I am proposing to build a small (4mx3m) single story extension to my 1960's bungalow. Access will be through an existing external window converted to a door opening. I obviously need to continue the existing FFL through to the extension and as such it would make things easier to use a suspended timber floor as the existing (albeit with insulation etc etc) . According to 'Approved Document C1' (Floors 4.14.a.ii) I could construct the oversite with 75mm concrete, on 1200gauge DPM, on (say) 25mm sand blinding. Although the joists will span the short width supported by joist hangers I want to put a 'sleeper' wall in the center to remove any floor 'bounce'. Will the above oversite construction be acceptable as support for the sleeper wall? or should I incorporate a 500mm wide x 100mmdp strip of hardcore under the wall area?
Oversite to Suspended timber ground floor
- Thread starter Blazeyboy
- Start date
A sleeper wall will require a full depth foundation.
Building Regs Approved Document 'C' Technical solution 4.14 (a.i) Diagram 5 shows a sleeper wall built off of 100mm concrete on compacted hardcore (no foundations) but doesn't indicate if this is acceptable for method (a.ii) (min 50mm concrete on 1200g dpm on sand blinding)
The short span is 2.9m so to span without sleeper wall will require 145mm joist. To maintain the minimum 150mm ventilation space below the joists and (as per regs) keep the oversite above the external ground level will mean that the external ground (paving) will be 175mm below dpc which gives a problem where it meets the existing paving which is 150mm below dpc. I either finish with a pond in the corner or rip out 6m of existing pathway and relay to match the level of the extension. Any suggestions appreciated
Build a foundation under the sleeper wall.
Assume the depth should be same as the extension foundations (as required by BCO) would 300mm width suffice?
Yes, if practicable to dig a narrower trench.
Good point. Probably going to be a lot of hand dig as Rear extension with only a wheelbarrow wide path access! but as retired now I don't have anything better to do!!!!
The oversite does not need to be concrete. It can be a permeable membrane and gravel. The level of the oversite relative to adjacent DPC or ground levels does not matter either, and it will not make a difference to water levels or retention of water.
A micro digger can access a gap as narrow as aboot 34" (the machine itself is only 30" wide). We're doing one at the mo'. I managed to shoe-horn one in and had to wriggle around downpipes, overhanging porches and gallows brackets and gate posts etc. My determination not to hand dig, knows no bounds.
The two lads on the wheel barrows were not so happy about the fact that dumpers are all a lot wider though.
Thanks 'Woody' but can you clarify as B Regs Doc 'C' Tech solution 4.14.a.ii (50mm concrete or inert material etc) says "To prevent water collecting on the ground covering, the top should be entirely above the highest level of the adjoining ground" i.e. in my case above the concrete path running around the extension. I'm also still concerned that the 2.9m span of even 145x47mm joists will allow some bounce in the dining room floor, would 5 rows of solid blocking help to reduce this? (floor covering is 9mm ceramic tiles on 6mm HardieBacker Brd on 18mm WBP plywood - another reason not to allow any deflection)
Blazeyboy, It might be worth checking with your BCO what he wants, as sometimes the regs can be applied differently in different parts of the country.
Where I live there are many places with a high water table so a concrete oversite needs to be above ground level unless laid to falls with a drainage system in place.
On all the sites I've worked on in the past the sleeper walls were built off a concrete oversite, although nowadays a sleeper wall is usually a course or 2 and a timber plate.
Timber floors on new builds are quite rare now as block and beam is preferred.
Where I live there are many places with a high water table so a concrete oversite needs to be above ground level unless laid to falls with a drainage system in place.
On all the sites I've worked on in the past the sleeper walls were built off a concrete oversite, although nowadays a sleeper wall is usually a course or 2 and a timber plate.
Timber floors on new builds are quite rare now as block and beam is preferred.
Thanks 'Stuart45' don't think water table is a problem as in the existing floors the oversite is way below ground level and they are as dry as a bone. I am veering towards the 'Technical solution' given in Diagram 5 of the B Regs Doc 'C'. Any idea on depth of hardcore as this is not given.
6 inches of hardcore is usually enough. In the 60's and 70's they used 6 inches of concrete for the oversite, but the sleeper walls were a bit taller then.
Nowadays the oversite is usually above ground level. Make sure you have at 6 inches from site to bottom of joist.
Nowadays the oversite is usually above ground level. Make sure you have at 6 inches from site to bottom of joist.