Replacing flat roof, like-for-like

[Gallah]

Live in East Lincs and believe my27ft x 7ft flat roof was fitted in 1975 so lasted over 35 years before the felt deteriorated.Have left it a bit too long since problems began as was planning to pass problem on when selling up.Change of plans so need to replace as cheaply as possible which rules out rubber and fibreglass.Am I right in assuming felt is the cheapest method?Do I have to invest in gal bucket,tar blocks,propane burner....is that the way it`s still done?
Fitting trimmed down 8x4 ply should be straightforward once I`ve double-checked/replaced any rotten joists.At 16" joist centres what thickness ply sheets would I need to prevent any flex when walking on roof?Is there a cheapo one-stop flat roof supplier incl delivery on the net?
Thanks.

 

[noseall]

You should also consider a warm deck insulation system which is likely to be another hefty cost.

 

[Gallah]

In an ideal world, excellent suggestion .Unfortunately there are plenty of other things to catch the critical eye of a buyer looking to purchase this property before the pluses of a bit of insulation are considered.Very old cottage which I`m not prepared to spend a penny more than I have to.
In this instance I`m not looking for more outlay as doing this on a tight budget.Aim is to spend as little as possible and get out.
Any ideas to this end appreciated.
Thickness of ply?Type of ply,eg waterproof?Type of felt?Felt fitting process for a 27x7 foot area?Best place to buy cheapest price materials?
Cheers.

 

[noseall]

Fit furrings, 18mm roofing ply and cover with three layer felt system.

 

[Gallah]

Thanks Noseall,so that`s what they`re called then,those tapered strips.Hopefully that`ll help the run-off of water.
I will google the 3 felt layer system to see how it`s done.
Cheers.