On 31/05/2018 10:41, Roger Tonkin wrote:

I have installed a water butt to collect the rain water from a
flat felted roof over the garage. (Only place I could get to a
downpipe that would not obstruct access).

Now the butt is full, water coming out quite yellowish, so I am
concerned that may be roof felt may contain things harmful to
plant - particularly in my case orchids.

Any one know?

Mine runs clear of a clay pantile roof. I have had mine go green if the
lid blows off but it is otherwise pretty clear. Could be rust possibly
as rain brings down iron micrometeorites in the gutter sludge.

You can extract them to look at under a microscope using a small Nd
magnet to grab onto the iron content.

--
Regards,
Martin Brown