Living in Seattle, you likely have an abundance of evergreens, and they create an acidic condition, not particularly suitable for all grasses. If you over seeded with a fertilizer high in nitrogen, that can compound your issue.
Fix the soil, overseed, aerate regularly and top dress. Save your dough on sod. Your grass will come back. Besides, until it does, you have weekends free from mowing!!!
+1
If you go with overseed, better do it now so the roots have time to get deep enough to withstand Aug/Sept.
No need to thatch. Just cut the lawn as short as your mower will safely take it.
Fertilize: Scotts Starter Fertilizer
Overseed: Scotts Pacific Northwest mix w the hydracoat.
Dress: Peat Moss. 1/4" - 3/8"
Keep it moist. Water in evenings after the sun is off it.
Let the outer layer of the Peat Moss dry crust during the day...it'll be like an oven underneath to help germinate the seed, but it needs to be moist under there. If the area is small enough, hand water it. Sprinklers have too much pooling and runoff.
Winters are tough here. Rain and moss. Mine just came back. Followed the same process.
dolomite lime is a good soil acidity reducer.
