Transparency when overlapping?

Hey there construct community. I'm trying to make it so that when a player enters a building the roof texture becames very transparent. How do I edit the opacity of a layer through expressions?

You use the function:

System > Layers > Set opacity. Then enter the name or number and in your case, 0 for transparent.

siron, but doing this would cause the entire layer to become transparent...

It would be helpful to use the System event for "is overlapping" and set:

Event: when the "roof" sprite and the "character" sprite are overlapping

Action: "roof" sprite Opacity=30

...this way, only your roof is effected.

the_chosen_byte

I meant if he used layers like

1-HUD

2-Roofs

3-Level

He could make an entire Layer (only the roofs) invinsible.

He said Layers in his text, but maybe meant sprites <img src="smileys/smiley2.gif" border="0" align="middle" />

Thanks siron and the_chosen_byte.