Ideas how to avoid the void

[MortarThePoint]

We're having a precast concrete hollow core floor as our first floor. The Architect has designed it so that there is a ceiling void and as I see it this is 'only' to allow the routing of services and drainage:

1. mains cables - to whole house, but I guess these could be routed up to the first floor ceiling and then down from there
2. hot and cold water - pretty localised at one end of the house in the first floor. There is one bathroom at the far end of the house, but I guess pipes could pass above the first floor ceiling and then back down?
3. drainage - except for showers, this can easily be dealt with above the floor as although their is a separate loo at the far end of the house, its drainage exits through an exterior wall. At that far end we would have to have a raised shower tray, but at the other end everything is very close together so could be routed through a cast in hole in the precast slab
4. lights - down lighters all need a void to sit in as far as i can tell. regardless of whether the light itself needs a void, if the light is in the ceiling then a mains cable needs to reach it. Could I route this through the first floor screed?
5. mains powered smoke alarms - the alarm itself but also its mains feed.

 

There's a lot of work and cost required to install the ceiling void and I am starting to think it could be designed out. The biggest two hurdles would be ceiling lights and mains smoke alarms. I guess if these are kept low voltage, then I could have some pretty flat cables that go into the plaster??

 

What do people think, any good ideas?

 

The obvious solution is below, but it's not acceptable aesthetically:

[Russell griffiths]

If you don’t have the lowered ceiling how are you thinking of having any ceiling?

have you seen the underneath of a concrete floor ?

it will be the most un level thing you have in the house, without the dropped ceiling you will have nothing to fix the plasterboard to. 

 

You will I’ll need some form of frame be it metal or timber to fix the ceiling to, just reduce the void to 100mm. 

[MortarThePoint]

5 minutes ago, Russell griffiths said:

If you don’t have the lowered ceiling how are you thinking of having any ceiling?

have you seen the underneath of a concrete floor ?

it will be the most un level thing you have in the house, without the dropped ceiling you will have nothing to fix the plasterboard to. 

 

You will I’ll need some form of frame be it metal or timber to fix the ceiling to, just reduce the void to 100mm. 

 

You don't think that wet plastering the underside of the concrete floor could take out the unlevelness?

 

Any void size effectively comes with the same cost and effort consequences.

[Russell griffiths]

1 minute ago, MortarThePoint said:

 

You don't think that wet plastering the underside of the concrete floor could take out the unlevelness?

 

Any void size effectively comes with the same cost and effort consequences.

Have you seen this done?

how do you stop it cracking?

 

 

[MortarThePoint]

Just now, Russell griffiths said:

Have you seen this done?

how do you stop it cracking?

 

I haven't no. Sounds like this could be the killer to the idea

[Russell griffiths]

I think you will find the floor comes with a minor camber built in at the factory,

 

all the ones I've ever seen have an underside like the surface of the moon, I believe they also have a bevelled edge on the joins so you end up with a big void that needs filling. 

 

Lets ask @nod have you ever wet plastered a concrete plank floor as a flat ceiling or do you normally fit a suspended ceiling. 

 

Why not ask the floor manufacturer. 

[MortarThePoint]

1 minute ago, Russell griffiths said:

Why not ask the floor manufacturer. 

 

I will ask the designer, but I'm sure it's not the normal way to go.

 

[MortarThePoint]

I haven't managed to speak to the flooring designer, but I did see this link and this link which make sense.

 

We're not using them, but Longley say:

"Textured paint finishes or plaster finishes may be applied to Hollowcore planks in accordance with the manufacturer’s instructions. Plaster manufacturers may require the application of a bonding agent. (Plastered finishes are not recommended for longer spans)."

https://www.longley.uk.com/products/hollowcore/finishes-service-runs/

Not clear what classes as a longer span, but that's likely to do with the camber the planks have.

 

I'll ask our guys if it is possible to chase slots in the underside of the hollow core as even a 5mm deep slot would be enough to hide the wire to a light or smoke alarm.

[the_r_sole]

Surface mounted services are going to look awful unless you really go for it and try to achieve that industrial aesthetic, I would definitely keep the service void and ceiling!

I've done a house where all the services were surface mounted in galv conduit, but in the end it probably wasn't any cheaper than hiding them in the walls/ceiling - turns out electricians don't like having to be mm perfect with all the cable runs... who knew!?

[MortarThePoint]

3 minutes ago, the_r_sole said:

Surface mounted services are going to look awful unless you really go for it and try to achieve that industrial aesthetic, I would definitely keep the service void and ceiling!

I've done a house where all the services were surface mounted in galv conduit, but in the end it probably wasn't any cheaper than hiding them in the walls/ceiling - turns out electricians don't like having to be mm perfect with all the cable runs... who knew!?

 

I agree, we don't want the industrial aesthetic. If the ceiling can be wet plastered and if the mains cables can be recessed into rebates in the underside of the slabs, then it might work. That's two big ifs and a might though. ?

[the_r_sole]

Just now, MortarThePoint said:

 

I agree, we don't want the industrial aesthetic. If the ceiling can be wet plastered and if the mains cables can be recessed into rebates in the underside of the slabs, then it might work. That's two big ifs and a might though. ?

 

I think you'd probably spend a lot on getting a decent finish on the underside of a slab tbh - that's why they usually use the horrific textured finishes on them!

[Russell griffiths]

Do you have a problem with height?

you will spend as much plastering that ceiling as fitting a dropped ceiling with plasterboard 

what about duct pipes. 

[nod]

5 hours ago, Russell griffiths said:

I think you will find the floor comes with a minor camber built in at the factory,

 

all the ones I've ever seen have an underside like the surface of the moon, I believe they also have a bevelled edge on the joins so you end up with a big void that needs filling. 

 

Lets ask @nod have you ever wet plastered a concrete plank floor as a flat ceiling or do you normally fit a suspended ceiling. 

 

Why not ask the floor manufacturer. 

Yes I have 

Quite a lot of hotels would have the underside of the concrete floor skimmed We would simply fill the joints with bonding and Paint gypbond on the whole ceiling 

and skim with two coats of multi finish the following day 

[Russell griffiths]

There you go @MortarThePoint bonding coat and a bit of skimming. 

 

 

[Mr Punter]

You could probably get a wire between the planks but you will struggle to go across them.

[MortarThePoint]

2 hours ago, nod said:

Yes I have 

Quite a lot of hotels would have the underside of the concrete floor skimmed We would simply fill the joints with bonding and Paint gypbond on the whole ceiling 

and skim with two coats of multi finish the following day 

 

Interesting thanks! Does it give a good finish you'd be happy with in your own home or can you see the camber of the planks or anything?

Edited September 25, 2020 by MortarThePoint

[MortarThePoint]

3 minutes ago, Mr Punter said:

You could probably get a wire between the planks but you will struggle to go across them.

 

I think it would be possible for me to limit myself to going along the long axis of the planks. I think I can chase a channel into the underside of the plank and route the cable in there. I asked if they could cast a channel in, but no cigar. Some other manufacturers allow that I think. Alternatively, I can drill an up to 68mm hole through the surface into the core and use that as a conduit. You have to do it in the correct place to not hit a rebar and reduce the plank's strength. That said, my designer was happy that any single bar could be knocked out without an issue.

 

If the finish can look top notch I think it's really worth considering as I'm not keen on voids in general. Might have something to do with having lived in places where mice have enjoyed running around in them at night. Saves a lot of effort and gives a bit more ceiling height, though I don't think we need the height.

[AliG]

We have concrete planks on the first floor.

 

We had to frame them out with wood and then plasterboard them.

 

Luckily we had enough ceiling height to deal with this. We had allowed for up to 2.8m ceilings.

 

Over a length of 5 or 6m they have a considerable camber, the combination of this and the gaps at the joins would make it a bad idea to plaster the underside in my opinion. You could end up with places where the plaster had to be over 50mm thick.

 

The space ended up being used for water pipes, cabling, downlight etc.

 

What heating method are you planning for upstairs?

 

We have EPS and then screed on top of the concrete planks for the UFH. This adds another over 100mm on top.

 

 

Edited September 25, 2020 by AliG

[Mr Punter]

We did a suspended ceiling in hollowcore and almost every core we drilled held about a bucket of water!

[nod]

16 minutes ago, MortarThePoint said:

 

Interesting thanks! Does it give a good finish you'd be happy with in your own home or can you see the camber of the planks or anything?

Exactly the same as any other surface 

The Gypbond leaves the surface like textured paint

Then skins as normal

[AliG]

3 hours ago, nod said:

Yes I have 

Quite a lot of hotels would have the underside of the concrete floor skimmed We would simply fill the joints with bonding and Paint gypbond on the whole ceiling 

and skim with two coats of multi finish the following day 

 

Does this work because hotel rooms tend to have small spans so the camber is less of an issue?

[MortarThePoint]

4 minutes ago, AliG said:

What heating method are you planning for upstairs?

 

We have EPS and then screed on top of the concrete planks for the UFH. This adds another over 100mm on top.

 

We're going for UFH with air source heat pump. We won't need quick heat so I'm happy to not insulate between the UFH and the hollowcore. Do you think that's a mistake?

[Mr Punter]

Just now, MortarThePoint said:

 

We're going for UFH with air source heat pump. We won't need quick heat so I'm happy to not insulate between the UFH and the hollowcore. Do you think that's a mistake?

 

Yes.  The heat will dissipate downwards.

[MortarThePoint]

6 minutes ago, Mr Punter said:

We did a suspended ceiling in hollowcore and almost every core we drilled held about a bucket of water!

 

I plan to seal the cores at the cavity end so hopefully that will help

[AliG]

Assuming planks plus screed plus possibly plaster underneath you would be heating around 250mm of concrete. This would be very bad for the response time of the heating.

 

You would effectively be heating the ceiling the room below as much as the room above. This would reduce the efficiency of your heating.

 

My compromise was to use EPS rather than PIR upstairs as it is a lot cheaper and I just want to separate the UFH from the concrete planks. It also provides a sound break.

 

All of this means that with concrete planks and upstairs UFH and the suspended ceiling you are looking at a 400mm gap between floors.My architect had allowed for 350mm, hence our ceiling height ended up being around 2.75m rather than the 2.8m I had asked for.

 

 

 

 

Edited September 25, 2020 by AliG