CEMP-ET
TI 809-53
01 May 1999
b. Low-Sloped versus Steep Roofs. As slopes increase greater down-slope loads are placed on
supporting clips. They must be designed to handle such loads. Sliding clips will depend upon method of
restraint as gravity force can cause slippage. Architectural panels are common on systems all the way to
vertical; curved panels are also available.
c. Low-Sloped Roofs. Some structural hydrostatic panels have been used at slopes as low as 4%
(1/2 in./ft.).
d. Steep Roofs. Usually colored panels are used since steep roofs are highly visible. Sliding snow
can be a big problem. For simple shed, gable and hip roofs without penetrations or obstructions, it may
be possible to let the snow slide to the ground. However, sliding snow can peel apart seams in valleys,
tear off vent pipes, damage other roof features and create hazards. Snow guards distributed over the
surface of the roof must be quite strong to hold back snow. Guidelines are presented in EI 01S011,
Commentary on Snow Loads.
e. Structural Considerations. With nonstructural metal panels the deck below provides the
diaphragm needed to resist lateral loads. Structural panels supported on clips do not resist lateral loads.
Either cross bracing or a supplemental deck is needed.
f.
Expansion Joints and Seismic Joints.
(1) Joint Location. When the longitudinal dimension of the roof exceeds 30 to 40 m (100 to 135
ft.) step expansion joints are used (figure 9-3). The maximum allowed run will depend upon clip design
(permitted movement), color of panels (dark colors absorb more solar load), climate, and metal used.
(Aluminum has roughly double the coefficient of thermal expansion of steel.)
(2) Panel Movement. Expansion in the transverse direction is accommodated within the
trapezoidal or vertical side lap itself. Panels must be anchored at some point to resist the down-slope
component of design loads. Panels are usually pinned at the eave. They expand up-slope towards
flexible ridge covers. Pinning or fixing at eaves makes waterproofing a simpler task at that critical
location over which all water flows and ice dams may form.
(3) Restrained Panels. Through-fastened panels, as opposed to clipped, are restricted to
relatively short runs. As runs increase, the accumulated thermal movement may increase stresses at
through-fasteners resulting in elongated fastener holes or panel buckling. With time this results in leakage
and corrosion.
g. Re-entrant Corners. At such locations points of fixing may shift among adjacent panels. Design
details are needed; special closures may be necessary.
h. Roof Access. Roof hatches must be treated like other curbed penetrations. Most metal roofs can
resist only light foot traffic. Walkways may be needed if more than casual traffic is expected.
i. Roof Venting. Architectural systems are relatively easy to ventilate when an attic space is
present below the deck or by airways created in a cathedral ceiling. For hot, humid environments
installation of a plastic film directly beneath the roof panels minimizes the potential for bottom-side
corrosion. Structural systems that contain batt insulation between the panels and the frame are difficult
to ventilate. This limits their applicability in very cold regions and for high humidity occupancies.
j.
Roof Decks. Architectural systems require decks for continuous support; structural systems do
not. However, decks may be used with structural systems for other reasons (e.g., to provide diaphragm
action).
9-4
Previous Page
