Ian said:

@oranjeboom

Re the potential for cracking along the line of the joint I was thinking more that you have 2 different floor slabs next to each other - one has underfloor heating and one doesn't(?) + the extension is founded on piles and the original building is on ground bearing strip footings(?). If laying floor finishes that are in any way vulnerable to cracking it would be useful to have a linear movement joint in the floor finish along the line dividing the 2 slabs.

Thanks Ian. Something new to consider then. It will be one continuous slab, all with meshed UFH. Finished floor will be bamboo. So if there is any movement between the two areas, then i guess it's best to induce that along this 'line'. Best to do a wet cut 20-30mm deep? Just wary of cutting the UFH loops! Probably best to do this when cured which will be harder to do but safer.

[Ian]

11 minutes ago, oranjeboom said:

Thanks Ian. Something new to consider then. It will be one continuous slab, all with meshed UFH. Finished floor will be bamboo. So if there is any movement between the two areas, then i guess it's best to induce that along this 'line'. Best to do a wet cut 20-30mm deep? Just wary of cutting the UFH loops! Probably best to do this when cured which will be harder to do but safer.

@oranjeboom

Is it possible to have 2 u/f heating loops? - one loop for each part of the slab. That way you could design in enough of a gap to have a (safe) saw cut along the dividing line between the 2 areas.

 

Edit: if you are planning on installing the bamboo flooring as a 'floating' floor on an underlay there should be enough tolerance in the build up to cover any minor cracking in the slabs at the junction between the 2 areas.

Any type of hard flooring that is bonded to the slab is the type that really needs a movement joint.

Edited April 11, 2017 by Ian

[oranjeboom]

4 hours ago, Ian said:

@oranjeboom

Is it possible to have 2 u/f heating loops? - one loop for each part of the slab. That way you could design in enough of a gap to have a (safe) saw cut along the dividing line between the 2 areas.

 

Edit: if you are planning on installing the bamboo flooring as a 'floating' floor on an underlay there should be enough tolerance in the build up to cover any minor cracking in the slabs at the junction between the 2 areas.

Any type of hard flooring that is bonded to the slab is the type that really needs a movement joint.

It's quite a large area, so I think I am having to go for 4 loops here, so I'll make sure that I have a loop each side of the 'divide'. Good thinking! : )

[Dudda]

3 hours ago, oranjeboom said:

It's quite a large area, so I think I am having to go for 4 loops here, so I'll make sure that I have a loop each side of the 'divide'. Good thinking! : )

Just to note you said it was going to be one large slab. Will it be reinforced with steel mesh? If so they creating a saw cut in the concrete won't work unless you cut through the steel mesh or ensure the steel mesh stops short either side of this movement joint and then a 20-30mm deep cut is fine the day after the pour. This happened recently on high profile council offices where they effectively reinforced the crack joint with steel mesh and the floor cracked everywhere except where they wanted it to crack. It was a polished concrete floor as well so couldn't hide it with bamboo like you.

 

As you plan on having a floating bamboo floor you could just ignore all this and accept cracks in the slab as you won't see them but I think it's a good idea to put in this movement joint should you ever want to change to tiles or have something bonded to the floor as Ian said. 

[oranjeboom]

9 hours ago, Dudda said:

Just to note you said it was going to be one large slab. Will it be reinforced with steel mesh? If so they creating a saw cut in the concrete won't work unless you cut through the steel mesh or ensure the steel mesh stops short either side of this movement joint and then a 20-30mm deep cut is fine the day after the pour. This happened recently on high profile council offices where they effectively reinforced the crack joint with steel mesh and the floor cracked everywhere except where they wanted it to crack. It was a polished concrete floor as well so couldn't hide it with bamboo like you.

 

Yes, Dudda. It will be one large slab, so will ensure that the two areas are meshed separately to encourage any cracking where we want it to crack - good tip. Thanks.

[Onoff]

I would introduce some "slip" at the joint to avoid lifting at the joint. You get the idea below. You could use some say M16 bolts and put the threaded end into the concrete or resin anchor into an existing slab. CUT the bolt head off and leave the plain shank in a sleeve.