Can I use the Supports directly on GRP (Fibreglass) waterproofing?

Yes, however we would advise the use of a Sound Suppressant Pad as a medium between the support and the GRP.

As GRP is a hard membrane. The Pad reduces vibration as well as suppressing sound travelling through the floor to rooms below.

In addition, it will also ensure the reduction of audible gravel crunch when walking on the raised deck, should grit be washed under the support at any time.