Ok! Let's get one thing right, straight away. You do not get a level lawn by using a roller. Using a roller, simply gives you hard bumps and hard hollows - and all the bits in between.
In fact, unless you are going to carry out all of the other operations necessary for a first class lawn, you can get rid of the roller altogether - although I might concede that it is useful for a light run over before the first cut after the winter.
In actual fact, a lawn rarely if ever, 'grows' a bump! There are no logical reasons for a lawn to rise up in places to provide you with a bump! What generally (always) happens, is that the lawn sinks over time - sometimes taking a few weeks as is the case with new lawns, or sometimes over a period of several years, as soil naturally sinks in places as a result of natural earth movement (rare) and underground roots and organic matter decomposing.
Most lawn bumps and hollows are cause by natural soil settlement or erosion over years in the case of mature lawns, and over just a few weeks with a badly prepared surface for new lawns.
Sinking of the lawn can also be as a result of animal actions - not least moles or even ants burrowing under the lawn. In both cases, soil is removed. As in the case of moles, the soil is burrowed away to make tunnels, and deposited on the surface in the form of molehills. In the case of ants - to a lesser extent, the soil is bought up to the surface to form ant hills - thereby making room for the ants nest underground.
Mature and Established Lawns
Usually however, it is simply an 'uneven' lawn that is the problem; so take the easy way out, and bring the hollows up to the level of the bumps.
Result = Even Lawn!
(For small hollows - which materialise as 'large bumps', simply carry out the last stages of the article on 'Top Dressing Lawns'.)
As you can see here, the lawn has 'hollows' of about 2 inches (remember them?) deep. (50mm). If you are a lateral thinker, then I suppose you could say that the lawn has 'bumps' which are 2 inches high! As we are dealing with hollows; it has 'hollows' and not bumps. It is much easier to view the hollow and bumps in the lawn by the use of such a straight edge. Sometimes a length of batten will be enough to determine the depth pf the hollows.
The hollows can be filled up to the level of the bumps, with the use and application of a suitable top dressing mix. Now we need a decent straight edge length of timber. (4x1in (100x 25mm) will be suitable - as is used in the picture. Use the straight edge to spread the top-dressing mix (described in the Top-Dressing article) to a rough level by screeding the straight edge back and forth.
Once level, then compact the soil by treading with heels, until firm enough not to leave a foot imprint! Do this when the top dressing and soil underneath is dry and not soaked - as is sometimes the case with hollows. (If you do it on sodden soils, then you will need to read about 'compaction' in the lawn aeration article!)
Once the top dressing has firmed down, you will probably need to add a little bit more in stages until you get the top dressed hollow up to the level of the high spots. Rake to final level with as wide a rake as possible. Finish off the raking using the back of the rake as a screed tool. This should give you a good firm level surface on which you can apply your grass seed mix.
Spread the desired mix of seed at a rate of approx. 'a good handful per square yard'. Really scatter it about to ensure an even spread. Try to get the same type of seed mix as your existing lawn. Basically a 'normal' lawn will consist of Rye and meadow grasses. A fine lawn will normally consist of fescues, browntops and creeping meadow grasses. Don't be over worried - simply opt for a 'general mix' or a 'fine lawn' mix. Even if you can get the mixture absolutely right - highly unlikely - the newly dressed areas will take on a different appearance to the established lawn. After a year or so, it will all be virtually the same, for your normal maintenance schedule will have a large bearing upon how the lawn ends up.
Rake in lightly; protect with netting against birds; water when dry; and be patient for about 2 weeks. Hey presto, the grass starts to grow, and you now have a lawn that is (should be) more level than it was.
Large areas are best done with some help!