+1 what Jon says
I was a sectional buildings erector for several years, dealing mainly with garden sheds, summerhouses and garages. Also slab bases.
First a good solid level base is key to long life of the building. Slabs laid on dry mix sharp sand and cement is perfect, as we used to lay the bases and erect the building straight on . Some spade work to get your ground level before laying your slabs on a good bed saves faffing on relifting/relaying your slabs because you ain't dug out enough.
Regarding sheds, you get what you pay for, cheap ply roofs or that OSB crap, waste of time. For a start the weather will twist it, heat and cold during the seasons, causing your felt to sag and come away. Also you can't knock s good felt nail in it without it falling to bits. Often they pop the nails out in no time. Your ridge felt then ends up ripping off in the wind.
Floors are crap too, no strength at all, so useless for bikes and stands.
Often with cheap sheds and on larger buildings, they don't have a roof truss to brace the roof, so they sag and you can't get tight felt runs, meaning it will flex and the felt won't last.
Often the felt supplied is like tissue paper.
A good shed company is Albany sheds in Northampton. We used to erect tons of these, heavy duty range, ideal for bikes. They are not cheap but great quality.
I also recommend using roof shingles on top of felt, I did this to mine and they last for ever.
Mine are fibre type, and have tar strips that the heat of the sun sticks them down. Looks like a tile roof. Fit and forget job.
Wickes sell them now.
Bet your looking at £1000 now for a good 10x8
cheers jacko