For a typical UK domestic gutter, aim for a gap of about 30mm or less where practical. Treat 50mm as the maximum only if your gutter manufacturer allows it. Neither measurement guarantees that the gutter will catch the water; its width, position beneath the tiles and the roof pitch also matter.

FloPlast’s installation guide says to fit the outlet no more than 50mm below the roof tiles. This limit applies at the low point of the run; the gap elsewhere should be smaller. Without seeing your 30mm guide, I can’t tell whether it gives a stricter recommendation or covers a different system. [FloPlast installation guide](https://www.floplast.co.uk/uploads/related-documents/vqChg2jTaorA9euuY0SruczmrL0E1yXqi5peXcAm.pdf)

The main reason to limit the gap is to stop water overshooting the front of the gutter. In heavy rain, water leaves the tiles with forward momentum. The lower the gutter sits, the farther the water can travel outward before reaching it. Marley warns against lowering the gutter far enough for this to happen. [Marley installation guidance](https://www.drainageonline.co.uk/Content/files/11025/Marley%20Installation%20Guide.pdf)

For your installation:

- Use a modest fall. FloPlast specifies a maximum gradient of 1:350, or approximately 3mm per metre. Over an 8m run, that gives about 23mm of drop. If the tiles were level, a 20mm gap at the high end would become approximately 43mm at the outlet. [FloPlast FAQs](https://www.floplast.co.uk/company/frequently-asked-questions/rainwater-faqs)
- Set a string line and check the gap along the whole roof edge, including any uneven tiles. Keep a consistent fall towards the outlet, as following each tile’s irregularities could introduce dips.
- If the fall puts the gutter too low, consider a central outlet or another downpipe to shorten the falling runs.
- Check the horizontal overlap as well. The tile drip edge needs to sit comfortably over the gutter opening. A small vertical gap won’t compensate for poor horizontal positioning.

I’d also fit the felt support trays while replacing the gutters, if practical. They support the underlay and direct water from beneath the tiles into the gutter. They won’t stop runoff from the tile surface overshooting it. Make sure the tray edge will drain inside the gutter once fitted. [BMI underlay support tray](https://www.bmigroup.com/uk/p/underlay-support-tray-2479515345/)