Attached to the end of our house is a small shed. It has four (newly built) brick walls and a bitumen corrugated sheet roof. The shed was finished about eight months ago and is fairly waterproof. However, in the last few months we've noticed, what I presume, is mould on the inside of the roof. Although it appears dry to touch, all our belongings are covered in drops of brown liquid (from the roof).

Could the mould be caused by a lack of ventilation or is there another plausible reason? BTW, I'm calling it mould for lack of a better term.

Response to the comments

I don't think it's tar (surely tar would be black?) Also, we've had this type of shed roof before (the old shed collapsed last year due to a burst water main), and this is just a replacement. Admittedly, the brick work is far superior in this shed.

Some pictures

  1. The first picture is of the inside of the roof. All the roof is now like this. When new, it was black.

  2. The second picture shows some of the brown drops of liquid on a box.

The underside of the roof. A box with drops from the roof.

  • 1
    What are you looking at on the inside of the roof? The bottoms of the corrugated panels? Is there sheathing? A ceiling? If it's just the panels, could the drops be melted tar dripping?
    – DA01
    Commented Apr 6, 2012 at 20:36
  • It's coming from the inside of the roof. Here is a picture of the roof: m.wickes.co.uk/mt/www.wickes.co.uk/roofing/… Commented Apr 6, 2012 at 20:50
  • What makes you think it's mold? Could it be the tar, itself? Unless that's a super air-tight shed, I can't imagine there being enough moisture, nor that being a surface mold would thrive on.
    – DA01
    Commented Apr 6, 2012 at 21:37
  • @DA01 It's not what I expect the tar to look like (see new pictures). Commented Apr 7, 2012 at 9:51
  • 1
    I had a leak near my chimney once, whoever installed the roof did not use flashing only tar. Anyway the water that leaked through was a similar brown color, not black as you might expect. So I wouldn't rule that out. Is this localized to a small area, or is it throughout the entire shed?
    – Tester101
    Commented Apr 7, 2012 at 13:53

2 Answers 2


It most certainly looks like tar residue to me... There are multiple latex paints that you could use to seal the tar in or you could attach a polyurethane sheeting of at least 3mil to the underside of the roof to protect your belongings.


It turns out that moisture was the problem. The water beaded on the ceiling and dripped down. The shed needed additional ventilation.

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