Building up floor in garage conversion

[LuckyBenski]

So I'm halfway through a garage conversion. We had a builder do the major works, laying foundations and bricking up the garage door and porch. Unfortunately he laid a floating floor without a damp proof membrane, just placed PIR insulation boards down with OSB on top.

Building control won't sign off without a DPM, so the plan is to lift the floor and insulation, lay a membrane, and rebuild the floor. The floating floor doesn't feel very solid so I would like to create something a bit more robust. The room will be a workshop so it needs to hold machinery etc.

Currently we have the original concrete garage floor around 120mm below the finished floor height in the hall. Then 55mm of insulation and 22mm board on top, so the garage floor is too low at present anyway. I plan to put in timber joists for a bit more strength, but there isn't the height for a proper suspended floor. In fact I can only realistically get 95mm/4" joists in!

So effectively, I think I'm trying to work out if a floor made of joists sitting on the solid ground will work. I'm at the mercy of how flat and level the concrete is (it wasn't perfect). I'm also not sure if the joists will act as a cold bridge where the current insulation doesn't. The garage is eminently warmer since bricking it over. In fact I've never felt it was cold in there despite having no heating yet.

Garage is about 2.8m x 5.3m. I was thinking to fix a stringer along the long walls and use joist hangers to fit joists between them. Then to maximise strength and compensate for the small timber height, use 3" joist widths and close spacing. And probably support them in the middle of the span somehow. But then they might be resting on the floor there anyway?

TBH I have more questions than answers at this point I've booked some time off next week as I really need to push this forward, I'm paying £LOL to store my garage crap in a self storage unit.

Maybe I'm missing an obvious solution? Is a timber floor sitting on concrete just a terrible idea? Or am I overthinking it and I should just whack the joists in and shove PIR between them?

 

[dlockers]

Do some research on the strength of PIR - you'll be surprised. You could always "counter" board it in 22mm giving you a 44mm deck.

 

[dlockers]

Also if you want a workshop build a posh shed durrr

 

[LuckyBenski]

Do some research on the strength of PIR - you'll be surprised. You could always "counter" board it in 22mm giving you a 44mm deck.

Thanks, it's not so much the strength as the stability, I want it to feel rock solid. When I stamp my feet, stuff rattles in there.

I know plenty of people are happy running washing machines on floating floor/PIR, so maybe the doubling up is a good idea. Maybe I can blind the surface with sand before relaying the PIR also?

 

[LuckyBenski]

Also if you want a workshop build a posh shed durrr

I spend a portion of each day resisting starting plans for my garden room I just can't fit it in this summer I think... Trust me though, my YouTube is in full shed build mode!

 

[mickyflinn]

Trust me though, my YouTube is in full shed build mode!

You need some entertainment in your YouTube between shed videos , Kermit omegle

 

[radderfire]

I'm not a builder but what's to stop you laying more concrete? There is also such a thing as deep fill floor levelling compound, but that only gives you 75mm.

 

[LuckyBenski]

I'm not a builder but what's to stop you laying more concrete? There is also such a thing as deep fill floor levelling compound, but that only gives you 75mm.

So the "Right" way to do a new floor would have been 120mm insulation. But to do this you'd also need to pour a 50mm screed on the top. To get that depth I'd have to dig down and break up the existing concrete floor - which would the require a sand blinding layer or another 50mm screed. Basically it's lots more work, noise, and would have to clear a couple of m³ of rubble.

I am still slightly tempted to get a concrete breaker and jump in though. My stepdad is coming to help and he assumed that was the plan.

Effectively the limit is not having enough height for a concrete pour plus insulation.

 

[LuckyBenski]

Ok so doing some reading, planning etc... I have a plan A and a plan B.

Plan A: 95mm joists direct onto concrete. Remove insulation, lay DPM, fit runners on long edges of garage. Use joist hangers and lay joists in place on floor. If they're too tall or uneven, trim wood or concrete until they sit happy. Lay subfloor. Job done.

Plan B: If BC won't approve undersized joists direct on floor, I will lay a layer of sand blinding to level/smooth the surface then put DPM down. Then put the insulation back down and subfloor.

All in all I will probably add some insulation board to build the height up to where I need it.

Tonight I will email Building Control with the plan and wait for their response. We start lifting flooring Thursday and I can order materials to come Monday.

 

[dlockers]

At this stage can't you just whack it full of concrete?

 

[LuckyBenski]

At this stage can't you just whack it full of concrete?

Needs at least some insulation!

BTW where should I be looking for timber/sheet materials? Materials market looks ok but tempted to uprate the timber grade.

 

[dlockers]

Needs at least some insulation!

BTW where should I be looking for timber/sheet materials? Materials market looks ok but tempted to uprate the timber grade.

Not sure how thin you can get away with but sounds like you may have enough for 50mm PIR and 70mm concrete? I don't know what I'm talking about tho!

Yep MM for wood imho.

 

[LuckyBenski]

Not sure how thin you can get away with but sounds like you may have enough for 50mm PIR and 70mm concrete? I don't know what I'm talking about tho!

Yep MM for wood imho.

My calculations put me at 115mm overall to match finished carpeting the hall. If we ever go back to solid in the hall, it'd take me down by 10mm or so.

You're not wrong actually - I could likely get a pour in, might be more solid than any board based solution! Now I've got to think, hmph. Question is how level/smooth can I get concrete.

 

[LuckyBenski]

So yesterday we pulled up the floating floor installed by my builder without a damp proof membrane... Currently got the concrete garage floor drying out over the weekend. Building Control should come to inspect Monday and I will build a floor by whichever method they approve. Ideally 2x4 timber direct onto the DPM on concrete.

Fingers crossed they don't tell me I have to dig down, lay hardcore and a concrete screed...

First board up and the first sign my builder was full of ****: The "proper" foil tape I'd asked him to confirm, is just gaffa tape.


Chopped all the tongue and groove joints with the circular saw. Well worth taking the time and draining 3 batteries as we just picked the boards up after. If we'd gone at it with the crow bar, all the boards would be wrecked and we'd have been later and tired-er.


Floor under the boards is quite obviously very wet - I had a disagreement with the builder who said a DPM would trap moisture and the garage would smell of damp. No, the floor has ground water coming up as it's below the DPC so we need to seal that off before building the floor up.


The damp was causing mouldy patches and also ruined the foil layer on the insulation boards.

Just need to let the floor air out a bit now, should dry quite fast if it doesn't rain.


Nothing to do til BC arrive now!

 

[danlightbulb]

Dpm then insulation then bury the lot in concrete?

 

[LuckyBenski]

Dpm then insulation then bury the lot in concrete?

This might be what I get told to do, and it might work. I could place 55mm PIR down and pour 65mm concrete and just about get away with it.

 

[Mysterae_]

If no financial recourse is possible with the builder it's at least a message back to him telling him I told you so. Why are the trades becoming so untrustworthy?

Anyway, considering your use-case for the room I'd think as long as the existing concrete floor is sound, a quality screed over insulation over a DPM would work, keeping a slight gap between your walls for expansion.