How to raise garage floor height 130mm?

Question

I have an internal garage that i want to utilize as an additional room. I need to raise the floor height 130mm to match the rest of the house and the new entry door (nib was poured when garage door was removed)

Can i use a structural Polystyrene to add height?

Ideally i would lay damp proof membrane, Expol Structural polystyrene, tape the seams, lay OSB or ply, then flooring on top.

Does any one have experience with this using Structural Polystyrene instead of concrete?

*I have checked that there will still be 2400mm height after raising the floor

Nailbag · Answer

Hi @inthebuninging

Though not having any actual experience with polystyrene bases, I would agree with both @MitchellMc and @Dave that it would not be a suitable foundation structurally. The cost and effort to level such a low level difference to me outways the benefit. I have had a similar situation and chose to make a decent sized landing so that when stepping through the doorway there wasn't a sudden step in my case up in this small mancave.

Nailbag

Dave-1 · Answer

Afternoon @inthebuninging

I would strongly suggest not to use any Polystyrene as a support. Over time, compression, decay rate and a bunch of other factors would come into play.

I would install a false floor with osb and proper underneath supports. Install the Polystyrene in between the beams for imsulation but as a support it would be a strong nope.

Dave

MitchellMc · Answer

Welcome to the Bunnings Workshop community @inthebuninging. It's wonderful to have you join us and many thanks for your question on Expol Structural polystyrene.

I had a look at the Expol NZ website and see they offer a range of under-slab insulation products. These appear to be designed to prevent cold from penetrating up through a concrete slab, rather than for raising floor heights above a slab.

While it might technically be possible to use the product for your purpose, it doesn’t seem to be what it’s designed for.

Let me tag @Nailbag and @Dave-1 for their thoughts.

Please let me know if you have any questions.

Mitchell