**Use your gutter manufacturer’s maximum, and aim for a smaller gap where practical.** For a typical UK PVC gutter, FloPlast specifies fitting the outlet no more than **50 mm below the roof tiles**. A 30 mm limit gives more allowance against water missing the gutter, but neither figure guarantees that every roof arrangement will work. [FloPlast installation guide](https://www.floplast.co.uk/uploads/related-documents/vqChg2jTaorA9euuY0SruczmrL0E1yXqi5peXcAm.pdf)

The main reason is **water overshooting the front edge**. Water leaving a tile has outward momentum; a lower gutter gives it more time to travel outward before reaching the gutter opening. Roof pitch, tile overhang and gutter width therefore affect the acceptable gap. The tile ends should discharge approximately into the centre of the gutter. [Marley roofing guidance](https://cm.marley.co.uk/-/media/files/brochures/marley-roofing-sitework-guide.ashx?rev=775fbfcd691d423799e9e964d70922fc)

For your replacement, I would:

- **Aim for about 30 mm or less where practical**, subject to the chosen system’s instructions and clearance from the tiles.
- **Check the gap along the whole run**, particularly at the outlet and beneath tiles that sit higher. Uneven tiles can make the largest gap occur somewhere unexpected.
- **Use the specified fall.** FloPlast recommends 1:350, approximately **2.9 mm per metre**. An 8 m run therefore needs about **23 mm of total fall**. Set brackets with a taut string line or laser.
- If that fall makes the low end too far below the tiles, consider a central outlet or additional downpipe to shorten the drainage runs. FloPlast also permits level installation, provided the system has sufficient capacity. [FloPlast technical guidance, §15.10](https://www.floplast.co.uk/uploads/pdf-links/lZzGnZ9wTJYz2weWPdcKdZ5AvctlvPPugYLvcRWn.pdf)

**Plan the felt support tray now, ideally fitting it with the gutters.** Its purpose is to support the underlay and discharge water from beneath the tiles into the gutter. It must overlap correctly beneath the underlay and reach the gutter; it does not solve water overshooting from the tile tops. [FloPlast eaves protection guidance](https://www.floplast.co.uk/uploads/related-documents/floplast-roofline-brochure.pdf)

A hose test before finalising the installation can reveal obvious misses and pooling, though heavy, wind-driven rain remains a tougher test.