PART XI SCAFFOLDS, STAGES AND WORK PLATFORMS — Inclination and support
152
Where a portable single or extension ladder is in use
152(a)
the ladder shall be placed so that the horizontal distance from the base to the vertical plane of support is approximately one-quarter of the ladder length between supports; and
152(b)
the lower ends of the ladder side rails shall rest on a firm and level base and the upper support of the side rails shall be rested on a bearing surface strong enough to safely withstand the applied load.
Not legal advice. Verify the cited article against the signed agreement.