Create a tapering thickness with surface.

Hello,

I'm having trouble creating a thickness for this roof surface. I know conventionally i could use offsetsurface. but that'll give me a constant thickness. i need to have it thicker at the top and taper outwards as it goes down. 

in the model i've attached i have the section of what i want it to look like drawn in red curves. the thickest portion wants to be something to the tune of 2~ meters and and the thin edge to be about 300mm. (even tho in the model it is 900~mm)

just wondering if y'all had any ideas.

Thanks!