1/ Do the garden sheds you are considering have an all timber roof or it is the man made chipboard, OSB board or sheet materials? Chipboard is by its name made from wood chips stuck together - OSB (sterling or wafer board) is made from wood shavings and stuck together- which is fine for kitchen and bedroom cabinets covered in melamine - but it is NO USE whatsoever in a shed in the long term. OSB board can be adequate in certain circumstances however if any of the board is cut (which nearly always has to be to meet the size of the shed) then this removes the special waterproof paint which had been applied to the edges. The cut edges of the roof is then prone to swelling when it gets wet and then that is the beginning of the end of the roof. A roof should be solid T&G timber to ensure a long life.
Also notice that most companies offering chipboard or OSB boards describe them as solid sheet materials as they are too embarrassed to tell you what you are really getting and they normally exclude the roof from any warranty over the minimum period as they KNOW these boards won't last (Ask them the question).
2/ You have to cover it with the roofing felt - how do you do that? - you put clout head nails through the roofing felt into the chipboard or OSB boards which again - potentially if you do not align the nails correctly - allows water to get into the chipboard or OSB board. When this happens you can expect the roof to shows signs of letting in water and disintegrating. (literally).Why take the chance??
3/ Also check the grade and thickness of the roofing felt and also ensure that all fascia and barge boards are included to finish your shed off. Felt should ideally be at least 40kg weight.
All our roofs are made from solid T&G timber boards, we do NOT use chipboard, OSB board, sheet material or any of these other cheap alternatives as they are NOT suitable for outdoors use. On the Platinum we do offer the top of the range Onduline roofs which has a design life of at least 15 years. All fascia and barge boards are included.
Further articles about garden sheds, workshops and the type of timber cladding you should use to follow. Also about the truth behind so called 10 year guarantees