In this question, the Questioner has a damp spot below but seen no sign of water on the framing/sheathing. Roof Leak without any Visible Shingle Damage or Issue

What would be a good material to dust onto the framing that would leave a track when water runs over it?

I thought perhaps chalk dust, or talcum powder. Flour seems inadvisable as it's pretty much spreading bug food. I know there are fancy "water indicating paints" used to show water at the bottom of oil tanks, but those seem rather expensive and specialized for the purpose.

  • Think any fine dust that is cheap would do, that sticks well to dry wood. Problem would be covering the surface without having a lot of dust in the air. Imagine this would be for a known time that the leak will occur. Chalk line containers might be best, limited dust, good lines laid down. Water should make a trace though the lines.
    – crip659
    Apr 14, 2022 at 0:34
  • cool-aid drink mix .... sheet of paper with text printed by an inkjet printer should also work
    – jsotola
    Apr 14, 2022 at 2:10

1 Answer 1


In your question you suggest talcum or chalk, and this is what I use under sinks, in cars etc..

Kid's chalk comes in a variety of colours, which is handy if you need to trace against a white background. You can dust it on or draw it on in horizontal lines.

Sometimes I'll wrap or tape on a piece of tissue or kitchen towel, when I don't have the above powders at hand or when it's difficult to blow the powder on with proper aim such as deep under a sink, or all around a piece of pipe.

I also use soot dust (from burning some paper or a scented burn stick from my spouse's yoga department) or graphite from a dispenser in areas where I need strong contrast against white, of if I have neither of the above at hand.

For a long lasting mark you could use drywall setting compound, which would mark the water trail and then harden for inspection much later. I haven't needed this, but always thought this trick could be of use somewhere. Possibly cement or grout could be used too.

The benefit of talcum or chalk, and to some degree soot, is that they can be used in small amounts without too much worry about toxicity and lung irritation. Uncured drywall, cement and grout powder are corrosive and the dusting should not be breathed in.

  • Not that it matters, but kids chalk, at least the fat ones used for drawing on the ground, is actually plaster
    – Chris H
    Apr 14, 2022 at 11:12

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