Garage Conversion - Sanity Check

[LennieB]

Hello!

I am in the early/mid stages of converting an integrated garage to a liveable room.

The construction of the garage is standard brick/block cavity wall, so far the garage door has been removed and bricked up leaving space for a window and the side access has been completely bricked up, in both cases only the outer leaf of bricks has been constructed. I've also cut an internal door.

Next jobs are to add timber joists to raise the floor and infill the inner leaf in studwork.

I had building regs out just before Christmas to inspect after all the structural work was done, they were happy and the guy offered to sanity check my plan for the next stages. I emailed and chased but I have yet to hear back from him, as really want to get on I am hoping you good folk will give it a look over for me?

For the in-fills where a single leaf of brick has been constructed, I am thinking 100mm x 50mm studwork with 100mm PIR between the studs. The front side of the insulation would be completed with foil tape over the studs to complete the vapour barrier and then covered with 12.5mm plasterboard.

The gap between the back of new brickwork and the internal face of the blockwork is around 160mm, so this would leave a 60mm cavity. Am I right in thinking that I won't need any sort of DPM plastic behind the entirety of the studwork? The studs will be fixed to the internal blockwork, so I don't see that I should need DPM between the studs and where they fix to the blocks either?

For the floor I am planning joists of 150 x 47 C24 treated timber. If I understand things, the dead load should be less than 0.25KN/m2 given the floor will not be supporting any of the building fabric above and therefore it seems this should be suitable at even 600mm centres. The width of the room is 2750mm and will be used as a study/spare bedroom and downstairs loo? Does this grade of timber seem right for the application? I don't want a bouncy floor. I was thinking 2 rows of noggins to more evenly spread any load.

The next joist size up at my local timber merchant is 200 x 47 C16 but there are some internal concrete steps starting at about 160mm down from floor height (where the side access was) that I was hoping to not need to remove, they would block at least one of the joists. They also come in lengths less divisible by the room width.

Joist would be hung by joist hangers off wall plates the same size as the joists, connected to the wall using M10 concrete bolts. Floor insulation will also be 100mm PIR between the joists.

Any help appreciated, thanks in advance.

 

[Notch7]

150 x 47 has a span limit of 3.0m at 600 ctrs

but given you want a loo that will probably be tiled and you don’t want it bouncy, I would go 400ctrs.


given that it will be a suspended floor it needs ventilation and a minimum air gap below the joists

 

[LennieB]

150 x 47 has a span limit of 3.0m at 600 ctrs

but given you want a loo that will probably be tiled and you don’t want it bouncy, I would go 400ctrs.


given that it will be a suspended floor it needs ventilation and a minimum air gap below the joists

There is about 400mm from finished floor height to the original concrete slab, so plenty of room underneath.

Modelled it on Sketchup earlier and 450 centres fits well in the length, so will go with that and add noggins or reinforce around the bathroom if needed.

 

[^woody^]

Two noggins won't make things twice as rigid. One down the centre span will do. You could use thinner joists and drop a strut down to the slab every other joist or so.

That's a deep floor void, how is condensation risk or venting being dealt with?

You should insulate across the face of the wall timber not just between.

You don't need a vertical DPM on a cavity wall.

 

[LennieB]

Thanks for the advice @^woody^, ordered the joists today and decided on one noggin as well, good suggestion on the struts, will try that if it feels bouncy.

Floor void is so deep because the house is on a hill and the garage is on the low side of the plot. I have put a couple of air bricks in the front wall but not got a lot of options on the others and ideal placement is an internal wall. Building control already okayed that and said that the two in the front would be fine, despite my concerns to the contrary.

I was wondering about the need to insulate over the timber, the insulation on the walls is only 6m² in a room with around 34m² of wall area. I was hoping to get away with it, I understand it isn't best practice but is the practical issue with heat loss from these areas or cold spots causing potential condensation points? If it is worth it I can set the studwork further back and use insulated plasterboard on these areas.

 

[^woody^]

It only needs to be a thin bit of insulation over the studs. 10mm will do, or possibly two layers of that polystyrene that comes on a roll like wallpaper. It's just to take the chill off the timber.

 

[LennieB]

It only needs to be a thin bit of insulation over the studs. 10mm will do, or possibly two layers of that polystyrene that comes on a roll like wallpaper. It's just to take the chill off the timber.

Easy-peasy then, thanks.