Thanks for stopping by Don M!
From the primer: http://www.hdtvprimer.com/ANTENNAS/siting.html
If an indoor antenna is not as reliable as you want, an attic antenna is the next step up. If you are in a neighborhood with moderately strong signals, an attic antenna might work. But you are wasting your time installing an attic antenna in a poor-signal neighborhood. Most successful attic antennas are within 20 miles of the transmitter. (30 miles often works if you are on a hillcrest.) The problems with attic antennas are:
1. The antenna might not be high enough above obstacles outside the house such as trees.
2. It is hard to estimate the signal loss caused by the wood and other construction materials.
3. Metal objects in the attic can block the signal.
Estimating the signal loss in ordinary construction materials requires knowledge of their water content. Exceptions are aluminum siding, stucco (which has an embedded metal screen), and foil-backed insulation, all of which totally block all signals. Concrete and most bricks have moderate water content, but their thickness is enough to block all signals. In a desert, plywood becomes so dry that it causes no signal loss at all, even for UHF. In any other place, there will be some moisture. Exterior wood is generally always wet inside, especially in north facing surfaces. (Paint does not prevent this.) The amount of water varies with the weather. Dry asphalt shingles are mostly transparent to TV signals, but the way they overlap encourages water to persist between them. The vapor barrier is often wet on one side or the other. The bottom line is that there is no way to predict the signal loss in these materials. It is usually a mistake to point an antenna through a surface that gets totally wet in rain.
Metals reflect signals. A metal object 8 inches long is big enough to reflect UHF. Smaller objects, such as nails, are of no concern. Wires and metal pipes effectively reflect VHF, as do plastic pipes containing water. If these reflecting objects are positioned to the side, to the rear, above, or below the antenna, they will have little effect on it, provided they are not too close. These objects should be further away than 2 feet for UHF, 4 feet for VHF-high, or 6 feet for VHF-low, and an even larger separation will help a little. (Some might wonder why these numbers are not proportional to the wavelength. It is because the lower frequency antennas are lower in gain. An antenna’s aperture depends on the gain as well as the wavelength.)
There should be no horizontal or diagonal wires or pipes in front of the antenna. A perfectly vertical metal vent pipe is invisible to TV signals, but its flashing at the roofline might not be.