Sloped Concrete Garage Floor - Joists vs Screed (Conversion)

[EViS]

We are trying to decide on our floor structure within our garage conversion. The existing floor is concrete, lower than the rest of the house and sloped. The highest point of the floor is 200mm lower than the rest of the house.

What is your advice on whether to go with a screed (on Celotex and DPM) floor or joists (infilled with Celotex) and finished with 22mm t&g?

If joists will work out quicker and cheaper (will they?), then I have a couple of questions:

a) what size joists should we be using?
b) what is the best way to fix them to the walls? (joist hangers off a wall plate?)
c) what gap do we need underneath?
d) do we need to install any airbricks?

 

[theoldun]

All your questions revolve around internal width of garage. Post internal width of garage and will give you all the answers to comply with BRs
Regards oldun

 

[EViS]

2.8 metres in width.

 

[PlanningandBuildingRegsMa]

200mm Is that awkward height between to high for a floating floor, yet too low for a suspended floor.

I would go with a 50 x 150mm C24 Timber frame with strutting and insulation, this will leave you with a 50mm gap above the concrete floor. I would then try to find a way of venting the timbers and floor.

Sorry I cant give anymore advice on the vents without seeing a few photos or the job.

 

[freddiemercurystwin]

Cheaper and far easier to just whack 50mm polystyrene insulation under the existing joists to fill the void.

 

[EViS]

Cheaper and far easier to just whack 50mm polystyrene insulation under the existing joists to fill the void.

I don't follow by what you mean there. Which existing joists?

 

[freddiemercurystwin]

Sorry should have typed new joists.

 

[theoldun]

If integral garage with solid house floor then no through air flow, so suspended floor will not comply with BRs
If span is 2800 then, to comply with regs joists will need to be regulised C16 47x145 at 400 centres or ditto 63x120 ditto, how ever, to comply with BRs you need a 150mm air void below either joists or insulation. Insulation would need to be on a P/A ratio of .06, 70mm of Celotex FR4000. As you will not be left with 150mm void, suspended floor will not comply.
Our approach to this floor would have been to have bonded with a cement slurry to existing slab a new concrete oversite to a level of 150mm below existing house floor 1200g membrane, and with the same P/A as above 75mm Celotex GA4000 with 20mm of celotex TR4000 as upstand to exposed perimeters to achieve current BRs U value of 0.22 W/m2K. Finished with 75mm fibre reinforced sand, cement screed.
Alternative. Lay new concrete oversite over existing to a level approx 107mm below existing floor level, DPC membrane, 85mm GA celotex and 22mm T&G floating chipboard floor. Will also give U value of 0.22W/M2k
Regards oldun

 

[EViS]

Thanks the oldun, very informative . Will go with the screed approach.

 

[EViS]

Also, another question, if I opt for screeding, at what point do you guys tend to screed? After the front wall (and window) is installed or before whilst the front is still open allowing for good access to get the screed in?