Question
QUESTION: I live in Phoenix.  I have planted a shamel ash about 4 feet from a sidewalk and driveway.  Now I understand the roots are invasive.  What can I do to minimize the damage in the future?

I have a raywood ash 8 feet from my house.  Same question, although I understand they are not as invasive.

ANSWER: The shamel ash roots will  give you a problem with the side walks. Their roots are fast growing and aggressive and will extend put twice the width of the tree foliage. If these are newly planted trees I would recommend moving them from near the sidewalk and driveway. You can install a root barrier along the sidewalk and drive. This would mean digging a trench along the walk way and putting a metal barrier in the trench to divert the roots. The over hanging limbs can also be a problem and will need to be watched and pruned as needed.
Raywood ash near house.

The roots should not be as great a concern as the branches. If the branches grow to rub against the house, it will damage the tree and perhaps the house. The tree should be kept at least 20 feet from the house, this should be sufficient to avoid problems. This is farther than the branches will spread because this tree has a very upright growth form. The roots will extend much farther from the trunk than the branches; however, if your home has no leaks under the foundation, the roots will not grow in the dry soil under the house. The worst that would happen is that the roots will grow up to the foundation and then run parallel to the foundation.
Realizing the tree over time will get large as it grows and the limbs extend to the house the branches that touch the house will need to be pruned. This will be a future concern not one now.

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QUESTION: The shamel ash is about 4 years planted from a 24" box.  I want to avoid moving it at all costs.  How bad can the damage be?  If I deep water, will that help to drive the roots downward somewhat?  If the roots just crack my driveway, so be it, but it is raises up a foot, well that's a different story.

Do you have any idea how big a shamel ash can get in Phoenix?

As for the Raywood, It's trimmed up and a one-story house, so the branches should be clear of the house as it grows.

Thanks for your help and quick response.

Answer
The roots may just crack the driveway and normally will not lift the drive at the most a couple of inches and then this may happen when the tree reaches maturity.
The Shamel Ash becomes a very large tree, up to 80 feet tall (24 m) in 30 years and almost as wide, usually around 45-50 feet tall and wide. It is shallow rooted, and its surface roots may be disruptive. It is very well adapted to Phoenix climate and if you have irrigation, and a back yard large enough, it is probably one of the best trees you can grow. Deep watering may help encourage the root system to be deeper in the soil. Roots will grow toward water so the % of roots should be deeper than only watering normally. There will still be lateral roots since these are needed to support the tree. If you want the tree where is is I say go for it , chances are you will not see any noticeable root problems since most folks do not live in one house for 25-30 years.