Advice on lowering finished floor level

[WWilts]

Just at the stage of foundation masonry being laid atop trench fill footings, with lintels in the cavity wall to allow drains to pass under slab. Not too late.

 

It may be that the trench fill came up too high, relative to the permitted finished floor level (long story). Will know for certain by measuring level difference from a survey AOD reference point that has just been discovered. Not certain that the permitted ffl will require any lowering, but looking for ideas just in case. 

 

If & when there is a need to lower the ffl, what is the simplest way to do it?

 

Would be good to keep a 1 in 40 drainage fall across 11 m width of footprint. Thickness from underslab sand blinding to flooring top surface is 317mm. Includes 50mm sand blinding at bottom and 65mm screed with under floor heating. 

Perhaps a channel could be drilled in the margin of the (deep) concrete footing to form a haunch for the lowest part of the drain? Alternatively, if the difference is small, perhaps switch to 40mm liquid screed that can take underfloor heating. And switch from 50mm sand blinding to 30mm?

 

 

[Dave Jones]

if the FFL has been ballsed up cut the conc for drains. Have you already put in conduit under the conc for mains water ?

 

I'd also ditch the slab, hardcore and sand for block and beam.

 

Another tip ditch the 100mm celotex which doesnt work brick and use the cheaper 150mm eps insulation with 75mm screen (225 or a block). Save yourslef some brickie cost and look at trench blocks  instead of the 2 courses of 7n's you have below ground.

[WWilts]

3 minutes ago, Dave Jones said:

if the FFL has been ballsed up cut the conc for drains. Have you already put in conduit under the conc for mains water ?

 

I'd also ditch the slab, hardcore and sand for block and beam.

 

Another tip ditch the 100mm celotex which doesnt work brick and use the cheaper 150mm eps insulation with 75mm screen (225 or a block). Save yourslef some brickie cost and look at trench blocks  instead of the 2 courses of 7n's you have below ground.

Cutting concrete for drains makes sense.

2 courses of concrete block (3 for inner) put in already around most of the perimeter.

Conduit under concrete already in place for mains water, but that involves a circuitous route. Considering shalloduct and bringing the water in nearer where the services reach the footprint.

[Dave Jones]

17 minutes ago, WWilts said:

Cutting concrete for drains makes sense.

2 courses of concrete block (3 for inner) put in already around most of the perimeter.

Conduit under concrete already in place for mains water, but that involves a circuitous route. Considering shalloduct and bringing the water in nearer where the services reach the footprint.

 

Bit late to fix stuff if already built out of ground.....

 

water has to be 750mm deep.

[WWilts]

2 minutes ago, Dave Jones said:

if already built out of ground.

Longest underslab drain approx 11.25m

Not too difficult to set its origin in the wall a bit lower than the existing. The exit from the opposite end of the footprint can be lowered by cutting concrete as suggested.

 

Shalloduct is reputed to allow less than 750mm depth for water pipe
https://www.bes.co.uk/shalloduct-water-service-pipe-insulation-835-x-32mm-19170/?gclid=Cj0KCQjw1dGJBhD4ARIsANb6OdmKISJLneP8bQfbEhDyABYg_nZDqb-ZvNFO6S_2zzFnjUNXHelrD4MaAjQjEALw_wcB

[Dave Jones]

should have enough fall to get that drain out 11m @ 13mm/M fall.

 

I'd run that duct past your waterboard, Severn Trent wouldn't accept it. If you already have a duct in for water it doesnt really help you to make the run shorter.

[WWilts]

55 minutes ago, Dave Jones said:

If you already have a duct in for water it doesnt really help you to make the run shorter.

Digging a deep trench and purchasing approx 16m barrier pipe likely to add £1k to cost compared to shalloduct. Shalloduct claim that water authorities accept it. Perhaps that claim is exaggerated.
Water on site already, near footprint. Taking it into the house is hopefully up to us. 

[Dave Jones]

if you forget to run it under the footings then sure you can roll the dice on an un-inspected solution. Should be ok ......