Window Fitters Guide to Window Installation 
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“H” Mulls and Snap Mulls
are Mechanical Mulls,
designed to join two or more
windows where the windows
do not need to be structural
or support any weight.
Stacking two windows, one
on top of the other, where
those windows can be
anchored to the surround-
ing rough framing is accept-
able for most residential
applications where window
pressures are not near hur-
ricane or even tropical force.
When windows (and doors)
are joined in an area near
the ocean or where high
winds can be expected, a structural mull is required to meet most
building codes.
A Structural Mull is usually a piece of steel with legs at top and
bottom that allow the mull to be securely fastened to the header
and sill - or even to the jambs where a horizontal mull is required.
Then the window units are anchored to the mull with high perfor-
mance fasteners so that the finished assembly is a structural unit,
secured to the surrounding framing and forming a weather and
wind resistant fenestration unit.
Where codes demand it, the structural mull is used in testing of 
the window and door units themselves in order ot get a rating 
which then is used to determine compliance with the required 
performance for the climate and region.
When it is necessary to install window and door assemblies this
way, there are usually engineering drawings showing the proper
mull size and anchoring details of both the mull(s) and the window
or door units.
Structural Mulls
SAMPLE MANUAL

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