garage conversion

[bounce111158]

Hi,We are converting our garage into a utility room by building a deviding wall and knocking through from the kitchen.
The garage floor is 3 bricks below damp so i have decided to build a small wall up to damp across the garage to sit my new stud wall on.I will be laying
timber joists and chipboard for my new utility floor.
I need to lay a damp proof membrane down and insulation.
2 Questions please.

1. what thickness membrane should i use.
2. Does it go under the wall i am about to build or do i build the wall first then
lay the damp proof up and over it on top of the damp proof strip which i will lay on my top layer of bricks under the stud wall.
I may not of got this across very well,sorry to sound thick but i'm not a builder.

 

[DevilDamo]

Firstly, have ascertained what approvals you require (planning/building regulations)?

 

[bounce111158]

Hi,We have had the guy from the council round and we only require building regs for which we paid £117.50
for the work without any plans.He said build a small wall up to meet the damp proof layer of the house and put down a DP membraine,but i wasn't sure weather you build the wall ontop of the membrane or just build the wall up of the concrete garage floor,put a damp proof layer ontop of the bricks then put down the membrane and take it over the wall ontop of the wall DP.

 

[DevilDamo]

Ok. Normal building practice is to lay a DPC and the build off it and then with the new DPM, form a physical lap with the existing house DPC.

 

[bounce111158]

Thanks for replying,I'm sorry but when you say physical lap with the house dpc what do you mean.
Bring the membrane up the walls of the house?
Sorry to sound dumb.

 

[DevilDamo]

Correct. It's something BCO's like to see. You'd run the DPM up the side of wall and lap it with the existing to maintain a continuous membrane. If your BCO hasn't asked for this, then there's no reason to do it, but I'd be a little surprised if they didn't.

 

[bounce111158]

Thanks for all this info,you really are kind,but if i can pick your brains a little longer.Do i lay the membrane directly onto the old concrete garage floor.

next question. if i want to use timber joists and a chipboard floor do i lay morter and bicks directly onto the membrane to get the joists to the correct hight so the kitchen floor matches ok.

 

[DevilDamo]

Well... as you're converting an existing garage to a habitable room, then you will need to upgrade the existing floor, walls and roof to comply with current u-values.

With an existing concrete floor slab, you would lay a DPM on the slab, then lay insulation, then another DPM (as a VCL/Vapour Control Layer) and then screed on top. That's the traditional and most common way of achieveing the values. There are other forms of construction (e.g. timber).

As for using timber, you would lay a DPM on the concrete slab on the line of the new joists. You'd then lay the timber deck on top but provide insulation between the joists to achieve the u-values. If you were going down that route, your BCO may ask you to provide cross ventilation in the form of airbricks, which may not be practical.

What difference in level are we talking about between the existing slab and existing kitchen floor?

 

[bounce111158]

we have 2and 1/2 bricks difference between levels.
The only reason i'm saying timber is because i have to get a tiolet
waste in somehow and out to the pipe on the outside wall.

 

[DevilDamo]

So you are talking about a difference in level of approx. 175mm? Why don't you measure it?

That can be easily made up using insulation and screed or insulation and timber.

If you're going down the screed route, then you would only need to lay a 65mm screed and the make the rest of the difference up with insulation. If you increase the thickness of the screed, the longer it will take to dry out hence why I recommended to increase the insulation thickness.

If you're going down the timber route, then you can use 50 x 150 joists at 400mm c/c with 22mm thick T&G flooring on top, which would take you up to or near enough the kitchen floor level.

Check with your BCO as to the insulation thickness required to achieve the current u-values. If you had plans drawn up by an architect/technician, then this would have all been detailed and spec'd for you. It sounds like you were asked to submit a Building Notice (i.e. no plans). These applications are normally for those who know what they're doing, know what insulation to put in, etc... and can just proceed.

And you should speak to your BCO about all those queries above and any other as that is what you have paid them for

 

[bounce111158]

wow! Thanks for all that.
If i go down the concrete route could you please explain the order i do things,i.e. first lay the damp proof membrane....

 

[DevilDamo]

I've just told you

From the bottom up...

- Concrete slab
- DPM
- Insulation (with perimeter insulation on exposed edges/walls)
- DPM (VCL)
- Screed

Or were you asking for a method statement as to how to actually construct the floor? If so, then I can't do that. Well, I can but I won't because that is what drawings and spec's are for. And... as mentioned previously, this is why a BN (Building Notice) was submitted. Maybe you should have gone down the FP (Full Plans) route and submitted detailed plans/spec's.

Speak to your local BCO. After all, that is what they're there for.

And also... don't be afraid to use the

button on here. You may get more help in the future if you use it wisely