If it's full of tree roots & you till it you'll either break the tiller or kill the trees.

Use mulch around the trees. Seed cool season grasses (fescue, bluegrass) in the fall. Do not till.

Bare grass?? Do you mean bare soil?

Best practice is to never rototill in preparation for a lawn. This has been the advice published in the garden books since the 50s, so I'm not sure who's trying to change it. If you have bare soil then just scatter the seed, walk on it or roll it down to make good contact with the soil, and start watering.

Also best practice is to never seed in the spring or summer. Does it get warmer than 80 degrees in the summer? If not then you might be able to seed all season long, but normally this results in dead grass except for the crabgrass. Seed in the early fall if you can wait.

I rototill if needed for compaction, as well as if the soil would benefit from the addition of a large amount of organic matter. Rototilling is a "it depends" proposition.

But I would be cautious of damaging trees with a rototiller.

"Fall seeding" is really late summer seeding. You can seed as early as Aug. 15 in zone 6. I wouldn't put in a seeded lawn now. Make a plan to do it right in a couple of months.

In zone 5 I usually seed at the end of August unless the weather is unusually hot. Most grasses need a good month of growth to be winter hardy. It's best to not plant late to avoid the risk of an early frost.

Compaction is a very specific thing that almost no homeowners will ever experience. It involves completely saturating the soil with water and repeatedly plunging a tool (or animal hooves) into it until all the air is driven out.

Hard soil is a very common thing, that almost every home owner will experience, and sometimes many times. Hard soil is caused by the depletion of the beneficial fungi which normally keep soil soft. This can happen from extended dry conditions (many months), extended wet conditions (just a few days), or the repeated use of fungicides or sulfur.

Compaction is best treated with ripper tines on the back of a tractor. Hard soil can be treated with shampoo or as a very last resort, a core aerator.

When preparing for seed, all you should need is a soil surface. It does not need to be six inches deep of fluff.

Hi all. Sorry for the incomplete and inaccurate post. (I was really tired.) Didn't mean tilling - no tilling or digging involved. My first priority is protecting the large trees on either side. Current plan using prof lawn service involves dethatching the remnants of the existing grass, adding adequate topsoil and then re-seeding. All as low-impact as possible. I've spent a lot of time defending the mature trees on both sides of my lawn from various neighbors, and have paid to have them trimmed and maintained (even the one that isn't mine) to keep them healthy. So: tree welfare uppermost.Thanks all.

Rethink adding topsoil. You don't want to create drainage problems (water draining toward your house). You don't want to smother tree roots. They are close to the surface and need oxygen. Putting too much soil or mulch around trees can impair their health. Get a soil test and see what amendments you may need. If you have low spots or holes you could add topsoil just in those spots. Grass will grow in your existing soil. You want to train the new seed to grow roots deep into your existing soil. If you put a layer of "better" soil on top, the new grasses' roots may stay mostly there instead of probing deeper. You want deep roots, not shallow ones.

No problem with draining toward house; the lawn is a very noticeable steady slope to the street. Very hard to keep water on the lawn long enough to soak in; it all runs rapidly out into the street.

I know the goal is for nice deep roots, so the grass can withstand the vagaries of the weather.
But for the first time this spring, after a really rough winter, the grass is totally missing in the front yard. Not a matter of the grass growing in the "existing" soil - there is no soil there anymore. Tree feeder roots are visible.

History: have tried for years to get the grass to grow in the existing soil, except that there is almost no soil at all. Soil around the trees routinely runs into street. For several years I would add a little soil on top of the net of tree roots to hold the seed, and the grass would sprout - then deep watering to encourage it to root more deeply. Worked pretty well - not perfect, but I don't need a perfect lawn; just so it doesn't look neglected. With regular deep watering I could coax a kind of lawn to grow - just enough to keep the soil from running into the street all summer. Until last year, I always had some kind of lawn. Don't mind if it's patchy or thin, as long as it's not bare dirt and tree roots, and streams of my topsoil running into the street.

But this year no grass came up at all in the front area; just bald dirt So I'm just trying to reset my very "light-impact" lawn growing program again.

How much shade do you have? Grass needs a fair amount of sunlight. Sounds like mulch under the trees instead of grass might be better. Unless that just washes off too. How about using a ground cover, something like periwinkle? Or hostas? Can you post a picture of the yard so people on here can see what you're working with? They may have some ideas to help with your erosion problem. If your slope is steep, you might want to consider terracing. Or doing some shrubs or other plantings to interrupt the flow of rainwater washing down the hill.

If a lot of the water washing down the hill is coming from the gutters, you can install drainage pipes connected to the downspouts and redirect that water to the sidewalk or street or to somewhere you might want to do a rain garden. Rain barrels would save some of that water and keep it from running down the hill. You can install edging to keep mulch from washing away. If you really want grass, sod would have a better chance of getting established than seed if you have so much water washing down the hill.

Gutter drainage system is underground. There is runoff on the slope but not violent.
Lawn is edged on one side with a bed of periwinkle, with ivy on other; the trees are surrounded with beds ivy or periwinkle around the base.
Again, I have had a successful lawn every year before this - not hugely lush, (it is growing under the trees) but green and decent. Can be a bit thin, but still there.
It's just this year that the lower 5 ft from the edge just didn't come back.
I can't afford sod. So realistically, I'm just going to go with some carefully applied new topsoil and then seed and water. I'm not willing to risk injury to either of the mature trees, so will go with the lowest impact plan, and plan ahead for a more permanent plan in the spring. Thx for all your time and advice.