Let's get some of the basics right here- slates (natural and fibre cement) are not suitable on low pitches, that means anything below 27.5 degrees. Concrete tiles (depending on type and interlock) can be used on lower pitches, subject to manufacturer's details. Any roof pitch lower than 15 degrees is technically a "flat roof". Flat roofs have a bad reputation in this country, mostly due to poor design, substandard workmanhip or materials and inadequate maintenance. Flat roofs require regular maintenance (at least an annual inspection is generally recommended), this why flat roofs aren't well suited to a domestic build, where maintenance can be unscheduled and sporadic at best. Also flat roofs don't have the typical lifespan of a more traditional material like slate or tile- both of these can last for 60 years or more, flat roof memebranes can have a life of 15 to 30 years. Myself personally; I don't have a problem with a flat roofs in a domestic build where the roof is properly designed, the roofing product is a quality material with an Irish Agrement Cert., laid by a registered, accredited contractor and where the completed job is covered by an insurance backed guarantee. The problem is most domestic flat roofs won't meet this criteria- mickey mouse roofing contractors abound, using cheap materials for which there are no guarantees. Quality manufacturers like Bauder, Sarnafil, Sika Trocal, Moy Materials etc give insuarance backed guarantees when their products are fitted by their registered and accredited contractors.