The weather was varied from about 55 to 80°F. I gave the ground a good soaking, not just a layer of dampness, stopping short of flooding it though (not sure how to measure how much I watered the lawn). The packaging boasts that the seed can withstand skipping an occasional day of being watered, so I didn't think it would need watering more than once a day.

If I "try again" with new seeds, should I rake the patchy areas and risk hurting the little grass that did grow? Would spreading a layer of new potting soil before seeding help, and how does the new grass react when the roots reach the old, hard soil?

I assumed sod would be significantly more expensive but I'll check out that route too.

It sounds like you did everything right. Maybe the seed was old seed. I would complain to the store. They might give you a new bag of seed, or not but it doesn't hurt to try.

If you only have patches to do you might look into bits and pieces of sod. These are the ends that are too small to make it into the rolls. They are quite a bit cheaper than the sod rolls. You can cut the pieces to fit with a utility knife. Water the sod roots and wet the ground well, and butt the edges together tightly.If they do give you new seed then use it to patch.

Any plant has a hard time getting their roots in clay which is why you have to water often to keep the clay soft.

You can help in future years by top dressing the lawn with top soil. You don't want to cover the grass completely just put some around the roots It will wash down and the grass will grow up through it

You can't allow grass seed to get dry. It's right on the surface, so it dries out fast.

You also can not water enough to float the seed up off the surface of the ground.

So, a deep watering to get the soil wet down below the surface. Then you seed. Next you cover the seed with a little layer of something to protect it. I use oat straw. You can use screened compost or even dirt, as long as the layer covering the seeds is thin.

Then you water for 5-10 minutes at least twice a day, and more often if the weather is hot and dry. How long depends upon how much water your sprinkler puts out. You can not have any standing water, even for a second, or it floats the seed up off the surface. Also, you want to use a sprinkler that puts out a fine spray, not a stream heavy enough to move the seed.

Many lawn seeds germinate when the nights are cool. But maybe you got a warmer weather variety since it was sold in your area.

Can you still see ungerminated seed or has it disappeared? If the latter I'm wondering about birds??? Sparrows or crows can destroy a newly sown lawn. Or is that ridiculous?

If you're a beginner and want to try again, there is a product such as lawn patch, which is sort of bluish green thatchy like material that you apply to prepared soil. The grass seed in such products is mixed in with the material, so less chance of drying out or floating away, and nutrients are usually a part of the mix also. Still, keeping the material damp until seed germinates is essential. More than once a day watering(unless you're getting rain each day at another time) is still needed. Good soil prep is important in clay soils particularly. Scott's has a decent product and this approach is good for beginners and for smaller areas. My primary objection to this is I think the color is rather weird looking until seed germinates because it's that bluish green color, and it can be more expensive than normal seed, but still usually less cost and headache than sod. Color of the product until actual germination is not a big deal to some people as to others. Here is a link to other forum answers regarding using lawn patch products.

Good luck--and personally I've always had excellent results with Scott's products, though there are other brands folks seem to like, also.

Here is a link that might be useful: Garden Web on lawn patch products and use

flora_uk, I can still see ungerminated seeds

delightedinco, I tried the Scotts EZ Seed too - it sometimes worked, and it's very convenient to use (I like the built-in spreader on the smaller containers - the gardening newbies this product is aimed at are not going to own seed spreaders or roto-tillers, don't know what kind of soil or fertilizer to use, and have no idea what 5 lbs. per 400 feet looks like when spread. But it is so expensive for what you get). I bought it only because Scotts used to have $5-off discounts on it. Pennington has a similar product as part of their Smart Seed line that's a bit cheaper (but not much), although I wonder why they claim much longer germination times than the Scotts product. There's also Scotts Patch Master which is cheaper and I haven't used yet.

I think I'm going to try just regular seed, and watering more often (a real pain here - there's no outside garden hose faucets, so I either need to use big watering buckets or run a hose from an indoor faucet. I have a 50' coiled garden hose with a sink-faucet adapter at one end and a trigger sprayer at the other for that purpose). I have a bag of Vigoro Starter Fertilizer 20-27-5, and have applied quite a bit of new seeding soil (of several varieties depending on location) since the ground is mostly hard clay. I did my best to loosen it with a metal gardening rake before adding a new layer of topsoil.

Any recommendations as to what type of grass I should use? I'm near Washington DC, which means both humid 100°F+ in the summer and subfreezing icy winters. The big-box stores around here seem to favor tall fescoe, which is also what the combination seed/mulch/fertilizer products use in this market. I like the bright green color, how fast it grows, and its tolerance for all sorts of bad weather, but not the thin straight grass blades that get matted down like cheap carpeting when walked on. The second most popular here seems to be the generically-labeled "sun and shade mix". The real nurseries have more variety to choose from but also higher prices. I don't want anything that goes dormant or turns yellow or brown in the winter. The side yard is in heavy shade due to closely-spaced houses and lots of old trees, but the backyard (which I'm more concerned about since that's the main socializing area) gets alot of sunlight.

I also hear some people say I shouldn't even try to grow from seed during the hot summer. But I'd like to have a lawn by fall if possible, when I plan to have a new renter. I could plant sod - but it's inconvenient given how thirsty the stuff is when new (given the water situation I mentioned earlier) and hard to transport for someone with a small car and a bad back.

Any ideas?

Just a few more ideas that I have used over the years to establish lawns. I know I live in AZ. But out here we have hard clay as well. The best thing to break up that clay is with Gypsum. It comes in a bag like fertilizer. You water it and rake it in it will help loose the top soil enough for seeds to grab the clay. Your watering is a issue as well. You don't want a spray nozzle that will push seeds around. If there is a lot of birds in your area that like the seed. The best thing to do is put down bird seed with it as well. The birds will go after the bird seed instead of the grass seed. I hve done this plenty of times and come across a great lawn.

I used the Gypsum clay-breaker in one area, and might get another bag (it's inexpensive). Not many birds here, althought they're always lots of ants all over the new seed and soil.

Should the seed just be placed and left on top, or raked in or even submerged slightly within the soil? I just added some head-resistant seed in some areas yesterday. Trying alot of approaches to see what works best.

You do not need to rake it in. I use a leaf rake to evenly spread it around after seading and put a thin layer of manure over it. You do not need to use the manure, if you don't want to. I use it to get nutrients into the soil to further break up the clay.
The ants are not a problem as they just take the seed down to their holes. It will still sprout. If its in area that gets water. If its not in an area that gets water it will not sprout.

I seeded a few areas in early Spring (Southern Minnesota) and got poor results. I recently (08/01/2016) did another few areas with much better success. I used Scott's Landscaper Seed and Greenview Accelerator.

The biggest difference was applying the accelerator (not used the first time) and using a garden shovel to break the dirt up to a depth of about 3 inches. If the ground is too hard you need to go deeper than a 1/2 inch to establish a decent root.

Planted on a Monday afternoon and I could see tiny sprouts surfacing on Thursday morning. I would rent a tiller if doing a large area.

Great information Kyle.

Great comment about Kyle's great information Kyle.

Kyle

I have great success growing grass from seed, using this method: I til the soil down an inch. I add about 1/2 inch new topsoil on top of that, then I spread seed using a broadcast spreader as recommended on the bag. Rake in lightly. Next, I add a ton more seed using my hands! I mean a ton more! Rake in lightly. Next, toss a very fine layer of peat moss over the area to hide seed from birds, but not to completely cover it. Less than 1/2 of peat moss. Now, water it three times a day for less than 10 minutes. Just get the ground moist. You don't want puddles on your seed. After the majority has sprouted, then you water deeply every other day, either early morning or late evening. I prefer evening. If you are experiencing a heat wave, you would need to look at the grass and see, if it needs more water. Best time to do this is by me is April and very early May. Also, in the Fall. But my favorite time is to winter sow in Jan and February though. Then, you just toss the seed down onto the exposed ground (if you have no snow cover currently), just before a snowstorm The snow covers it and nature does all work for you. The year I did it, I didn't water once and had beautiful new grass in April already. The snow and spring showers took care of all the watering!