My own insurer was interested in whether the flat roof exceeded 25% of the roof area.
Many new designs of roofs at the start of the modern movement relied on the traditional craftsmen of the day in terms of their competence.
This resulted in several variations, including concrete shell roofs, layered felt roofs, shingle and slate roofs, brick vaults and metal roof coverings.
Asphalt became the economical covering of choice, laid on a separating layer or not, protected and reinforced or not.
Asphalts poor relation was the torch on felt roof covering.
Torch on felt has another relation, nailed on felt used for garden sheds.
IMO relying on a single application of torch-on felt or unreinforced asphalt or nailed on felt in Irish climate is a disaster waiting to happen.
Asphalt needs protection from structural movement, UV light, differential movement due to moisture [timber], heat [solar gain generally] and poor falls [cool areas under water adjoining hotter areas that are dry.].
Torch-on felt needs renewing on a regular basis, since it suffers many of the problems that asphalt does and has a far thinner layer for protection from the elements.
Nailed on felt is literally full of holes and even overlapped layers can work loose and admit water in a country where rain can blo horizontally [yesterdayin Dublin was one example].
Asphalt has been largely replaced by various rubber based roofing materials like Trocal etc, which have problems of durability and micro-penetrations, allegedly by builders foot traffic or nails.
The current vogue for new build domestic work rests with metal sheeting like zinc [or the old style copper or lead sheeting work] and the cheaper option of fibreglass.
Ths zinc is a more robust material but suffers visually of not protected from site traffic and damage before during and after installation.
Visual appearance is general held to be excellent but there are limitations as to the fall and the standing seam arrangement.
The fibreglass is somewhat less resistant in theory, but can take a knock without dinting or cracking, is painted with UV resistant paint and can be laid almost flat.
Visual appearance is arguably less impressive than a zinc roof, but equally doesn't register as a lack fo visual amenity either - its visually neutral.
Careful detailing in terms of fixing and dealing zinc sheets is matched by the need to correcting install the fibreglass on a suitable timber.
Depending on the extent of the roof, in both cases the size of sheet and the roof structure need to be carefully matched to the covering and its support.
Both zinc and fibreglass have their proponents and swear-by's but they have one similarity.
Insulation is under the roofing membrane and should therefore be installed with a continuous 50mm air gap between it and the timber substrate supporting the weathering layer [metal or fibreglass].
I will not go so far as to recommend any method - each has their merits and I have seen instances of both which seem to be well-regarded.
One contractor I spoke with has seen fibreglass roofs installed for up to 14 years without problems.
That at least matches a felt roof and is on course to exceed it.
That having been said I know of one felt roof that was instaled to traditional details using several layers of felt that has given good service since 1987 or thereabouts.
The torch on jockeys nowadays may no tbe able to match this.
That particular roof was overseen by a well-respected Architect when it was installed to the best of my knowledge.
The general principle is, like most things on a building site, its not so much what you do, its how you do them and who is inspecting the work that matters.
ONQ.
[broken link removed]
All advice on AAM is remote from the situation and cannot be relied upon as a defence or support - in and of itself - should legal action be taken.
Competent legal and building professionals should be asked to advise in Real Life with rights to inspect and issue reports on the matters at hand.