If you want to build a flat roof, then you will need to build a flat roof truss. These are designed to span the space between two load-bearing walls, and take the weight from the roof onto these walls. A flat roof truss is also known simply as a flat truss and is not completely flat, but rather slightly sloped in order to allow water to run off of the roof. In order to make this correctly, you will have to work out exactly what you need to do with your truss in order to create a flat roof.
Step 1 - Draw Out Your Plans
You should begin by drawing up some plans which you will use to work with. Begin by measuring the walls which you want to use as the braces. Some designs which might prove useful are known as the hip truss and the pergola truss. Both of these are designed to help install flat roofs, so you should research these shapes. If you wished to do a hip truss, you would need to have the internal walls sloping towards each other slightly, as well as a broader upper area. Make sure that you include the slight incline of your flat roof, in order to ensure that it doesn't develop pools of standing water.
Step 2 - Start the Brace
Start your brace by constructing a square of timber along the top of the outside walls, and then building your first support from this. Once the square of timber has been created, cut two pieces of 2 x 4 so that they incline towards each other. Cut in half, with one beam slightly lower than the other. Add a beam between these two slopes. Adjust this until it fits in with your design.
Step 3 - Continue the Truss
Once you have this first piece of the truss completed, you can repeat the shape again half a foot away. Make sure that the slope is the same all the way along. With the second beam to go in, you will need to construct a support beam between the two 2 x 4s. This should be screwed into the square of timber. Repeat for the rest of the truss until you get to the other side. On the final structure, you will not need the beam going between the two walls.
Step 4 - Add More Support
Once the basic structure of the truss is complete, you can then proceed to support the truss by connecting the upper and lower beams. Start by attaching 2 vertical beams to the upper length of the truss, and then bringing it down to the supporting beam. Follow this by attaching smaller timber to the lower corner where the vertical and horizontal beams meet, and then attaching it to the sloping beams. Add bricks to the support walls until you reach the upper edge of the truss, and then you can attach your roofing boards to this.