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You never design for D + L. You design for 1.2D + 1.6L.
Structural codes do not let you add loads and call it a day: each load type gets a factor reflecting its uncertainty, and the designer checks every combination to find which governs. This calculator runs the common ASCE 7 LRFD combinations on your dead, live, snow, and wind loads and flags the governing case — including the easily-forgotten 0.9D + 1.0W uplift check.
Structural engineers use it as the first step of every member design — which combination governs sets everything downstream. Students use it to stop mixing LRFD factors with ASD capacities, the classic exam and rookie error.
Uncertainty. Dead loads are knowable to within a few percent, so the factor is 1.2. Live loads vary wildly, so 1.6. The factors come from reliability analysis calibrated to a target probability of failure — they are statistics wearing a hard hat, not arbitrary padding.
When wind lifts the roof, dead load is the only thing holding it down — and assuming you have MORE dead load than reality is unconservative. So the code reduces the resisting dead load to 0.9D while applying full wind. Underestimating your anchor is how roofs leave buildings.
LRFD (strength design) factors loads up and compares against reduced member strength; ASD keeps loads unfactored and divides capacity by a safety factor. Both are legal under ASCE 7; steel and concrete codes each have both paths. Do not mix factors from one with capacities from the other.