Do you have drainage issues such that water stands for a period of time after a rain?

Hi! I'm not too sure - never paid too much attention to drainage when it rained. Would that be the reason why there are yellow patches / mud?

Any insight into how to fix potential drainage issues? Would it require tilling / rolling of the lawn?

Your problems could be from a number of things and you have to start somewhere diagnosing. I suggest you pay attention to where water goes during and after a rain. Trying to solve drainage issues by remote can be difficult and time consuming. No point to speculating about it if we don't know they exist.

Resolving bumpiness will be a matter of grading -- cut and fill ... smoothing. Probably by hand, being careful not to CAUSE drainage problems by filling a way that water uses to escape.

Where do you live?

What kind of grass do you have?

- Dethatch

No. Thatch, if you have it, is the least of your problems.
- Cut the grass short
Oh heck no. Always mow at the mower's highest setting or nearby.
- Scarify
Scarify is something you do to seeds that need to be roughened up. What does it mean in your context?
- Aerate
Probably not. Wait until you have the major issues resolved.
- for the muddy areas, apply triple mix soil to flatten and level the lawn
Not yet. Assuming you have low spots, you need to address those. Fixing those should get rid of the muddy parts.
- overseed / over the mud spots, I will apply more seed and
Wait for seeding (preferably until September), but until you have the bumps and dips sorted out.
- put triple mix soil on top of seed
No. You probably already have too much soil in the yard. Almost nobody is short of soil. There are tons of people who put 1/4-inch of topsoil on the yard EVERY year. Well, in 4 years that's an inch of soil you don't need.
- Rake and spread the soil around
NO! No more soil.
- add water

NO! You already have a water problem. I live in Texas and have not watered my lawn yet this year. Well, a little on the southern bank, but then it rained, of course.

The big thing is to identify any drainage issues. Water should drain away from all the buildings and out to the curb or a ditch somewhere. If you have low spots anywhere, or if you have soil higher than the concrete walkways or driveways, then you have too much soil already. Scraping it up and hauling it away is a huge deal that I've rarely seen done, but that solves so many issues. Fixing the drainage will NEVER involve tilling the soil. It might involve scraping surface soil away to form a swale or ditch, but rototilling is never in the solution to drainage. Rolling is similarly useless in profiling the surface.

It looks like your yard is bumpy mostly from lumps of grass interspersed with dead spots. I would not jump to any conclusions about the soil being bumpy until you get an even stand of grass.

Hello dchall! Thank's so much for your response.

I live in Toronto, Canada. I believe the type of grass I have is a combination of Kentucky bluegrass, ryegrass and tall fescue.

" It looks like your yard is bumpy mostly from lumps of grass interspersed with dead spots. I would not jump to any conclusions about the soil being bumpy until you get an even stand of grass."

Yes - this is correct. I might have not used the correct terminology when posting my question. It is lumpy with dead spots and grass interspersed. I also have very shallow roots in my grass (is the reason for this most likely to because of grubs?)

Can you advise on how best to fix this?

Another thing I would like to mention is that last fall I noticed two ant hills in my backyard. I removed the ant hills and poured hot water over where they were, and removed parts of the soil. As a result, water is now sitting in the divot where I removed parts of the soil. This is why I want to add soil to level the yard and then overseed.

Thanks very much. I would really appreciate any advice.