Step 1: Plan It Out
How deep you'd like your seat to be is the next question. I build this one to be 24" deep so we could use a rug runner instead of cushions. Runners are 21"-23" wide. How deep your seat is will also determine how wide it will be. At 24" deep, mine turned out to be 8' 5" long. Since sheets of plywood are 8' long it would have been easier to make the front shorter than 8' long and let the depth work itself out. In that way you could rip an 8' piece of plywood for the front whereas I used 2 pieces.
On www.askthebuilder.com I found a sketch of the framing for a window seat (pic 2). The height they recommend is 19" with a 21" deep seat. You can see from the drawing that a window seat is actually a short floating wall achored to 2x4 cleats on the wall. The front can be wood, sheetrock or other materials since it is only decorative. The one thing I did differently from the drawing was to place the 2x4s on edge as opposed to typical wall framing. This makes a sufficiently strong wall and also creates a bit more storage space inside the bench.
Step 2: Supplies You'll Need.
Other things you'll need for this project are construction adhesive (Liquid Nails), silicone caulk, wood putty, sandpaper, primer, paint, 3 1/2" wood screws and nails. A trim nail gun and air compressor are ideal and worth renting if you don't have them. A miter saw is also very nice for the many angle cuts. In addition, you'll need the usual carpentry tools such as a level, stud finder, hammer, nail punch, circular saw, etc.
You'll see as we go along that I used pocket screws for constructing the 2x4 frame and also the poplar trim. Pocket screws are a quick and easy way to make strong tight joints and are applicable to virtually every type of woodworking. If you aren't using pocket screws, check out http://www.kregtool.com/pocket-hole-jigs-prodlist.html and join the fun!
I don't hate painting if painting is my project (e.g. paint the kitchen). What I hate is building my project and then having to paint it. I get that "You did it!" feeling and then realize I have to paint it. To remedy this problem I paint before I start my projects when possible. While I'm scratching my head and measuring and figuring, I run to the shop periodically to lay down a coat of paint. It only took me about 30-45 minutes to paint the boards and plywood each time. As it dries, I go back to scratching and starring blankly at the walls. When the project is built, just dab some wood putty in the holes, touch up the paint and you're done. It is so much easier it's insane.
Step 3: Building the Frame
The next step is to remove the baseboard and build the front wall frame. I chose to build the wall next so I could insure it was parallel to the back wall. Alternatively you could mount the other 2x4s on the side walls, however, this seemed a simplier way to determine where the wall would actually be located. Both the top and bottom 2x4s have 45° cuts on both ends to match the wall. The front wall is only 18 1/8" tall so shims could be placed underneath it to bring it to level with the wall mounted 2x4. True to form, my floor had a crown in the middle and it was necessary to shim both ends.
My design was for 4 front panels so I placed five 2x4 blocks in the front wall. These blocks are located behind where the poplar vertical trim pieces will be which allows for nailing. I'm not sure why I didn't have the furthest left & right blocks all the way at the ends of the wall, but I ended up adding a block on each end for trim nailing (pic 3). Note the pocket screw holes in the blocks.
The final steps are to add the 2x4s on the side walls and add some bracing for the top. The side 2x4s have a 45° angle on the front end so it can butt up to the front wall. Once they are screwed to the walls using 3 1/2" wood screws you can also place screws through the front wall and into the side 2x4s to lock the corner. Three 2x4 were installed between the front and back walls using pocket screws. The middle 2x4 is centered and the outer 2x4s are located where the end of the lid will rest.
Lastly, this is a very good time to reinstall the baseboard & quarter round on the inside of the box.
Step 4: Building the Faceframe
Picture 2 shows the finished faceframe. Doesn't it look nice painted!! You may also notice that the 2 inside rectangles are slightly smaller than the outside rectangles. This is by design to make the box look less symmetrical and "exact". When something you build is very precise and regular, it doesn't look natural. Nature is much more random than it is regular which can make a large symmetrical piece appear odd and less pleasing to the eye. When I have the opportunity, I try to add a bit of asymmetry to projects. No one will be able to tell you why they like it better, but they will like it better.
Step 5: Cover the Front Wall
Step 6: Building the Window Seat Top
Next I measured the distance from the front edge of the 5" piece to the front of the box to get the width of the lid. Since the piano hinge requires almost 3/16" space I cut the lid to 19" which was the actual distance measured. This gave me 3/16" overhang on the front of the box when the hinge was installed. I chose to make one large lid, however, you could easily make two. Alternatively, you could also make a solid top and doors on the front. In this case, I made one lid which extended to the center line of the outside 2x4s. In this way, the side edges of the lid rest on the 2x4 for support. The lid measures 19" x 51".
Before you glue & nail the 5" back piece in place, it's much easier to go ahead and attach the lid to it. This is a job you don't want to do from inside the box. Picture 3 shows how a piano hinge is attached. A piano hinge is a long continuous hinge which is very strong and a perfect solution for this project. PIano hinges commonly come in 30" and 48" versions and 48" would have done the job. I chose instead to use two 30" hinges so the entire gap would be filled. This was easier than mortising a space for the 48" hinge. Piano hinges can be cut to the desired length using a hacksaw or portable grinder. The hinges I used have sides that are 3/4" wide so I mounted them flush with the bottom of the 3/4" plywood. In picture 4 you see that the hinges protrude a small bit but it's not noticeable when the rug in laying on top of it.
With the 2 pieces hinged together, there's a couple things that need to be done before permanently attaching them to the box. First is to mark out and cut the 2 smaller side pieces. This is most easily accomplished with the lid & back piece in place so you can measure size. Leave a 1/16" gap between the lid and the side pieces in case the wood expands. Plywood doesn't move much but a little gap is needed. Once the side pieces are cut, the second thing is to attach the 1 1/2" trim to the front of the lid and 2 side pieces as in picture 5. Some wood glue and nails is sufficient.
Now put it all together with some Liquid Nails and some solid nails. Lay down a bead of Liquid Nails on the 2x4s which are under the 5" back piece and nail it in place. Then do the same with the 2 side pieces. Make sure to avoid getting the adhesive under the lid --especially when positioning & nailing the side pieces. The last picture is the 3/4" cove molding along the top edge of window seat. I like cove molding here but you could also use quarter round or a larger piece of trim.
With everything in place it's time to touch up the finish by filling nails holes with wood putty, caulking the seams and giving it another coat of paint.
Step 7: All Done!
Thanks for looking! I look forward to any questions or comments.