why don't you have 6" cement deck instead and have a nice surface put on it . theres lots of bricks,slabs etc you can use on top . you have a frame your asking for trouble .

The birds are not short of food at the moment so stop feeding them until you are sure the rats have moved off to somewhere else. Before you start feeding them again use a bin lid or something similar suspended under your bird feeder. If you or your neighbours throw out bread etc for the birds tell them there is a rat problem and advise against the practice. I think a solid base as suggested by Np is probably your only answer. Do you know where the rats are coming from. When foundations were dug for the house next door we saw rats for the first time in over thirty years. We took appropriate steps to get rid of them and they seem to have moved to more lucrative hunting grounds.

I'm sorry to say that, even if you blocked all round the deck with something, rats and mice will still get under there, they love a nice deck to live under, they just tunnel in through the ground. What were you intending to put on the soil beneath the deck, assuming it is soil? And is there a reason why it needs to be a deck rather than paving (I'm assuming there is, that maybe its a sloping area or very much lower than your house, or maybe the relative cheapness of a deck compared with paving)?

I mean a concrete pad with hard core under it and just pretty the top up .

Thanks for all the replies, I should have mentioned that I already have a concrete base where I wanted to put the new deck. The broken glass or small stones would have been put between the concrete pad and the bottom of the deck.I think I'll have to go with a stone patio instead. Snoopdog your photo of the two Jack Russells are the image of my two, thanks again for the replies.

its the void that that makes a perfect home for rats so if you want to feed birds then you will be best making a concrete deck . your welcome .