I purchased an older home, 1920's style about 4 years ago, and apparently one of the valleys has been leaking into my attic for a bit unnoticed and looking at the general condition of the roof, it's time for a replacement.

I had a contractor come out and quote me on replacement, and while the cost of shingles, installation and flashing, etc, all came out to be expected pricing, when he looked in my attic he said there would be some additional expense to redecking the house - to the tune of $3000.

The way he explained it to me is that the current decking is spaced 2 inches apart, and that the cedar shakes are nailed onto those separated decking boards - new shingles need decking with at the most half an inch of space between the decking boards, and so he said in the process of tearing out the two layers of shingles and the cedar shakes, he'd need to redeck before placing the new shingles on.

Does this sound like something really necessary to complete the reroofing on my house? Does redecking a house REALLY cost $3000? We're not talking about a lot of roof, my house is a 25x30 one and a half story single family home, very plain. Couldn't one just tear off the existing shingles and shakes, affix a layer of treated plywood or somesuch to the existing decking, and then place the new shingle layer, etc, on top of that?