4A.6.18 FLIGHT IN RAIN
Performance deteriorates in rain; this applies particularly to the take-off distance and to the maximum horizontal speed. The effect on the flight characteristics is minimal. Flight through very heavy rain should be avoided because of the associated visibility problems.
WARNING
Poor maintenance condition of the airplane, deviation from the given procedures, uneven runway, as well as unfavorable external factors (high temperature, rain, unfavorable wind conditions, including cross-wind) will increase the take-off distance.
CAUTION
The figures in the following NOTE are typical values. On wet ground or wet soft grass covered runways the take-off roll may become significantly longer than stated below. In any case the pilot must allow for the condition of the runway to ensure a safe take-off.
NOTE
For wet grass, an additional 10 % increase in take-off roll must be expected.
Other then the note on wet grass the DA42 manual has NO wet runway performance numbers.
If it's paved we don't need to worry too much
Care to share what this nugget of wisdom is based on?
The manufacturers are not required to provide the performance numbers.