Raising garage floor / Stud Wall Partitions

[franktank]

Hi All,

I'm looking to do a partial garage conversion to add a laundry room to my house. The house is a late 80s Bovis (not sure if that's good or bad...) The house has a semi integral double garage (i.e. one side sits under the house, the other half under a pitched roof) Hopefully my 2D floor plan below explains this. Currently there is a firedoor between the garage and the hallway (All signed off by building control)

I want this to be compliant with building regs.

The room in turquoise is what I would like to create (2 new walls, 1 door and a raised floor)

Currently the garage slab is ~130mm below the finished floor level of the house, I would like the new laundry to be at the same level of the house. I have no idea how thick the slab is, nor whether it is insulated. I will say, it is dry in there.

I have a few questions which hopefully the experts here can give me a steer on before I start the conversation with building control:

1) As I want to raise the floor, what is the best way to do this? In my mind I need to build a small dwarf wall (2 courses) to bring it to the level of the house, then add a stud wall on top of this? I assume I can not simply go from the concrete slab with the stud wall as how would I create a new door threshold? Can I build a small dwarf wall direct onto the garage slab?

2) I assume I will need to add some kind of damp proof membrane and insulation on top of the floor slab, then add insulation and finally some screeding. Could I use timber joists instead with insulation between and floorboards? It would mean the joists would be very close to the existing concrete slab. The rest of the house is concrete floors.

3) I'm aware I need to comply with fire regs, which will mean the 2 new stud walls will need to be rated at 30 mins, and also the new door, I don't believe I would need to do anything with the existing internal walls?

4) What kind of thermal performance do I need from the stud walls, I'm not clear if they are classed as exterior walls as they are enclosed by the existing garage wall, if that makes any difference

Any guidance on this would be appreciated. I have deliberately not gone to the end of the house with the conversion to ensure I can still get a car in the left side and open the door without hitting the new laundry. Plumbing/drainage isn't an issue as the WC sink waste comes into the garage before returning back to the soil pipe in the bottom left corner of the WC.

 

[noseall]

Hi All,

I'm looking to do a partial garage conversion to add a laundry room to my house. The house is a late 80s Bovis (not sure if that's good or bad...) The house has a semi integral double garage (i.e. one side sits under the house, the other half under a pitched roof) Hopefully my 2D floor plan below explains this. Currently there is a firedoor between the garage and the hallway (All signed off by building control)

I want this to be compliant with building regs.

The room in turquoise is what I would like to create (2 new walls, 1 door and a raised floor)

Currently the garage slab is ~130mm below the finished floor level of the house, I would like the new laundry to be at the same level of the house. I have no idea how thick the slab is, nor whether it is insulated. I will say, it is dry in there.

I have a few questions which hopefully the experts here can give me a steer on before I start the conversation with building control:

1) As I want to raise the floor, what is the best way to do this? In my mind I need to build a small dwarf wall (2 courses) to bring it to the level of the house, then add a stud wall on top of this? I assume I can not simply go from the concrete slab with the stud wall as how would I create a new door threshold? Can I build a small dwarf wall direct onto the garage slab?

2) I assume I will need to add some kind of damp proof membrane and insulation on top of the floor slab, then add insulation and finally some screeding. Could I use timber joists instead with insulation between and floorboards? It would mean the joists would be very close to the existing concrete slab. The rest of the house is concrete floors.

3) I'm aware I need to comply with fire regs, which will mean the 2 new stud walls will need to be rated at 30 mins, and also the new door, I don't believe I would need to do anything with the existing internal walls?

4) What kind of thermal performance do I need from the stud walls, I'm not clear if they are classed as exterior walls as they are enclosed by the existing garage wall, if that makes any difference

Any guidance on this would be appreciated. I have deliberately not gone to the end of the house with the conversion to ensure I can still get a car in the left side and open the door without hitting the new laundry. Plumbing/drainage isn't an issue as the WC sink waste comes into the garage before returning back to the soil pipe in the bottom left corner of the WC.

Raise the floor with 1200g dpm, insulation and screed.

Yes - dwarf wall off the slab.

100mm Stud walls filled with 100mm Celotex and another 50-75mm layer across the studs. Foil tape the joints. Use pink fireboard on the garage side. We like to add a layer of ply behind the plasterboard, to aid fixing (later on).

 

[firapara]

I have done a similar conversion in my house.

1) Small wall two courses of brick on top of the concrete slab, then DPC on top of the wall then studwork.

2) I laid a DPM on top of the concrete, then 50mm celotex jointed with tape, then another DPM, then expansion foam around the perimeter, then liquid screed 30mm. You can do 60mm celotex and dry screed 70mm. Speak to your Building Inspector, he will tell you what he will accept.

3) I did a 60 min rated fireboard on the outer (garage) side of the wall plus 3mm skim , and insulated (52mm) plasterboard on the inside (laundry room side). Your Building inspector will want an OSB or a Plywood on the outer side of the wall behind the fireboard, just in case if you accidentally hit the wall with a hammer from the garage side. Not sure about the door, just use a fire door, to be safe.

4) You garage is not heated, so the walls will need to be insulated properly. 100mm stud wall with 90/100mm celotex between the joists and an insulated plasterboard on the internal side of the stud wall. I used 52mm PIR insulated plasterboard.

You should go via Building control, Building Notice (no Building application is required). When you will be informing them about the commencement they normally do not do a site visit, but you can ask them to send an inspector, when he is onsite, ask him what he will be happy with and just do what he wants.

 

[franktank]

Thanks both, so something like this? I've added ply to both sides so to facilitate hanging on units on the inside, and protecting the outside. Would you take the outer ply and plasterboard down to the slab level or the top of the dwarf wall as shown?

 

[noseall]

Thanks both, so something like this? I've added ply to both sides so to facilitate hanging on units on the inside, and protecting the outside. Would you take the outer ply and plasterboard down to the slab level or the top of the dwarf wall as shown?

View attachment 346627

We just fitted (18mm) ply on the non-garage side. But your drawing is accurate. Don't forget to foil tape all the PIR joints.

 

[firapara]

You need DPM under and above PIR on the floor.

DPM underneath PIR must overlap with DPC

Plasterboard and Ply must be above DPC level. I would build I thicker brick wall, single brick not half brick as on your drawing.

Some inspectors want the ceiling to be fire rated as well, your garage should already have double plasterboard, so this should be fine.


And don't forget to use structural timber framing screws when building the stud wall.

 

[franktank]

Thanks again both of you.

So something more akin to this?

Double width wall 2x2 courses?

Would there be an expectation from building control to tie the new DPM back into the existing brickwork of the house? Like they do with lead flashing.

 

[firapara]

Yes, this looks good

No, you do not need to tie the new DPM to the existing DPC.

 

[franktank]

Finally after 2 years and various other priority jobs I'm getting back round to this. Building controlled notice lodged and I'm just about to kick it off with them.

I've been thinking about the wall build up and I think I'm going with 4x2 studwork, 90mm PIR in between studs + 50mm on top which gives a u Value of 0.17 W/m²K which is OK.

My question is regarding the interface between the walls and the floors, and what is the best option so I can go in prepared when I have the initial conversation with BC. I think I'm probably over thinking this and making something much more complex then it needs to be! I've tried to slim down the wall depth as much as possible to increase the internal area of the laundry whilst maintaining a useable garage space.

I understand I need some perimeter insulation between the screed and dwarf wall to stop the cold bridge between the wall and the screed? Are 10mm strips OK or could I use some of the left over 50mm? I plan to put some self levelling compound to take up the last 5mm or so to FFL, but that would mean there would only be 5mm of self level compound, then celotex on the edges, not sure if that is sufficient. Finished floor will be LVT if that makes any difference.

Secondly, how should I position the wall relative to the floor? I could sit the complete wall build up all on top of the dwarf wall bar the internal plasterboard, which would sit over the perimeter insulation? OR should I plan to sit the 50mm celotex part of the stud wall build up over the floor? The issue with this is then I have 215mm width wall, with only ~130mm of studwork sitting on it, leaving me with 85mm of exposed dwarf wall on the garage side?

Any help much appreciated, even if its a steer to do some more research!

Thanks

 

[noseall]

Why is there a 215 mm wide wall there? 100mm would suffice. Or double timber sole plate straight off the floor with DPC stapled to the underside. You can allow the surface wall insulation to travel down to the floor insulation. The DPM gets stapled to the studs as it travels up the wall the thickness of the screed etc.

 

[franktank]

Why is there a 215 mm wide wall there?

Thanks noseall. I think I'd doubled it up on the advice from firapara in reply #6

Or double timber sole plate straight off the floor with DPC stapled to the underside. You can allow the surface wall insulation to travel down to the floor insulation. The DPM gets stapled to the studs as it travels up the wall the thickness of the screed etc.

Get you, so something more akin to this (Ignore gap between 50mm and 90mm PIR that's just my excel drawing with a super thick DPM!):

As always I appreciate any advice so thanks in advance.

 

[noseall]

Thanks noseall. I think I'd doubled it up on the advice from firapara in reply #6



Get you, so something more akin to this (Ignore gap between 50mm and 90mm PIR that's just my excel drawing with a super thick DPM!):

View attachment 421574

As always I appreciate any advice so thanks in advance.

I'd position the insulation on the garage side. The plywood is not a requrement but it is something we often fit prior to fixing the plasterboard, particularly in kitchen or utility zones.