I wouldn't til the yard.

The yard will remain lumpy and bumpy for a long time, unless you do something to level it. Also, tilling the yard will just bring dormant weed-seed to the surface, giving you more weeds to fight next year.

You're in zone 8, so I have no idea if you still have time to seed your lawn now, or wait until next year.

Assuming you still have time, I would leave your existing yard in place. But I would use roundup to kill the lawn.

After you install the irrigation system, I would then aerate the heck out of the yard (being careful to mark the location of the sprinkler heads so you don't crack them with the aerator.)

Next, I would seed with a powerseeder (you can rent from home depot or Lowes) directly into the dead grass. Then rent a roller and roll the seed to get even better seed to soil contact.

Some will say that step is not needed, if you are using a power seeder. But my experience is that using both a power seeder and a roller gets better results than using just one or the other.

(The dead grass will help prevent seed runoff and erosion and acts like straw to keep your seed moist. The dead grass also becomes a soil "amendment" as the roots from the old grass decay).

Finally, I would apply a very light layer of compost.

I would strongly advise against tilling no matter how badly the home depot rental guy insists that it's necessary. Maybe you can tell us a little more about the project and why you want to replace the existing grass and what's going in.

If you are in Zone 8, you are dealing with some type of Southern grass such as St. Augustine, bermuda, or perhaps zoysia. Since I am in Zone 8 Texas, would be helpful to know which type of turf you have and what is your purpose for renovation. BTW, I installed my own sprinkler system about 20 years ago.

Zone 8, what kind of grass?

You are in warm season grass teritory, and most of the warm season grasses are from sod only, at least any worth having, and those that can be seeded you would have to wait til nex May or June to seed.

Do not till! You will understand why after you put in the sprinklers. There will be lines of weeds where they embedded the water pipes and where the sprinkler heads are set. You'll have enough weeds to contend with then.

Capt Tom

If I don't till...

How do I get the compost/amendments mixed into the soil...

Part of the challenge is that I have clay/poor dirt.

Can I till then roll with a water filled roller?

Jangell, answer the question. What kind of grass? Seeing how you are in zone 8 your answer should be either Bermuda, St. Augustine, Zoysia, or maybe Centipede. None of these require good soil to have a great looking yard.

No a roller wil not level anything out after tilling, only regrading will do that. Admending clay soil is a lifetime project of top-dressing with compost, or my favorite, sand once a year followed up by mulch mowing.

First...Grrass is a blend of:
48.49% Creeping Red Fescue
39.57% Perennial Ryegrass
8.75% KBG

One of the reasons I wanted to do this was to fix some of the bumpiness of my lawn...

I think the original owner used dirt and rock to fill places...

If I want to till the soil to get some of the uneveness out...What do I need to do to grade the space?

OK, need some more follow up, where do you live? Zone 8 does not normally grow Fescue, Rye, and KGB. I suspect CA somewhere.

we live in Seattle, WA

Being that you are in the PNW, you might try one of the improved varieties of Velvet bentgrass or colonial bent. I don't know much about either but that would be a unique choice and I think your climate could comfortably support it. This of course would be if you enjoy working on your lawn and want to mow it short for a look similar to a putting green.

You asked about what to level the ground. If you've got extreme problems you would want to hire a tractor service to use a box blade. You might be able to rent this equipment.
Otherwise you can do it manually by bringing in soil and shoveling and screeding and smoothing and packing it into place. This is a very time consuming task. So much so that *I*, as a very determined person, didn't even have it in me to completely do my back yard.

You asked about ammending your soil and claim to have "poor clay soil". I am of the belief that clay soil is not poor soil. Jam packed with nutrients and retains water in case you are in eastern washington somewhere that is very dry, this clay soil will help with drought and establishment. The only reason I would suggest doing it is to improve drainage if that is even necessary such as if you were constructing a home putting green or were using some grass that really needed well drained soil.

I also would not till your existing grass into the soil since you probably don't want this existing grass to come back. Even if you ignore all the advice not to till, you still need to rake out the dead grass and this will be back-breaking work, I assure you. You would want to contact your local prison and see if you can get a chain gang out there to do that!!!

Now if you want to add organic matter to the soil, you simply topdress the lawn with it. You don't blend it into the soil. It will find it's way down there eventually.

One more thing. Having dead grass there (mowed very short) when you reseed is very helpful to hold the seed in place in the event you plan to use seed. A heavy rain shower can ruin the entire project if you're unlucky.

water in case you are in eastern washington somewhere that is very dry, this clay soil will help with drought and establishment.

Quirky, Seattle is the wetest region of the US. Sunshine is a rare event:>)

There goes that selective reading again.
I could have sworn it said washington state, not seattle specifically. I guess I figured seattle would be a lower zone. Disregard the part about eastern WA since you are in Western WA but isn't your soil there kind of loamy?

I think the builder brought in pure clay for fill to level out this area...

Easy to find out with an empty glass jar and some water.
Take a glass jelly jar and fill it 1/3 the way with soil sans rocks or any other debris and fill the rest with water and cap tightly. Let it sit overnight. If it's clay soil, you'll have a layer of clay sitting at the top with a bunch of other stuff at the bottom and the water will be translucent to some degree. If it's loamy the water will be filled with organic matter and you will not be able to see through it.

You can also do a squash test and take some in your hand and squash it. IF it's clay it will pack into a ball and not crumble apart. Loam will fall apart.

And since the name of the game is drainage, you might do a simple test to evaluate your drainage and that can easily be done by digging a 1ft by 1ft hole in the ground then drop in a bucket load of water.

If the water drains through immediately, it's too fast draining and you might consider ammendments to help with that. If the water drains through in several minutes, you're good. If it just sits there for hours or days you might consider ammendments to improve drainage.

Then figure out what grass it is that you'll be planting and determine the needs of the grass based on that. Ask your local extension what they recommend to help prevent diseases. I'm sure you can grow the grass on clay... they grow grass without ammendments on every roadside in most of the country, but since you're putting in irrigation, you should do these tests to determine what changes to make if any so that you do not over irrigate.

****BREAKTHROUGH***
I think I figured out the cause for my selective reading...
Why the irrigation system for Seattle where it rains constantly? That's why I assumed you were in the more arid region of your state which is apparently the lower 2/3 only past the cascades.

Thanks for the thoughts on soil testing.

Seattle get a bad wrap...Left un watered...Grass turns very brown due to little to no rain in the summer months...

I'm tired of hand/portable sprinkler watering and thinik that sprinklers will also improve resale...

Good discussion here.

I am in the Seattle area as well and my backyard is bumpy and un-even. Someone mentioned just dumping dirt into the divots to even out the lawn. Will the grass be able to grow through this soil? I'd imagine after I was done filling in all of the divots I'd be left with a dirt covered yard, that was not at all pleasing to the eye.

Will the grass be able to grow through this soil?

About a quarter inch at a time, yes. Any more dirt than that and probably not. A lot more and no way is it coming through.

That having been said, I dumped 4" of dirt on part of the lawn to convert it into a new garden. Fortunately, I decided to let it lie fallow from September through May to settle.

Fortunately because the amount of grass that managed to come through it was unbelievable. I figured none would, closer to 5% managed to pop through and grow. I have no idea how.

Still, don't count on that. You can adjust the height 1/4" at a time, doing it again when that settles (call it maybe 2 months or so, depending on rainfall) and the grass has raised its crown above the new soil.

It takes a while. The other option is to dump it all at once at the correct time for reseeding and reseed it immediately.

If you fill in the divits where there is no existing grass you will have to seed those places assuming you don't have a type of grass that spreads on its own. If you have existing grass there you can put a little at a time, such as 0.25-.5" at a time and the lawn will grow through that.