Build a cross-gable roof or large dormer perpendicular to the main roof as a distinctive feature to your ranch-style home. Add a cross gable to create inside storage space in an attic; install a dormer to provide an upper-level window for additional daylight into the house. Follow the same basic construction for the cross-gable or dormer; the difference is mainly the size.
Decide on the pitch and width for the gable; look at design books and test appearances by drawing various outlines on a photograph of your house. Follow the same pitch of the existing ranch gable or create a steeper one, particularly on a dormer. Settle on a plan, either for a basic dormer or a larger gable and mark the outlines on your picture.
Go up to the attic and identify the rafters or truss chords on either side of the new gable. Drive 16d framing nails with a hammer through the roof outside those rafter edges to mark the corners. Install horizontal 2-by-4-inch braces to connect the tops of those rafters to corresponding rafters on the other side of the roof. Mark the rafter angles on the ends of those braces, cut them with a circular saw and fasten them with framing nails.
Climb up on the roof and snap a chalk line between the corner nails to mark the outline for the new gable. Use a pry bar to remove the shingles and roofing paper. Set the circular saw's blade depth to the thickness of the roof sheathing and cut out around the outline to expose the rafters. Keep a tarpaulin handy to cover the opening during construction.
Cut off the rafters inside the outline with a reciprocal saw. Cut the rafters at the bottom for a dormer; remove those rafters for a cross-gable addition, which will extend to the wall. Build a header to the width of the opening's top. Cut two 2-by-6-inch boards and a 1/2-inch spacer strip of oriented strand board to that width and nail it into a header panel with 8d galvanized nails and a hammer.
Install a second rafter board along the inside of the outer rafters, cut to match the angle of the basic rafters. Nail this to the rafters with 16d framing nails. Set the header in place on top of the doubled rafter and fasten it with nails driven into the ends through the rafters. Nail it also to the bottom of the cut rafters at the top.
Make two "valley" rafters to the desired pitch of the new gable. Leave a 1 1/2-inch gap between them at the top. Place these to connect the header to the doubled outer rafters and nail them to the header and the rafters. Use cut-off pieces of the top rafters to connect the header board and the valley rafters. Nail these onto the ends through the header and rafters.
Build a front wall inside the attic. Make a frame of top and bottom plates and end studs. Square it by measuring corner to corner until the diagonals are identical. Install internal studs on 16-inch centers; frame a window for a dormer but fill the wall for a gable end. Erect the front wall, set it plumb and fasten it to the top outside wall plate or to a bottom header for a dormer placed across the cut-off bottom rafters.
Fasten a top wall plate between the top of the front wall and the doubled gable rafters. Connect this to the rafters with studs on either side, cut at varying lengths to match the slope of the main roof. Nail these to the side wall plates on the top and the rafters on the bottom. Use framing nails driven in at an angle.
Cut rafters to the pitch for the new gable. Install the first pair onto the outside wall nailed to wall caps on both sides. Slide a 2-by-6-inch ridge board between the tops of the valley rafters and the end rafters and secure it with nails through the rafters. Use a level to make sure it is even. Install other rafters between the front wall and the valley rafters. Trim the rafter bottoms, as needed, to connect with the valley rafters toward the top.
Sheath the roof and walls with oriented strand board nailed to the rafters and studs with 8d galvanized nails. Lay roofing paper over the roof and install metal flashing with shingle nails at all edges and roof intersections. Lay shingles to match the original roof on the new gable roof. Wrap the walls and gable end with house wrap. Install siding to match the house on the front and sides of the new gable walls and ends.
Finish the new gable with 1-by-4-inch trim boards at the wall corners and along the slope rake edges of the new gable. Overlap vertical trim boards so the side of one covers the edge of the other.