Has anyone designed a dynamic roof block which is constrained by just by width and roof pitch. Just a simple triangle would do!! thanks. I do not know how to control line length when it is dependent on a variable angle.

If I understand your needs :
It's possible to deal with x, y ( and z ) parameters of a bloc ( CTRL + 1 to open the properties control pannel ).
If you want to have 1 dimension informaticly correct you can try the command : align with "adjustment" of the object scale ( you have to choose "Y" for yes just at the end of the command ( then you can come back in the properties control pannel to set the two other geometry scales to 1.
Hope this will be clear and/or usefull.

Thanks very much - what i am after is something which will draw a section through a roof or similar shape where you know the width and the angle of the roof but not its height. It doesn't seem possible to do it simply as a dynamic component with only two controls.