Reasons to not use tapered seams
This panel is correctly formed with a 90 degree bend for the turn-up of both the male and female legs.
A frequent misconception is that the legs should not be 90 degrees. The resulting taper leaves a gap for the panel to expand along its width. This is incorrect.
The absolute MAX allowable spacing for proper seaming is:
1" high standing seam = 1/16" gap
1-1/2" high standing seam = 1/8" gap
There are two reasons for this.
The First Reason:
Metal panels up to 20" wide will expand .0348" - that's under 1/32" on a normal width panel!
Note: This calculation is based on zinc - the metal most susceptible to thermal expansion. It is also based on a temperature change of 100 degrees F. You would need to install on the coldest day of the year to even get 1/32" expansion.
The Second Reason:
Tapered seams inhibit proper seaming.
When a power seamer is placed on the seam and locked down it applies pressure at the base of the seam for traction.
On correctly formed panels this is not a problem and R1 will start to bend the female down-leg.
When the seam is tapered pinching the gap at the bottom together will force the seam apart at the top. This also draws the male leg down and in - shortening the width across the top of the male leg. The seam also gets pulled away from the deck of the roof.
On a correct seam R2 will bend the female down-leg up and complete the "single lock" or "first stage" bend.
When the seam is tapered the male leg continues to get shorter when R2 forms the seam.
On a correct seam R3 comes along the seam and applies pressure to the female leg. The female leg, in turn, applies pressure to the male leg and they both bend together at the top of the seam.
When the male leg is not long enough the female leg does not have sufficient leverage to bend the male leg. Instead it will bend around the outermost tip of the male leg.
When formed correctly the final roll (R4) will complete the "double lock" seam.
On tapered panels the seam will look boxed or squared and the seamer may rise up or drive off of the seam.
