Shower drain

We have a problem with our shower drainage that appears to be different from other problems posted on here.
When I first begin to shower, water builds up in the tray to a depth of one or two inches. After a few minutes, however, it clears and drains normally. For that reason, it is not a real problem as such but I find it intriguing as to the cause. The plug is like this one (but without the small holes in the top!):

http://www.onlinebathrooms.com/chrome-high-flow-90mm-shower-waste.html

This is a long-standing issue and I regularly clean out the waste, but that doesn't solve anything. The strange thing is that, after the first few minutes, the water drains quickly so I presume it cannot be due to blockage. Perhaps it's something to do with the temperature or the hydrostatic pressure of the waste water increasing after a time.

Does anyone have any idea why this happens, please?

It sounds like the action of a hepvo inline drain valve. What is after the shower trap, how long and what size etc.

Pete

Fluffster said:

It sounds like the action of a hepvo inline drain valve. What is after the shower trap, how long and what size etc.

Pete

Click to expand...

Thanks for your reply.
I had to Google that(!) but, having done so, no I'm sure it isn't. We have a normal U-bend beneath the outlet.