I put a couple of sheets of newspaper on the floor under the wood shavings and straw. i tend to only put straw in the end where they usually nest as there's not a separate nest box. it's a bit more warmth/insulation & easier to clean out as you just roll up the newspaper with the mess inside!
I use a layer of easibed or similar with a sprinkling of Stalosan F underneath which helps keep the floor dry and clean, I then add a a thick layer of straw on top. I buy the straw at £4 a large bale which lasts a while. I just change the straw/top up the straw as the easibed stays mainly dry. The benefit of this system is also the fact that straw breaks down quickly in our compost heap!