Advice for mini extension damp issue..

tizzle6560

Will try to explain this as best as I can, but really in need of some advice here -

Attached to my ground floor flat, was a previously boarded up coal store/annex room, which I have now started to convert into a 'walk-in wardrobe' to the bedroom.

Having taking up the old flooring, half of which was just plywood boards above original joists (which looked to be in decent condition), the other half concrete/cement filled (as someone else before me had retro-fit an ensuite toilet in there).

The ground underneath looked OK at the time, no dampness but also no DPC.. I've since laid down new plywood boarding across the exposed joists but after some heavy rain I can see the plywood is getting damp from below.. The plywood I have used was NOT treated or marine ply (Yes i know.. :embarasse)

Short of ripping up the plywood, which would not be easy as have also fit plasterboard to all internal walls above it, what are my options to try to remedy this?

The only thing I can think of, is to make a small hole and fill the exposed under area with cement/concrete?

Any advice here would be much appreciated. And yes I know I messed up with not using marine ply..

FreeBear

Quite simply, you will not be able to fill the void completely with concrete through a small hole. In addition, without a damp proof membrane, you will only aggravate any damp issues - Building regs would also likely require 100mm or more of insulation to be incorporated in the slab.

To be able to give better advice, we need to know approximately how old the building is. Whether the walls are single skin brick, solid double brick, or cavity construction - On top of that, is there any evidence of a DPC, and has it been breached externally.

tizzle6560

FreeBear wrote: »

To be able to give better advice, we need to know approximately how old the building is. Whether the walls are single skin brick, solid double brick, or cavity construction - On top of that, is there any evidence of a DPC, and has it been breached externally.

Thanks for your reply on this.

Building from what I have been told, was built between 1910-20

It is solid double brick.

I do not believe there is any evidence of a DPC. Especially in this small annex room, which was simply boarded up and hidden away when the property was redeveloped into 3 flats about 30 years ago.

When you say i wouldn't be able to fully fill the void, the area in question is only about 1m squared in size.

TIA

tizzle6560

Was also wondering whether a couple of air bricks added to the external walls at ground level would go some way to remedy this at all..?

FreeBear

tizzle6560 wrote: »

Building from what I have been told, was built between 1910-20

It is solid double brick.

I do not believe there is any evidence of a DPC.

When you say i wouldn't be able to fully fill the void, the area in question is only about 1m squared in size.

Regardless of the size, you will not be able to effectively fill the void with concrete and include a DPM and suitable insulation. Being a solid brick wall, concrete would be an inappropriate material to use anyway. But then, the developers 30 years ago, and more recently, yourself, have probably slapped plasterboard & gypsum plaster on the walls.

Concrete floors and gypsum plaster on the walls will do nothing to help with managing damp in walls & floors that don't have a DPC. If anything, the materials will only aggravate the problem. That said, for a 1910-20 building, I would have expected to see a DPC - It may be slate or a course of heavy engineering bricks. Possibly covered by a cement plinth externally or buried under 100 years of increased ground levels.

If you really want a solid floor in this space, there are two options, both require removal of the timer & concrete floor.

Option 1) A layer of foamed glass, well compacted, and topped with limecrete - For more information, see https://www.mikewye.co.uk/limecrete-floors/

Option 2) 100mm of Celotx/Kingspan laid on a bed of hardcore & sand topped of with 75-100mm of concrete with a suitable DPM incorporated.

Option 3) Remove the existing concrete, fit suitably sized joists, and then board over the top (reclaimed floorboards to retain the period feeling ?). Add a couple of air bricks and make sure none of the timbers are in direct contact with brickwork or ground.