When you are creating the frame for your garage, you need to fit a garage door header. The header is the large wooden beam above the door of your garage, which supports the walls and ceiling above, and helps to support the weight of any extensions which go over the garage. Fitting the header properly can be a very difficult project to undertake, as a garage door header which is too small can result in sagging or even serious structural damage to your home. Framing the garage door correctly can ensure that the header is not put under any additional strain, although you will still need to have a lot of help in creating the right kind of fitting for your header.
When you begin your framing, you need to work out the right size and shape for the header. This will depend upon the type of garage door you are using. A roll-up door, for example, will fit against a rough opening, while the double garage door will need to be fitted into the opening. This can affect the position of the header. Carefully measure the length of your door, as the header must be at least 9 inches longer than that, and at least 1 1/2 inches higher than the door. This allows the header to take the weight of your wall. Go to your local building office, and ask about the dimensions of the garage door header. They should be able to offer you advice which will prevent sagging.
Step 2 - Preparing the Header
Take the pieces of board which will form your header, and place 1 piece on your worktop. Cover this with wood glue, and then press the next piece onto it. Do this until you have built up your header. Make sure that all the pieces are positioned closely together so that they make up 1 board. The ends and the sides need to be flush with each other, in order to achieve this. Then, nail the pieces of wood together. Take careful measurements every few strikes, in order to ensure that the wood remains level. Then, trim down the header until it is the right size, removing any corners that might not fit into the rough opening. Position the header into the opening, and make a note of further removal needed from the opening. Remove the header, and make the final trims.
Around the sides of your door, add enough lumber to support the weight of the garage door header. You need at least 2 rows of wood, to hold up the header on each side, making up 4 studs on each half of the header. The smaller studs should be placed under the header as supports, and the larger stud should be used to nail the ends of the header to the frame of the surrounding garage. Use screws to hold this wood in position. You need to have double trimmers, known as cripples, due to the amount of weight pressing down on the studs.