SWHouston

Jack,
How nice it is of you to want to help your folks out, and asking questions here, is the first step toward getting them good reception.

First...
What Piggie said was very important, you MUST know what Channels are available at their location, to even start the evaluation process. Going to TVFool and making a Chart on their "specific" location, is the only way to get even close to an accurate evaluation and subsequent recommendation.
I'll disagree with Piggie to some extent, that being to use...
5 Ft, for an Indoor (top of TV) operational height.
15 Ft, for a Roof Mount on a Single Story house, and
25 Ft, for a Roof Mount on a Two Story house.

You'll actually need to run two separate charts, 1 for inside, and 1 for outside, to see if you actually will be doing yourself any good, by mounting the Antenna on the Roof. One thing I'll say up front, is that it ALWAYS is better to have a Rooftop installation, but, you need to run a couple charts anyway. Just to make sure.

Secondly...
How thoughtful it is that people will come online here, and tell how a given antenna works for them. BUT, that's them, for the type, height and location that they are commenting on, and is NOT where you are interested in locating their (your folks) Antenna.

Once having seen the Chart, there are several people on the Forum here, who can comment on an Antenna that's just right for your folks specific location and reception needs.

When linking to TVFool, be sure to scroll down to the bottom of the first page, and enter the Height of the installation. If you want to print out the Charts for viewing, you must save them to your computer as stated, and print them out from it.

Please look above the Chart Graph and Channel List (on the second page), for the bold link, which you should copy and paste into your post here, where we can take a look at the chart with you.

Have a good Day !
S.W.