Summer House refurb - Warm Roof

[fatmcgav]

Afternoon all

We've got an existing Summer House at the bottom of our garden that's in need of some TLC.
The intention is to try and make the room more usable all-year round, and possibly use it as a small photo studio.
Therefore it will need permanent electrics, and ideally some insulation in the floor and ceiling to make it a bit easier to heat in colder months.

The Summer House was installed about 10 years ago, and is 3.5m wide by 2.7m deep internally, with a 1.2m front overhang.
From memory, I think it's made from 22mm tongue and groove pine boards, with a pair of french doors and a single top-opening window on front. The ceiling is also pine boards, 12mm or 18mm I think.

I've noticed some damp patches on the roof, and the side facia boards are rotten through and falling off, so those need to be addressed.
Then I thought that as I've got to remove all the felt, I might as well insulate from the top and make it into a Warm roof construction.
That has added benefit that I can fit some downlights internally without having to worry about compromising the insulation.

So, some questions on the roof:

1. Assuming the pine roof deck boards are all sound, is it worth fitting a Vapor Control Layer (VCL)? There is plenty of moisture in the boards already, so is that likely to impact?
2. Is 50mm Celotex overkill? Current intention is to use 50mm Celotex, just because it seems the easiest to get hold of, and isn't much more expensive than 25mm or 40mm. Added benefit is that any left-overs can be used for floor insulation.
3. Plywood or OSB for top-deck? My current intention is 9mm WBP Plywood.
4. Shingles - Currently, I like the look of Coroshingle based on price and ease of fitting, but any other recommendations?
5. Internally I plan to over-board the existing 90x45mm rafters to create a void, into which I can fit LED downlights. As the rafters are at 600mm centres, I presume I best off using 12.5mm tapered-edge plasterboard? Using tapered-edge so that I can easily fill the joints and sand smooth before painting.

So, current plan of attack:

1. Trim back all the overhanging tree branches.
2. Remove the existing felt roof and inspect the roof deck for any rot etc, treat roof boards or replace as appropriate.
3. If required, fit VCL.
4. Batten around gable edge and lower edge.
5. Fit Celotex boards, and tape joints
6. Top with plywood and screw down to roof deck
7. Fit roofing shingles as per manufacturer's instructions...
8. Fit new treated facia boards
9. Internally over-board rafters with 12.5mm TE Plasterboard, fill and sand.
10. Paint ceiling.

Anything I've missed, or any glaring issues with the above?

Cheers in advance for replies.

 

[flameport]

You are probably wasting your time.
For an structure to be usable all year, 50mm of insulation is woefully inadequate, even if applied to the roof, walls and floor.
It's unlikely the structure will be capable of properly supporting the substantial extra weight of the ply, plasterboard and insulation you intend adding to the roof.