Thanks for all of your help.

What I decided to do was to rout the dado into the rails, cut longer
spacers than normal, then trace the shape of the curved rail on to the
spacers (set up with the slats in finished order). This helped with the
spacers and the slats actually. A bit time consuming, but seems like
the best way to deal with the curve to me.

Chap


Chap wrote:
I am making an outdoor garden bench. Originally I was going to use a
straight top rail for the back, and use separate mortises for the slats.
I have decided to curve the back top rail, as I think it will look much
better.

Does anyone have a suggestion as to how I could mortise the slots to
accept the slats for the curved sections? I thought of routing a dado
along the bottom and filling in the spaces with wood spacers between the
slats, but I don't know how that would work in the curves. Wouldn't I
need to take into account the curve angle for the spacers? I don't know
how to do this.

Can anyone help?

Thanks.

Chap