How to Build a Level Post and Beam for a Carport
When building a carport, having the post and beam level is the number one priority. Here are three methods of setting the posts to do this.
Three Post Base Applications for a Carport
1: The post is placed in a hole and stabilized with concrete mix.
2: The post is placed on an existing concrete slab with a metal post base.
3: A metal post base is placed in a new hole with concrete mix.
No matter which method is used, it is very important that the base of the post is lined up with all the other posts, and that the height of the posts are the same. If the carport is to be attached to a house, start by determining how far away from the house the posts need to be. Measure and mark the location for the two end posts and square them up with each other.
The space between the posts will be determined by the size of the beam that you use. A rule of thumb for sizing a beam is that every inch in height it will span one foot. Therefore a 4×8 inch beam will span 8 feet. This can very depending on the amount of weight the beam will need to carry. Check with your local building department. If you are using fiberglass or other lightweight material for the roof, the rule of thumb should do.
If you are using method two, mark where the center of the post will be located on the concrete slab. You can use a snap line (chalk cover string) or just plain string stretched tightly between the first and last post locations. If there is nothing to tie the string to, drive a stake off the concrete at either end to line up the bases. Another method would to be to use two long concrete nails and driven in on the out side of where you want the post to be. Make sure the string lines up with either the center or one edge of the post base.
With locations now marked, use the correct size of concrete drill bit to bore a hole for the bolt to hold the base in place. Some bases have an adjustable bolthole in case the hole ends up not exactly where you wanted it. The bases will all be the same size, but make sure they are level by verifying with a string level. Starting out level is the key, and the concrete slab may not be.
Method one will require that you use pressure treated wood for the post to avoid rotting. The hole will need to be two feet deep and eight to ten inches across. Stretch a line to get the holes lined up. The center of the hole should match the spacing you decided on, but you have a little leeway. With the holes all dug you will also need another line at the height you want the post to be. If there is nothing to tie the string line to, you can build a temporary tripod out of 2x4s with one leg facing the line of posts. This will tell you when you have the bottom of the post at the correct depth.
Adding or subtracting a little gravel to the bottom of the hole can adjust this. The gravel needs to be in the bottom for firm base for the post anyway. Once the post is ready to be set, use two 2×4 or 1×4 boards and some stakes to secure it in a level upright position. Add the concrete mix to the hole. Be sure to check that the post stays level in the upright position during and after you have added the concrete mix.
If you are using method three, locate and dig the holes as in method 1 above. There are several types of metal post bases that you add to wet concrete. Use the string line to line them up, making sure you use the same spot one each base. Depending on the base, some will need support to keep them where you want them until the concrete sets up a little. Scraps of wood long enough to reach across the concrete will usually work well. Sometimes small would shims come in handy for fine-tuning this. Be sure you keep the bases at the correct spacing during this, as there is no adjustment on some of these.
This type of base will keep your post off the surface so you will not have to use treated lumber. You will also want to check the inside corners of the post bases. Some are single piece and the corners are not always square. If this is so you will want to shave a little off the post edges that will be against corners so that the post sits flat on the bottom of the base. Again set a string line to get the top of all the posts at the same level. Use lumber and stakes to get the posts vertical and then fasten them to the bases as per manufactures specifications (nails or bolt).
You are now ready to put your beams up. There are several manufacturers of post to beam caps, some come in pairs. These are good especially if you are working by yourself. You can put up one half and attach it to the post, put up the beam letting the half you already put up act as a stop for the beam. This is a good time to double check the beam for levelness. If it is not, add some shims to level it, not be much if you have been diligent up to this part. Then add the other half to secure the beam in place. Leave the post supports holding the post vertical until you have the rafters up.