Its ok for a shed lol!!! If your looking for a flat roof covering for an extension over a kitchen then it has to be EPDM (one sheet, no joins or seams) Or a high tensile felt.
I used a wickes self adhesive system on a shed in the garden. 6 years on, it's still in perfect condition and has NO leaks.
So, if you want to be cheap and save some money, it can be done and systems are available. Just keep in mind it should not last as long as proper systems.
OSB3
Paint Tar on it
Put on the two layer wickes system
However, whenever I do flat roofs now. I used a 3 layer system torched on.
I dont think you will get many years out of self adhesive felt, Id only use it on a shed.
As previously been said, fit an epdm roof on for peace of mind.
Although torched on felt done correctly will last many years.
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