oversite

[fastneattrowel]

hey noseall perhaps you might know this answer to.i have to do a over sight on the new build ,i have no plans or specs but after the inspector passed the footings i did miss a bit of information about the oversight ie the hard core part ,how much depth of hardcore? i know i have to bind the hard core with sand and then use membrane then 100 m concrete , thanks john

 

[fastneattrowel]

,OVERSITE,

 

[noseall]

a typical oversite for us would be:

100mm minimum compacted mot stone.

25-50mm sand blinding also compacted.

1200gauge dpm turned onto dpc.

125mm jablite* polystyrene insulation, and 25mm perimeter inso' to outside walls only

another covering of polythene to protect the jablite and not turned into dpc.

100mm tamped concrete.

75mm screed (laid when plastering is complete).

*can be substituted for 80mm celotex.

 

[JohnD]

you could have used the

button

 

[fastneattrowel]

you could have used the

button

yeah thanks

 

[fastneattrowel]

ok noseall i forgot to mention the floor is done in timber joists with a clearence of at least 400m to the oversite,building inspector never mentioned no jablite insulation just hardcore sand membrane and 100m of concrete cheers john

 

[^woody^]

Oversite is just really to stop the plants growing back up under a timber floor.

No need for a DPM as the DPC and airbricks will dry the underfloor void and keep the timbers dry, but you could lay polythene as a weed barrier, and not as a true DPM

Just remove any soft soil and vegetation, and no real need for hardcore unless you need to build it up a bit

 

[fastneattrowel]

Oversite is just really to stop the plants growing back up under a timber floor.

No need for a DPM as the DPC and airbricks will dry the underfloor void and keep the timbers dry, but you could lay polythene as a weed barrier, and not as a true DPM

Just remove any soft soil and vegetation, and no real need for hardcore unless you need to build it up a bit

i been told by inspector the over sight has to be hard core sand membrane concrete even though i have a void of 400 m from under sides of joists ,that was my question to noseall surely i dont need insulation as well?

 

[^woody^]

Ignore the BCO, or at least ask him the basis for his request (it can't be an instruction).

Oversite does not require a DPM, and damp-proofing to the timber floor is via a DPC in the supporting walls or sleeper walls

There is no need to put a DPM under the oversite - in fact this will effectively prevent drainage of the floor and could trap excess moisture in the floor void

I don't recall the BS for oversite under floors, but the only requirement is that it is trowel or spade finished smooth. It does not require a DPM, but hardcore is optional if you need to build the ground up

Also the minimum distance from concrete surface to underside of joists is 150mm, and the DPC under the wallplate must be minimum 75mm above the concrete.

Edit:
The above assumes there are no adverse ground salts which may attack the concrete

 

[fastneattrowel]

Ignore the BCO, or at least ask him the basis for his request (it can't be an instruction).

Oversite does not require a DPM, and damp-proofing to the timber floor is via a DPC in the supporting walls or sleeper walls

There is no need to put a DPM under the oversite - in fact this will effectively prevent drainage of the floor and could trap excess moisture in the floor void

I don't recall the BS for oversite under floors, but the only requirement is that it is trowel or spade finished smooth. It does not require a DPM, but hardcore is optional if you need to build the ground up

Also the minimum distance from concrete surface to underside of joists is 150mm, and the DPC under the wallplate must be minimum 75mm above the concrete.

i know that woody and so do you and i dont know why he said thats the regulations cant ignore something like that perhaps NOSEALL will know lol

 

[^woody^]

From Zurich guidelines

Avoid ingress of ground moisture into the building
Provide beneath all suspended timber floors either:
• A 100mm BS 5328/STI mix concrete oversite on
hardcore, or:
• A 50mm concrete oversite and minimum 1000 g
(250μm) damp-proof membrane on sand blinding

From NHBC

(b) resistance to ground moisture
Provision should be made to prevent
ground moisture affecting timber floor
construction.
This can be achieved by either:
• 50mm concrete or 50mm fine aggregate
on a polyethylene membrane laid on
50mm sand blinding, or
• 100mm concrete

 

[fastneattrowel]

From Zurich guidelines

Avoid ingress of ground moisture into the building
Provide beneath all suspended timber floors either:
• A 100mm BS 5328/STI mix concrete oversite on
hardcore, or:
• A 50mm concrete oversite and minimum 1000 g
(250μm) damp-proof membrane on sand blinding

From NHBC

(b) resistance to ground moisture
Provision should be made to prevent
ground moisture affecting timber floor
construction.
This can be achieved by either:
• 50mm concrete or 50mm fine aggregate
on a polyethylene membrane laid on
50mm sand blinding, or
• 100mm concrete

yea thanks woody similar to what inspector said, just as hes making me put a 100m cavity onto a 75m cavity built bungalow meaning i have to dub out inner walls, really cant upset them and ask there reason for this because this ones a lil hitler

 

[noseall]

i concur with woody, unless adverse ground conditions dictate a different view from b.c.

is it in a flood zone?

 

[NS215]

funny this how people brag how good they are then seek advice from others they have called

 

[fastneattrowel]

i concur with woody, unless adverse ground conditions dictate a different view from b.c.

is it in a flood zone?

no not all, the damp course is 4 courses of the out side ground level, so you say i have a valid point in asking why i have to use hard core sand and membrane and 100m concrete?