Have you noticed in the past how heavily marketed garden office products have hit their marks and effected the mindsets of potential customers?
A decade or so ago suppliers were crowing about their garden rooms being double glazed so that was always one of the leading questions I was frequently met with during early emails, but now everybody else has caught up with us on that one, so other highly marketed products have taken their place, Sedum roofs do still come to the fore but nowadays, more and more, people are asking about EPDM rubber membranes as a roof covering for their garden room or office.
The marketing claims are therefore working well, I am not saying that it is bad product, not at all, just more expensive and much harder and expensive to repair if something goes wrong.
EPDM adverts state a "25 year guarantee" so what happens there if you make a claim?
Let's be practical here, with a real life situation. It is 20 years on, even just 10yrs on, your roof is leaking and causing damage, you need it fixing fast. If you can find the guarantee first of all, then what happens? Is the number the same as it was a decade or so ago? Will they answer and send a specialist repairer out almost straight away? Is there no small print to be wary of? Will you find that the materials are covered but not the labour and call out or is it the other way round, leaving you a bill anyway?
As old Max Bygraves used to say, "Let me tell you a story".
As a builder in the past I researched into why some felt roofs failed and others did not, in order to be able to give a 10yr guarantee on all my work.
There were two common types of application, one was to melt blocks of bitumen in a portable cauldron and pour it hot over the roof between layers of thin felt, not much thicker than paper, then (theoretically) to inhibit ultra-violet degradation a layer of white granite chips was applied to reflect the sunlight.
Bad idea. in practice, the window cleaner got onto the roof on a hot day to clean the windows on an adjoining gable and his feet pushed the granite chips further into the warm soft bitumen and pierced the layers and secondly, the roofs were often laid flat or almost flat, the theory being that this would cause puddling of water and this would cool the bitumen down so it would not soften.
Then came a new version of the felt itself, it was called "torch-on" roofing felt and it was a lot better. The felt itself was of a better quality in the first place and instead of pouring molten bitumen onto the roof, the felt itself had the heat applied by a large blowlamp and as the felt was torched it formed a molten bead sealing the felt to the roof and to itself, all seamlessly.
After that came rubberised or "polymerised" roofing felts which are very different from old felts, finally signalling the end of the granite stone chips, which were replaced by something far superior; a solar reflective coating. This coating consists basically of bituminised aluminium powder which becomes part of the polymerised felt and reflects the ultra-violet light which is the part of the sunlight spectrum that degrades the felt over time. The felt never sees the sun after that, the coating blocks all the sunlight out altogether. This means the roof stays cooler in the summer, much cooler than a black surface does. If you put your hand on an EPDM roof in the middle of the day, it is burning hot, not so with a solar coated felt roof.
We've done many roofs, all with long guarantees, with pretty well no issues (apart from a couple of easy to repair, loose flashings).
So that is probably the answer, give the roof some fall, to stop water build up, use modern rubberised torch-on felt with a solar reflective coating and you are home and dry, literally. With one really great advantage. Repairability.
With a torch-on felt roof, if it goes wrong there are dozens of roofing companies local to you that can come and sort it our quickly and cheaply, no chance of being fobbed off by someone in the claims department after weeks of waiting only to find there is some exclusion in the cover, meanwhile the damage has continued.
So there are pros and cons, but a torch-on felt roof laid to a fall with a solar reflective coating so that your garden office can be remedied quickly by a large number of local people, does have its advantages.
By the way - just in case you were wondering! - Ethylene Propylene Diene Monomer (E.P.D.M)