If you're building a LEGO house, the hardest part can be assembling the roof. Fortunately, roofs are pretty easy to make if you have the right parts in front of you. There are even a number of alterations you can make to build the roof that works best for your model. With LEGO, there's a huge number of parts that you can use, and if you don't like the roof, you can always take it apart and start from scratch.
Build an even top layer on your house model, so you have a flat layer to build upon. Use similar bricks to those that make up the rest of the house, and fill in any gaps that make the roof uneven.
Place a layer of slanted pieces along two opposite edges of the house. Use bricks that slant 2 or 3 studs down, depending on the angle you want for your roof. Try to use as many 4-stud-wide pieces as will fit the width of the roof, filling in gaps with 1- or 2-stud-wide bricks. Most houses have an overhang on the roof--about 1 stud will suffice--although this is optional. On the ends of the house with no slanted pieces, extend the walls up by one layer.
Repeat the process of adding slanted bricks and an even layer until the the two edges meet in the middle--at the crown of the roof. Match the new angled bricks with the angles of the lower bricks. The bricks should come toward the center by two or three studs with each new row.
Place a row of angled flat bricks across the crown. These bricks are 2-studs wide, flat with a pair of angles that meet in the middle, so they lack studs across the top. Again, use 4-stud-long bricks for most of the roof, and fill in any gaps with 2-stud pieces.
Start with an even top layer as before. Along one edge, add 3-stud-long angled bricks. Overhang the roof by one stud if you'd like to. Fill in as much of the roof as you can with 4-stud wide bricks, and fill in any gaps with 1- or 2-stud pieces.
Build a layer of the house walls to meet the angled bricks. Level up the other three sides of the house, so the top is even again, using similar bricks to the rest of the house. Viewed from the side, the top of the house should be flat, with the angled row to one side.
Repeat the process of adding angled bricks and leveling the three other sides until the angled bricks reach the top. Place the new rows of angled bricks so they continue the angle of the previous rows.
Even the top of the house model as before.
Use flat plates to build a flat surface on the top of the roof. Flat plates come in a variety of shapes and sizes, so use the largest ones you have first to cover the house. You want to use as few pieces as possible to make a sturdier roof.
Add a second layer to the roof to make it stronger and sturdier. Cover as many gaps on the lower row with plates from this one.
Build a banister out of 1-stud wide bricks around the outer edge of the roof, if you'd like to make this into a roof deck or roof garden. You should also add a small square structure with a door that looks like a roof access to complete the effect--as in the photo. If you're content with the flat roof, you can finish here.
Wherever possible, try to place bricks across gaps between pieces on a lower row. This makes the structure stronger overall, and looks a little more realistic.