Thank you. Any idea on how much soil I would need to remove and replace if I was to do sod or reseed right away?

Tilling to 3-4 inches may break the pre-emergent barrier. Tilling is not recommended.

My recommendation would be to just wait 8-10 weeks and sod. Typically fertilizer/pre-emergent combos found at big box stores don't last as long as straight apps of pre-emergent. Ask your landscaper what he applied and how long the stuff is supposed to last.

Crabgrass is a summer annual plant that only returns by seed the following spring. It has a quick-germinating seed, so if you try to establish grass in the spring, the first seedlings you see will almost always be crabgrass. Then if you think the new crabgrass sprouts are your target grass seed coming up, there is a tendency to stop the daily watering. Once you do that the target grass seed just sits there while the crabgrass fills in. It is adapted to heat, low mowing, and dry spells. It's also yellow and shaggy compared to more popular turf grasses.

The best way to avoid getting crabgrass is to create a dense turf to begin with. The best way to do that is to make it dense in the fall after the crabgrass has stopped germinating seed. You can kill it out to create open space, seed into the open space, and the new grass will provide that density that keeps the spring weeds out. If you start early in the fall (in northern Utah that means August), then you will have time for the new seed to come in and still reseed if it was not dense enough. Continuing with the best way to keep crabgrass out, if you can withhold water in the spring and keep to a temperature driven watering schedule, that will deprive the summer annual weed seeds from the continual moisture they need to germinate. And the last thing you need is to provide as much shade on the ground as possible. Do that by mowing the grass at the mower's highest setting. Beyond those cultural practices you can use preemergent herbicide as has been discussed. Ideally you will develop a turf that is dense enough that you won't get crabgrass. I have not had crabgrass in forever with a dense St Augustine lawn.

Watering: Deep and infrequent is the mantra for watering. This is for all turf grass all over the place. Deep means 1 inch all at one time. Put some cat food or tuna cans around the yard, and time how long it takes your sprinkler(s) to fill all the cans. Memorize that time. That will be the time you water from now on. My hose, sprinkler and water pressure takes 8 full hours to fill the cans. Your time will likely be less. I like gentle watering. As for watering frequency, that depends on the daytime air temperature. With temps in the 90s, deep water once per week. With temps in the 80s, deep water once every 2 weeks. With temps in the 70s, deep water once every 3 weeks. With temps below 70, deep water once a month. Note that you have to keep up with quickly changing temps in the spring and fall. This deep and infrequent schedule works in Phoenix and in Vermont, so it should work for you. The reason for deep and infrequent is to grow deeper, more drought resistant roots and to allow the soil to dry completely at the surface for several days before watering again. If it rains, reset your calendar to account for the rainfall.

I don't know if I've said this before, but cool season grasses stand no chance against a warm season grass like crabgrass in the heat of summer, regardless of how high you cut it and how much water you withhold. Warm season grasses like St. Augustine have a much better shot at competing in the heat.

dchall - Thank you for the very detailed response! The main problem with our lawn is that the home was vacant for quite a while before we purchased it a year ago. The lawn had been neglected (except of mowing and watering) and had developed thin areas and thus crabgrass. Landscapers are coming again tomorrow, so will check with them on what they used as pre emergent. Most likely will lay new sod when appropriate amount of time has passed.

j4c11 - St Augustine does not do well here in St. George. Summers are hot, but winters are cold, with temps dropping below 20 degrees frequently at night. Most (if not all) lawns here are tall fescue. Pretty sure my best bet is to wait out the pre emergence, remove lawn and replace with new sod and keep it healthy to prevent crabgrass return. Thank you again for all of your input.

Sorry, I wasn't clear, my comment was directed more towards dchall, I didn't mean to imply you should use St. Augustine. My point was that it doesn't matter how healthy and dense your tall fescue is, it will not be able to outcompete crabgrass in the summer. As a warm season grass, crabgrass will always have the upper hand in sun/heat/drought conditions. You will need to maintain a pre-emergent barrier every spring to keep crabgrass under control.

Thanks. I don't have an issue with using pre emergent once the lawn is thick and established. My dilemma was what to do now with a lawn that just had pre E applied when I was planning on over seeding. The lawn is very thin and has many bare spots. I am still leaning toward new sod if I can put it in before the heat of the summer sets in. We entertain a lot and the lawn looks awful!

Overseeing in the spring isn't a good ideal either way. St George has too hot of a summer for the new seedlings to survive