Why build a roost box? We’ve had a sparrow roosting above our electrical box at night and, frankly, making a big mess. It gave me an idea. Our birds need a place to perch at night, out of the wind.
I went to the local lumber store and picked up a 1″ x 8″ (real dimensions 3/4″ x 7 3/8″) 8′ foot long piece of cedar (smooth on one side and rough on the other). Here’s a step by step.
I cut four 10″ long pieces. If you use a hand saw, remember to hold the saw like you’re holding a baby bird. 
You can also use your table saw to cut the 4 pieces. 
The 4 pieces are the back, 2 walls and the front door.
I use pocket screws to put the pieces together. You can also insert screws from the back.
I lay out where the pocket screws go. 
I use my kreg pocket hole jig to make the holes. 
Finally I add glue on the back wall and the sides that meet the back.
I measure the distance between the sides … 
.. and cut the door to that width or slightly more. Also trim the door so that there’s about 1/8 ” at the top (to open) and enough clearance at the bottom.
Make sure you put the hole in the door at this point. Lee Valley sells birdhouse portal protectors so the squirrels etc. can’t enlarge the hole. They are offered in four diameters to match the preferences of specific cavity-nesting birds: Eastern bluebird (1 1/2″), nuthatch (1 1/4″), chickadee (1 1/8″) and wren (1″). I used their guide to size my entrance hole.
Next, I clamp the door between the two sides .. 
and drill a hole about 3/8″ down from the top and in from the front. This will hold the dowel for the door to move on.
Drill the hole for the door dowel through the side and into the door. Make sure you drill them as straight as possible so the door functions. You can also use a long screw for this.
Measure the bottom and cut the bottom to size. I clamp the box square to ensure the box remains square. 
Then glue and screw the bottom in and … 
Glue and screw the top. 
Use the thin strip to build the ladder. I use 3/8″ dowels. 
Done!
Here’s what I found in my internet research that might be helpful.
References:
Convertible Winter Roosting House
How are Roosting Boxes Different From Houses?
Attract Birds With Roost Boxes
Small wild Bird Winter Shelter Roost Construction and Review Hot Glue?
Great idea!