Everyone waits until summer for their roofing projects or to build their new pool. This is a smart time of the year, for the weather anyway, but you need to consider contractors' busy times, the market fluctuation on material costs, weather, and a gaggle of other things, especially if cost and time are of great importance.
Standard Market Practice
After Christmas Sales. Going to lunch before 11 or after 1 to beat the crowds. Working an hour later to avoid traffic. Supply vs. Demand. Simple enough, right? Let this be your guiding principle for all major home improvement projects. What you have to acquaint yourself with is that the price of materialslumber, shingles, paint, cement, etcfluctuate during certain times of the year. On top of that, contractors have busy seasons where they are turning away work and times of the year when they can't find enough.
So the trick is that you can save large sums of money and have the project work to your advantage just by being flexible and planning ahead.
Contractors' Busy Times
There's not enough space to list out every home improvement professional and their particular times of the year that are busy or slow. After all, this will depend on a great many factors. However, you can intuit that deck builders, roofers, pool builders, and landscapers are going to be very busy during the summer. You can probably put it together that there are not too many window replacements or air conditioners being maintained during the winter. This brings me to my second point: materials.
Material Fluctuation
Just like contractors have busy times, the materials they use will also fluctuate with market demand. In some cases, market down tics can be to your benefit or to your peril, but in most cases, overhead usually needs to get moved quickly and, thus, prices typically drop during slow seasons.
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I know what you're saying, who is going to build a deck in January when the ground is frozen? or Why would I replace my roof during the rainy season? I'm not saying that you should. What I am saying is that you can purchase these materials while they are cheap, store them in your garage or rent a storage unit for $30/month (as long as the numbers make sense), and once the weather permits, start building. If you think about an average deck, you are going to spend around $2500-$6000 in materials alone. If you can save a few hundred dollars by waiting for the market downturn in the winter, that can help allay the price.
Putting This Knowledge to Work for You
Now that you are thinking about these strategies, it's time to put them to work. What projects are coming next? When are the contractors for this job at their busiest? When are materials at their cheapest? Using this knowledge to your benefit should not only save you money on materials, but it also gives you a point of leverage with contractors.
During their busy season, contractors can afford to charge more because they don't really need your business anyway, and they might as well try to get themselves a great price. On the other side of that coin, during their slow time, if you don't like their price, you have more of a foothold for negotiating. For big projects like window replacements or home additions or remodels, this could mean thousands in savings. Plus, they are likely to be ready to begin right away, which gets the project finished faster (might even make you their only customer during that time), and makes you a happy camper.
Lead Us Not into Exceptions
Of course there are exceptions. The weather will eliminate your ability to use this knowledge to your advantage, especially with landscaping projects. Some projects can only be done during certain times of the year (planting specific trees and plants, winterization). But for the most part, this type of knowledge can be used for most large home improvement projects (not to mention everywhere else that things are being sold). 
Matt Myers is a freelance writer for the home maintenance and remodeling industry. Formerly a contractor specializing in deck building and casework, Matt has written over 500 articles for both homeowners and contractors.