Should I replace the slab in a garage to bedroom conversion?

[sejj]

I’m in the process of going through with a garage to bedroom conversion.

In particular, the slab work is giving me a bit of a headache.

The plans (from the 70s) states a 4inch reinforced concrete slab but don’t state anything regarding a moisture barrier.

I had an engineer (remotely) do up some plans with a few options.

Option 1: rip up slab and lay new one with damp barrier
Option 2: lay new slab on top with damp barrier in between slab

Option 2 is obviously far cheaper and easier. My concern is there is some slight damp in the brick wall coming up. Would option one reduce this likely hood or is this much of a muchness?

Lastly, is a new slab even required? Is there a solution that does not involve laying a new slab at all. The current slab is in good condition. 

I’m looking to have it all certified and approved. 

[EricL]

Hello @sejj 

Welcome to the Bunnings Workshop community. It's sensational to have you join us and thanks for sharing your question about your concrete slab.

Because you are turning that place into a habitable room, the floor needs to have a moisture barrier. In regards to the moisture coming off the wall, is that part under the soil or is soil leaning up against it? If it is, then the water is coming from the soil moving through your wall. That has to addressed first before you have your floor done.

I suggest speaking to your engineer and asking them what the minimum thickness is allowable for your new floor. The rules for conversion are very strict and must be followed very well in order for the room to be certified and approved.       

Let me call on our experienced member @Nailbag and @Dave-1 for their recommendation.

If you have any other questions we can help with, please let us know.

Eric

[Dave-1]

 Good Evening @sejj 

Cant wait until we see some pics of your project  

First question tho, have you contacted your local council? I know local councils have rules to change a garage over to a habital room. @Nailbag specifically has mentioned it in one of his projects. I did check with my council for my garage when I updated it but its still an open room. They were more concerned about where the car would park parking wise, If the car that was in the garage was parked on the driveway then that car on the driveway would get bumped to the street to park and thats the main issue my local council had. But I didnt get it in writing.

I know that if you go to sell the house after you convert the room there could be issues with council regulations.

You could put down a moisture barrier before a hardwood floor. It depends on your height from the slab to the ceiling. It needs to be a mininmun of 2400mm for me. (could be differant where you are) My garage also has a slope to it. It was constructed with around a 5cm drop for every 4m by the look of it. An Engineer or builder would be able to help you out with that as @EricL suggests.

I have rubber backed carpet tiles on the concrete slab and have had no moisture issues in the past 2-3 years.

Dave

[Nailbag]

Hi @sejj this is a project I have done myself on a couple of occasions, though not to a bedroom but still to some form of living space. This is my more recent project.

Ok so here is the serious side of your project you will need to consider. You need to take the advice of your engineer as I did with my draft-person as they are qualified across all building regulations and requirements for your area. They know and can advise on your legal building guidelines, specifications etc to convert the space from a garage to a living one which, have strict regulations. This means approved plans and building permit requiring inspections and final sign-off. So, if you want to have this all above board the engineers advice will need to be followed. 

So, to answer your questions: I suspect both options are as effective as each other in addressing the rising damp or they wouldn't have been offered. My concern would be if you go with option 2, what's the flow on effect of the floor being raised another 100+mm to access the room? The engineer has stated the rising damp is an issue from the brick walls, which is not affected by the quality of the existing concrete slab, but its depth and construction.

Within reason, I would be going with whichever option offered the best result for the room access.

Hope that helps, Nailbag

[Nailbag]

Interesting side issue with the council being more interested in where you were going to park your car @Dave-1 Thats also a good point you raise with a minimum height reg, but I assume the engineer has taken that in to consideration. My issue was the external ground was higher than the level slab, so I only had to regrade the lead up ground 50mm lower than the slab and incorporate drainage.

Nailbag

[sejj]

Thanks for taking the time to reply.

The engineer I engaged was mainly completing drawings for a separate concrete porch. His work was not comprehensive regarding the slab (he did not inspect it), he was just providing the options for if that was required. He did not specifically address the rising damp at all.

There are no issues regarding the slab being raised 100mm as the ceiling height it currently close to 3000mm and the existing door openings are all 2400mm so there is plenty of room to play with.

Others have suggested just applying a waterproof membrane and then building a low subfloor. Thoughts on this?

I see on your project you have retained the existing slab. Is this because there was already a damp barrier underneath?

Thanks

[sejj]

I have a separate double car garage so parking shouldn't be an issue. 

Haven't specifically contacted the council (we're in the ACT so works a bit different). The main problem is ensuring the slab meets code. 

What kind of moisture barrier would you suggest?

[EricL]

Hello @sejj 

The store carries the CAgroup 2 x 50m 200um Medium Impact Polypro Builders Film. Here is a link to their brochure - Polypro Professional slab Underlay. Please note that it is always best practice to double check with your engineer regarding the thickness of the moisture barrier as they will be familiar with the local council's rules and regulations.

If you need further assistance, please let us know.

Eric

[Nailbag]

Hi @sejj I had no issue with rising damp on my existing slab which was perfectly level. I would still engage a daft person/engineer to inspect and draw up plans for council approval unless ACT is somehow exempt of this compared to all other states. Even if for no other reason that they will have the qualifications to advise what's actually required.

Regards, Nailbag