Extension floor screed - joining with existing

[liftfan]

I'm going to fit a membrane, insulation and then screed on the beam and block floor of my extension. What about the remainder of the wall that now divides old and new?

Do both walls remain? Or, is just the outer leaf removed? Or is the inner removed as well such that the insulation layer buts up to the old concrete sub base (which is no doubt not insulated and as no DPM!).

Thanks

 

[SpecialK]

I would remove both walls and span the insulation over the gap. You could get away with just removing the "outer" wall if there is no insulation under the existing slab...

 

[noseall]

I'm going to fit a membrane, insulation and then screed on the beam and block floor of my extension. What about the remainder of the wall that now divides old and new?

Do both walls remain? Or, is just the outer leaf removed? Or is the inner removed as well such that the insulation layer buts up to the old concrete sub base (which is no doubt not insulated and as no DPM!).

View attachment 311401

Thanks

Remove all the masonry (both leaves) so as to allow the membrane and insulation to travel across and butt up to the existing floor.

Is that being built to the latest (post June 2022) reg's? If so, you will have little room for an un-bonded screed. You may just nick a 50mm screed? 75mm would be ideal.

 

[liftfan]

Remove all the masonry (both leaves) so as to allow the membrane and insulation to travel across and butt up to the existing floor.

Is that being built to the latest (post June 2022) reg's? If so, you will have little room for an un-bonded screed. You may just nick a 50mm screed? 75mm would be ideal.

Thanks, I'll remove both walls. It's being done under pre June 2022 although I'm working to the new regs where it makes sense eg. insulation.

What is the change in the new regs for bonded / unbonded? Just had a search and Part E, 5.22 mentions something but other than that?


I have around 200mm from the block and beam to the FFL so DPM, 100mm insulation, Polysheet, then 100mm to play with for screed, levelling compound and vinyl planks (Karndean style) on top. I have the added complication that the original floor is out of level with a 2cm fall from one end to other!

 

[noseall]

What is the change in the new regs for bonded / unbonded?

There is no change in that respect. It's more of a thickness issue i.e. the more insulation you have to put down into a pre-determined gap, the less room you have for screed. Un-bonded screeds i.e. those that are not laid direct to the concrete, but rather on a (separation) membrane, means they need to be wee bit thicker, to be successful.

I have around 200mm from the block and beam to the FFL so DPM, 100mm insulation, Polysheet, then 100mm to play with for screed,

Perfect. It just looked a bit lean from the camera angle. We are now setting our B&B floors at three courses down now, instead of two.

oversite2.jpg

• noseall
• 20 Jun 2023

 

[liftfan]

Great info thanks.

I was wondering if getting a company in to flow screed it was worthwhile rather than me just doing it the old way?

The extension is 16m2 but given the existing screed has a 2cm fall to one side I may end up using a lot of levelling compound.

 

[noseall]

Great info thanks.

I was wondering if getting a company in to flow screed it was worthwhile rather than me just doing it the old way?

The extension is 16m2 but given the existing screed has a 2cm fall to one side I may end up using a lot of levelling compound.

Never used liquid screed. Always the old way (sharp sand/cement) for us. Should be easier to manipulate any indiscretions.

 

[liftfan]

Ok ta!

 

[SpecialK]

I used liquid screed at 55mm but didn't have any issues levelling to the existing floor as it was not too wonky that a wooden threshold couldn't take it out.

If you are bonding karndean then a thick screed will take months to dry and it needs to be very dry for that.

Are you installing UFH?

I used southern liquid screed and its was £1k for a 27m2 room at 55mm ish. Then £120 for laitance removal. This was around 2020-21 so prices may have changed.

 

[liftfan]

No underfloor heating. Fair point about the drying time. However, it takes me months to get stuff done and the vinyl will probably be one of the last things I do.

I'll get a price for the liquid screed to compare - but it would go against my current ethos of spending days doing stuff that a professional would take hours to do. I reckon my hourly rate is about 10p.

I've just removed the outer bricks and just looking at the inner. Looks like about 5 - 6 inches of concrete over 20mm shingle. No DPM or insulation in sight obviously but it was built in the 70's. It's built on sand so I never have any rising damp issues.

 

[liftfan]

I would remove both walls and span the insulation over the gap. You could get away with just removing the "outer" wall if there is no insulation under the existing slab...

I'm now thinking, as you say, there's no insulation so there is no point removing the inner block. Maybe I should just take the old mortar off the top and it is then pretty close to the level of the concrete anyway.

 

[SpecialK]

I'm now thinking, as you say, there's no insulation so there is no point removing the inner block. Maybe I should just take the old mortar off the top and it is then pretty close to the level of the concrete anyway.

Up to you I guess. The more insulation coverage the better.

75mm of screed can take 100+ days to dry. If you end up with more than its much longer. You can always put in my PIR to use less screed. Depends how quick you want to be before flooring goes down.

Have you considered UFH? Not too bad to DIY! Will be nice with the Karndean!

 

[liftfan]

I'd like underfloor heating - maybe a hybrid system with radiators as well, but I don't fancy digging up the existing 6mx6m concrete to put insulation in it. I know there are "thin" options but these things just escalate. My next project I'm definitely just knocking down whatever is there first!

Just had a quote for Cemfloor screed: 16m2 at 75mm = 1.2 m3: £1125 . Plus they'd need a skip. I could use more insulation as you say.
I reckon 2 bulk bags of sand and 25 bags cement would be £300. Plus levelling compound.

 

[noseall]

I'm now thinking, as you say, there's no insulation so there is no point removing the inner block. Maybe I should just take the old mortar off the top and it is then pretty close to the level of the concrete anyway.

Take the inner block out, continue the insulation, or it will likely crack.

 

[SpecialK]

I'd like underfloor heating - maybe a hybrid system with radiators as well, but I don't fancy digging up the existing 6mx6m concrete to put insulation in it. I know there are "thin" options but these things just escalate. My next project I'm definitely just knocking down whatever is there first!

Just had a quote for Cemfloor screed: 16m2 at 75mm = 1.2 m3: £1125 . Plus they'd need a skip. I could use more insulation as you say.
I reckon 2 bulk bags of sand and 25 bags cement would be £300. Plus levelling compound.

They need somewhere to wash out. I just knocked up a timber frame and some DPM to make a tank for them..

If you dont fit UFH and decide to do the whole lot later you might regret it? Maybe not though! I have an extension with UFH and the kitchen that comes off it is not underfloor heated. It feels weird walking to a cold floor but otherwise works OK.