Sometimes nature doesn't work in our favor and we have to shoot on overcast days, or on days where the sky won't cooperate. In these cases I do a quick sky replacement. There are many ways to do a sky replacement, but I've found the technique here to be my favorite, since it gives a very natural look, and doesn't require any special plugins or other software besides just Photoshop.
Before and After
Here are shots before and after sky replacement. Click on either picture to see full size. I could possibly deliver the before shot, but it's bland and the sky has excessive glare (photo was taken as fog was burning off behind the house that morning). I could darken the sky in Lightroom or Photoshop, but in this case a quick sky replacement really pumps it up and adds a wow-factor.
Step 1: Select a sky
I keep a folder filled with various skies I've shot in the area, and for this case I'm using the one below. Note the lighting angle also matches the before shot as well (lighter on the right). This sky also looks a bit flat, but that's because it's RAW -- we'll adjust that later -- but starting with RAW on these skies lets me have full control on how it will blend with the original shot.
Step 2: Place sky over photo
I then place the sky photo as a new layer over the original (background layer), and scale/position until I get just the sky where I want it. For example, I show this with a lower opacity, but once placed, turn the opacity of the sky layer back up to 100%.
Step 3: Make Layer Mask
Place a layer mask on the sky. There are tons of ways to do this, I do it with keyboard shortcuts to the menu items Layer->Layer Mask->Hide All.
Step 4: Select Color Range
With the background layer selected, go to Select->Color Range and you will see this pop-up dialog. Click on the sky, then move the Fuzziness slider until the sky is white and the trees and roof are black, and then click OK. Note this works best when the sky is one color, so blown-out skies work best.
Step 5: Feather Selection
Go to Select->Modify->Feather and feather the selection by 2 pixels. Note the marching ants showing the selection.
Step 6: Apply Gradient
The next step may look tricky if you've never done this before, but it really is simple and only takes a few seconds. Select the sky's layer mask, then select the gradient tool, and make sure your colors and the tool settings are the same as shown here. Then just drag/apply the gradient from just below the roof to just above the roof. You may have to play around with the gradient to get the correct blending. Also, the reason for this kind of blending is that skies are lighter the lower they are on the horizon (due to a greater volume of air for sunlight to travel through); thus, this gradient, besides blending nicely with the roof, gives a more natural look.
Step 7: Cleanup & Adjust
The last step is to use a low flow soft eraser to cleanup any of the sky left on other parts of the roof (and in this case the flag also). Make sure the eraser is soft, and that the flow is NOT 100%...flow should be around 10% -- you don't want to cut-out the leftover sky, just blend it away. I then add quick adjustment layers above the sky layer, with a clipping mask applied to each (so they only affect the sky, not the background/original layer). In this particular shot, I used a saturation layer and a levels layer, each with a clipping mask affecting the sky. Do these adjustments until you get the results you want.
Step 8: Done!
That's it. I realize it may seem like a lot of work, but once you get this down it takes only 1 to 2 minutes to process. Here is the final shot:
I suggest taking tons of photos of skies in your area, keeping these in your own sky replacement library (as RAW files). Then, practice practice practice! The more often you try this under various conditions (and roof lines), the more skilled you'll become, and the faster you'll be able to process these kind of shots.
If you want to see more, checkout my sky replacement page for some other examples.