Question
The roots are exposed about.... 3 or 4 inches, I'd guess.  The
area is quite large.  Either bringing in soil or sod will be a big
project.  I'm going to guess it's about 10' by 40'.  And yes,
it is clay.  If we have to bring in that much top soil, it seems
the easier path would be sod.  Do we have to bring in soil to
put under the sod or just sod?

Thanks!
Eileen
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Followup To
Question -
We have an area of our lawn which has over the last many
years seen its grass become thinner and thinner, then lose its
grass, and the soil started to erode and we have tree roots
exposed.  We did trim trees back, and seed after rototilling one
year, but the seed did not take.  The ground is on a gentle slope.  
We should have helped it more in years past, but there wasn't
time in our schedules.  Now there is!  Sodding seems like one
way out, but I'd hate to go through the expense if I am just
going to lose the grass again.  I have three questions:
1. Do we have to add topsoil before we sod (if we choose to) as
the roots are exposed pretty severely?
2. Is there any way to know if grass  will be successful there
before sodding?
3. What can we do if we can't sod?  We have kids, and I don't
think a ground cover would have a chance to take root before
they trampled it.   It would also be an odd place in our yard to
have a ground cover.

Thanks so much for any advise.  We live in Central NJ.  As you may
have guessed, it is a partly shady to shaded area.

Eileen
Answer -
Hi Eileen;
Sounds like you need to haul in quite a bit of soil.
what is the soil like that is there, say 6 to 8 inces down?
If you have clay soil, bringing in a few inches of top soil will not help much.
That clay won't let water drain through.
You can loosen clay by tilling some soil amendments into it several inches down.
I use bark mulch, humus, and peat moss, about 1 part humus and peat, and 3 parts cedar bark
mulch ( because cedar repells many harmful insects). I put that on about 4 inches deep, and till
to a depth of 8 inches. That gives you 8 inches of good gardening soil ( top soil).
That will be loose enough for grass roots to grow.As the bark mulch composts the nutrients from
that, the humus and the peat will seep through the soil, loosening and breaking up that clay
further down.
Using an organic gardening program and letting the beneficial microbes, and beneficial insects
do their job of enriching the soil, will maintain the quality of the top soil you have made.
Lizards, toads, grass snakes etc will keep the bad bugs eaten much better than insecticides will
get rid of them.
I have no annoying insects to deal with. Nort Texas, where I live has just about every bug pest in
the world, but they don't bother my yard. My lawn critters eat them. I never see them. I use
organics in the house too, and I NEVEr see an ant, cockroach, silverfish, spider or earwig.
My herbs kep them out.
Write me if you want to know these natural repellants, I am happy to share what I have learned.
You may need to do some pretty serious pruning and trimming to get some limbs out of those
trees to get more sun on the yard. grass needs at least 5 or 6 hours of sunlight per day to grow
wel, and more sun is even better.
If you amend the soil, and start with 6 to 8 inches of good loose soil ( not too loose, if sand is
the problem, different amendments are needed) and you have enough sun on the grass, sod
should be very successful.
Write if i can help further.
Charlotte

Answer
Hi eileen;
If you bring in sod and lay it on those exposed roots and the clay soil, it is just going to die right out.
the only way you can get grass to grow is to make the kind of soil conditions it needs.
Gee, hon, 10'X40' is just 400 sq,ft. that is a very small yard.
If you dn't break up the clay, the water will not penetrate the soil, and it will either drain out between the clay and the topsoil, probably taking the top soil away with it, or it will set under the top soil and grass roots and just drown out the roots.
Loosening up that clay is the only way to go. to not break it up is just putting good sog on bad ground.
Charlotte