Height of inner leaf with wall plate relative to outer leaf.

[Allex]

I have a pitch of 18deg. how do I work out the difference in height between the outer and inner (incl. wall plate)? The cavity is uninsulated (old build) and the rafters are 100x50. Want a minimum overhang. Thanks.

[nod]

I normally make a triangle of the relevant pitch and simply sit it on the wall plate and hang the long leg over the cavity and outer leaf and measure up