[TowerTalk] Forty Meter height question...

[PAUL924--- via TowerTalk]

Hi Guys,

I would appreciate some suggestions and guidance. I have a 40  foot Rohn 55 
tower mounted on top of a building. The top of the tower is 100  feet above 
the street.
 
At present there is a Force 12 XR-5 just above the  tower, which plays very 
well.
I am planning to place a Force 12 Delta 230/240 above the  XR-5 on the same 
mast. 
They will be 8 feet apart.
Question:
The two elements for 40 meters will only be about 48 feet  above the roof. 
Since this is less than a full-wave length in height  above the roof, low 
angle radiation would be affected, does it make sense to put  up ?   Would a 
slightly longer mast fill the gap ?
Thanks, 73,
Paul  W2JGQ