Adjusting Reference Lines
It is common that concrete foundations or slabs are not the exact dimensions that are shown on the plans. These need to be identified and corrected.
Common sense and experience are the best decision-making tools for approaching and correcting errors. Once you have your footprint sketch with the dimensions and square checks on it, you’ll be able to determine if there are any errors.
If a diagonal line is too long, then some of the lines at the end of the diagonal must come
in to make the diagonal the right length. (See “Square Correction" illustration.) Check the wall dimensions lines to see which lines can be shortened. Once you’ve determined the best way to make adjustments, speak to the superintendent about your suggestions. Typically, a fix will involve moving the wall in or out on the concrete foundation.
Depending on the finish, there is a certain tolerance that will allow for moving the walls without
affecting the appearance. It is common to have finish material that overhangs the foundation, so moving the wall out slightly may not be noticeable. It is also common to have the sheathing on the outside of the foundation, so that if the wall needs to come in, it can be adjusted in the thickness of the sheathing without affecting the look of the finish.
If corrections would cause visible errors in the finished building, then consider alternative measures. An example of a visible error would be if the concrete finish wall sticks out past the siding on the finished exterior wall. There are three methods that can be used to address errors in the foundation. These are as follows:
• Correct the foundation wall. This is the best solution, but often cost-prohibitive.
• Change the dimensions of the building. This is easy, but very often causes problems later on. Make sure to check that the change does not affect truss span if using roof trusses.
Also, check to see that the change does not affect dimensions of items such as bathtubs or cabinets. If a change is made, make sure it is made on all copies of the plans.
• Do not correct the errors. Correcting the errors might cause more problems or imperfections in the building than the errors will.
The “Footprint Sketch Dimensions" illustration is made on the job site. It will help determine how to best adjust your reference lines to make the building square. In this example, four dry lines are established to form a square. The diagonal distances that should be the same are then checked. Because they are different, the reference lines will need to be moved to make the diagonals the same. By comparing the actual and the planned dimensions of the walls that the reference lines are measured from, you can determine which reference lines should be moved. When you move a reference line, the other lines are affected.
If you have all the information down on your footprint sketch, you can come pretty close to knowing exactly how much to move each line, and keep making adjustments until you are comfortable with your accuracy. Once your reference lines are established, you can set all the other lines in the building from them. The measurements in circles on the sketch show the distance that the reference lines would be first moved. It is difficult to determine exact amounts because of the proportions, but if you study the footprint for a little while, you can come pretty close.
Checking level using a rotary laser.