It’s long overdue but I’m going to replace the leaking patched up felt with pond liner I think having read another thread.
Under is 18mm ply, unfortunately there is no insulation and I’m thinking of adding some.
So, what to do…..
It’s going to be a nightmare trying to get the old felt a bitumen off so was thinking of going over the top of it.
Would I need to put a dpm or something on top of the felt before the insulation or pond liner?
I was thinking of using 50mm Celotex or similar then putting the pond liner & fabric on top.
Then that brings up the question of how to fix the insulation board to the ply under the felt?

I got felt up in the shed once.

It’s going to be a nightmare trying to get the old felt a bitumen off so was thinking of going over the top of it.

Be mindful of the growing weight on the roof.

That said, a neighbour* fixed his leaky felt roof by felting another layer of roofing felt over the existing felt, because lowest cost solution.

* yes, really.

If its leaked I’d just replace with some new 18mm OSB3 then stick a EPDM on top and insulate underneath.

I got felt up in the shed once.

Hope he was worth it.

Be mindful of the growing weight on the roof.

Hadn’t thought of that…..

If its leaked I’d just replace with some new 18mm OSB3 then stick a EPDM on top and insulate underneath.

I don’t think it’s been leaking that much and the ply looks ok, but if I need to take the felt off I can inspect then. Would use ply rather than stirling board.
I did insulate inside uder the ply with celotex boards but got really bad condensation so took it out.

Last summer I tore down the old shed to build my new garden shoffice.

The old shed had 8 layers of bitumen roofing felt down – I think the owners jsut put a fresh layer on every couple of years.

I think you’ll be reet putting a second layer on – weight wise anyway

I put 25mm Celotex on top of t&g roof boards, with Onduline corrugated screwed down through into roof bearers – but obviously that’s for a pitched roof. Think I might be inclined to do what stevextc suggests, for a flat roof.

If its leaked I’d just replace with some new 18mm OSB3 then stick a EPDM on top and insulate underneath.

Can you explain the ‘insulate underneath’ please mate?

bang DPM over the bitumen as a cheap vapour barrier, then the celotex then some 12 or 18mm ply or OSB, then the bond liner which is highly likely to be EPDM rubber. Dont forget the edge detail. And you will need to glue the rubber down to the board.

How do you know the roof isn’t rotten?

I’d remove the current felt to check and replace any damaged wood first before I start spending any money on it.

It’s quite normal to overlay existing felt, provided that you’ve established the existing is well bonded and that there’s no trapped water underneath in the existing substrate. Just cut some cores where you suspect water ingress to check condition of the deck.
Why do you want to use a pond liner/EPDM?
I have seen some really bad installations of EPDM. Not many seem to be able to install it well, trickier to detail and do laps.

Modern felt is good for 30 years.Get advice from a reputable manufacturer like Iko .

Use a PIR board for insulation,Recticel works well.
Most new builds and commercial re roofs use a Pu adhesive to fix the boards. Soudal is perhaps the most commonly used.

EPDM is easy to install if it is on fresh wood and you can get a strip wide enough to do the roof in one piece. It is also great protection. We have a big oak tree over the cabin in the garden and I have watched branches fall 40 feet onto the roof and literally bounce off. Not sure how easy it will be if you have to glue lots of bits together but I wouldn’t have thought it was any worse than felt.

I think this was a 4m wide roll and fitted to a 3.6m x 4.8m cabin on my own. Placed there 1 day to relax and then glued down the next with all the plastic edge trim etc.

Thanks guys, will look into the info.

bang DPM over the bitumen as a cheap vapour barrier, then the celotex then some 12 or 18mm ply or OSB, then the bond liner which is highly likely to be EPDM rubber. Dont forget the edge detail. And you will need to glue the rubber down to the board.

This was my initial thought tbh.
Don’t want to use felt again, period. Want to try one sheet of roofing think it’s 7x5m but will measure up.