How much water roses need from your system is very different depending on the soil, the amount of sunlight, the humidity in the air, and of course how much rain they get (or don't get). I would find someone who gardens in your immediate area and ask them this question - it is not something that there is a general rule for.

Jackie

Ditto on what Jackie said...

Our soil is clay based here. We get plenty of Spring rains, so no extra water needed at all.
Being clay based our soil holds moisture well, we also use mulch around our roses.
Our roses that have been planted for 2+ years do not get or need extra water here as rains usually provide enough.

So it just depends on the factors Jackie listed...

Yes, it depends, but...Deeper, less frequent watering is better. It forces the roots to go deeper which gives them a more even temperature and provides a stronger foundation for the whole plant. Roots close to the surface dehydrate more quickly in summer and freeze more easily in winter.

I'd say you are watering too much and too often. I too am on the clay side. Most of mine are on 1-gal emitters. In "normal" weather, I give mine a gallon a week except for the babies. When the temps get into the high 80's and 90's I go twice a week at 45 minutes. Well-established roses (4+ years) rarely need anything, but if they are already on the drip I don't take them off. The best test is to stick your finger in as far as you can. If it is still moist at the tip of your finger, don't water.

I have one area that is not on drip. It depends on me getting around to watering and thus doesn't get watered nearly as often. The roses all survive, even thrive. Over the years, the only roses I have lost to too little water are tender little bands with barely any roots that I forget [because I planted them in odd, out-of-the-way spots]. But the well-rooted bands survive my neglect.

I give 3 or 4 gallons a week to medium-sized roses when high temperatures are in the 80s to low 90s and no rain for a week or more. This is equivalent to 2/3 to 1" of water. (More would be needed in areas with higher temps and lower humidity.) Usually my soil stays naturally moist until sometime in June. I do not irrigate once-bloomers or reluctant repeaters, as it just encourages unnecessary green growth. In medium to heavy soil and an eastern U.S. climate, roses can endure 6 weeks with no added water. However, they go semi-dormant, and this will cause fully-repeating roses to skip a flush.

With pots, shallow soils or fast-draining soils, the irrigation budget should be divided into lighter and more frequent applications, 2 or even 3 times a week. With deep, heavy soils, once a week is good, and would be less trouble than the 4 x 1 gal. routine you are following. However, under that regime I 'd guess the deeper soil is staying moist.

The issue that you raise about deep watering/deep rooting-- in my area the subsoils don't dry out except during severe droughty spells.