We have recently had a garden building built in our garden by a bespoke company. However the roof has no gradient at all so we have standing water when it rains. Is this ok?
No, this is not OK. Standing water will deteriorate any membrane and eventually cause the roofing and roof structure to deteriorate.
The wetting and drying process puts stresses on the roofing. The edge of a “puddle” has different coefficients of expansion and contraction. This different movement will reduce the life of the roofing.
Good roof design is intended to shed water. (There was a short time in the 1980’s that designed a “pond” on the roof to create an insulated barrier to the elements, but we soon learned that any imperfection will allow the entire “pond” drain through any pinhole.)
Steep roofs seem to perform better and lasts longer than flatter roofs. (Have you ever noticed that old barn roofs don’t seem to leak, even though you can look up and see daylight coming through the roof?)