Thin liner for fire place opening for shelves

I'm pretty sure that I've asked this before, but my Googling doesn't find it....
I have a space where a fire place used to be. It will be a HiFi/AV cupboard. The walls are part brick and there is also some old cement render which is reluctant to come off. The brick steps inwards by a small amount at the back for the top 200mm of the 80mm high opening above floor level.
Maximum width is about 520mm. Usable width after lining should be at least 460mm to accommodate HiFi separates. So I need to provide a smooth finish which doesn't come in more than about 30mm either side and which can also support shelving. I am picturing some kind of board which can have holes drilled in to take push in shelf supports - much like the Ikea chipboard cabinet I have just by me. I want a board because then I can drill a vertical row of holes for the shelf supports and have very flexible adjustment of whatever kit I have now or buy in the future.
So - very thin skim of render to give a flat surface then 18mm board (OSB, WBP or similar) screwed to the brick?
The back of the opening is double brick (no cavity) to the outside so there may be a small chance of damp penetration - in which case will a render with some kind of waterproofing liquid mixed in help at all?
Cheers
Dave R
--
Windows 8.1 on PCSpecialist box

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 10/01/2015 14:10, David wrote:

Forget the render, screw a couple of roofing battens vertically and screw the ply to that, leaving a small gap at the bottom so that air can circulate to prevent damp buildup? Roofing battens because they are pressure treated and very resistant to rot.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

That's unusual - the back of a fireplace in a non-cavity external wall is almost always a half-brick wall (4"), so it's recessed back further than the walls outside the fireplace.

Fireplaces (including the chimney and hearth) often have no damp course or damp protection at all - they don't need it with a fire lit in them every few days. Without a fire, they then get damp.
I did exactly what you're looking to do, and it has the HiFi in it, although sitting on the floor.
I dug out the base of the fireplace down a couple of feet (was just a layer of cement on rubble and earth). Lined it (sides and bottom) with damp proof membrane, and filled it vermaculite and mortar (almost dry mix to retaining plenty of air) to within 4" of the top (to provide insulation which would also be OK if it was ever turned back into a fireplace), and then a mortar screed on top.
On the back wall, I used 25mm celotex (what I had to hand), with plasterboard on top, edges of the plasterboard wrapped with waterproof tape to prevent them wicking in any moisture from the sides. (If the fireplace was put back into use, this would need stripping off.) For the sides, I use a scratch coat of render with waterproofer (as the chimney has no damp course and measures as damp), with standard gypsom finish coat plaster on top (and on the rear plasterboard), which has never abosorbed any moisture through the render and measures dry with a damp meter. Also fitted a mains socket to one side near the back.
I sealed off the chimney opening with a piece of plasterboard with fibreglass insulation on top. The fibreglass is not only insulation, but also a shock absorber for bits of masonary which occasionally drop from the insides of old chimneys. Above the insulation, I took out a brick from the flue to the outside and fitted an air brick, so the flue remains vented, but to the outside rather than the room. The board is recessed above the top of the fireplace opening, so it has room for a concealed light to light up whatever is in the fireplace. Aerial wires from roof aerials are dropped all the way down the flue.
Did this nearly 15 years ago, and it has stayed dry - no sign of any penetrating damp or condensation, both of which are risks if you don't get the construction right.
--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Sun, 11 Jan 2015 11:00:59 +0000, Andrew Gabriel wrote:

Thanks - very useful.
I'm pretty sure that the wall is double brick but I may be wrong - the rest of the wall is cavity but I as you say you don't need that behind a fire. Certainly no cavity behind the fire.
I was going much the same route as you for blocking the chimney, except planning an external door above the seal to allow limited maintenance access. I plan to run some cables up the chimney a bit then through into the room to allow concealed wiring for the TV. Potentially quite a few wires and other cables.
The fireplace is already completely cleaned out down to the concrete raft which gives me some space to work with.
There is a DPC around the fireplace.
I need wooden board instead of plasterboard or just render for the sides to allow flexible mounting for the shelving. I also need to try and keep the side linings slim to allow reasonable room. I suppose I could render then fix vertical battens and notch the shelves (or leave an air gap up the sides for ventilation).
I am reluctant to put much up the back wall because the fireplace is already shallow and the kit will project into the room - I want to be able to put glass doors on the front so I have to build some kind of wooden fire surround to bring the whole thing forward. Maybe something like Marmox Multiboard could be an option.
Anyway, thanks again for your help.
Cheers
Dave R
--
Windows 8.1 on PCSpecialist box

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.