All truss foundation unit, method of making the same and foundation unit kit

A foundation unit for supporting a mattress, said foundation unit comprising:

• a plurality of elongate cross trusses, each cross truss extending between a first end and a second end, each cross truss including a bottom slat having a first end and a second end and a top slat having a first end and a second end, said top slat having a generally flat top surface, said top slat being spaced apart from and above said bottom slat, each said cross truss including a first top notch at said first end of said cross truss adjacent said first end of said top slat, a second top notch at said second end of said cross truss adjacent said second end of said top slat, a first bottom notch at said first end of said cross truss adjacent said first end of said bottom slat, and a second bottom notch at said second end of said cross truss adjacent said second end of said bottom slat;
• a first bottom side rail and a second bottom side rail, each bottom side rail having a first end and a second end, said first bottom side rail adapted to be located in said first bottom notches of said cross trusses, said second bottom side rail adapted to be located in said second bottom notches of said cross trusses;
• a first top side rail and a second top side rail, each said top side rail having a first end and a second end and a generally flat top surface, said first top side rail adapted to be located in said first top notches of said cross trusses with said flat top surface of said first top side rail being generally coplanar with said flat top surfaces of said top slats of said cross trusses, said second top rail adapted to be located in said second top notches of said cross trusses with said flat top surface of said second top side rail being generally coplanar with said flat top surfaces of said top slats of said cross trusses, said flat top surfaces of said top slats of said cross trusses and said flat top surfaces of said first and second top rails forming a generally flat top surface of the foundation unit adapted to support the mattress;
• and
• wherein each cross truss includes a pre-stress and at least one center support located between opposite end supports, said pre-stress comprising said at least one center support having a vertical dimension greater than vertical dimensions of said end supports.