roof gully issue

PLain tile slips bedded in mortar have been with us since when. But i find no references to bedding fibre cement either as slips or as board lengths. Perhaps i'm wrong.
My understanding of HMT's post was that fibre cement boards were being glued with mastic to the valley lead surface and mortar was then being bedded on top of the board.
Your post confuses me. Do you mean attaching the fibre cement board to the lead itself, or bedding the board in the mortar?
You have misunderstood my suggestion ref. capillary action and the vertical batten business.

catlad said:

I have changed grp vallys for lead at least 5 years ago that doesnt make it a long lasting product to me

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Was this one you fitted? or by others? in what way had it failed?

I was called to newly built estate 18 years old at the time of the work as I was working on one roof I was asked to look at another both had the same problem split grp valleys they could have been badly installed in the first place but they were brittle when removed.

GRP valleys are practically indestructible. I have not comer across any that i have installed or any others that have arbitrarily 'broken down'.

This is not a dig at lead at all it is just that for the price and thickness of material they are surprisingly resilient. Many architects prefer them.

Even when off cuts have been buried in acidic soil they dig up as good as new!

Split GRP... either it's been folded in half or it's not GRP probably the old vynl valley trough used before GRP