Felted roof, loose grit coming off when raining

[jam_rs]

I have a lean to conservatory that had a plastic sheet roof, that was heavily leaking, so we replaced the roof frame and felted it.

When it rains I to get a lot of loose grit fall onto my patio. It seems to miss the gutter somehow, water into the gutter isn't an issue. (The gutter does get some grit, but not alot...)

I'm no expert in roofing, complete novice! Done underlay tacked down, torched on 2nd / 3rd layers & drip edgings. Other than this issue, completely happy with my result! And most importantly, no leaks!

My thoughts, possibly one / some of the following;
- Should expect a little bit to come off
- Top layer is meant to be a slightly different one
- Poor quality felt
- Missed a step when doing the job

Open to suggestions, its not a massive problem, but annoying when having to sweep up the patio all the time!

Thought i had a couple of pics of the finished job but don't so heres a few anyhow;

Old:

Halfway:

Allmost finished:

 

[blup]

I know from experience there are big quality differences in domestic shed felt, plus the weather is much harsher these days.

Blup

 

[foxhole]

If it’s new all the loose grit needs to wash off before it stabilises.