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How to measure the level & plumb position of a deck, floor, post, wall or other structure. This article summarizes methods to check that a deck or other structure is level and plumb - important measures to assure that a structure and its connections are safe and properly made.
This article series describes critical safe-construction details for decks and porches, including avoiding deck or porch collapse and unsafe deck stairs and railings.
Get the Deck or Any Other Structure Plumb, Level & Square
The cardinal rule of carpentry can be summarized as follows: build it plumb, level, and square.
The best materials, finest tools, and greatest design cannot overcome a structure that has been put together contrary to this principle, since errors in any one of these attributes can quickly multiply as the project progresses.
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“Plumb” means true to a vertical plane. A plumb object takes maximum advantage of gravity, transferring loads directly to the earth. “Level” means true to a horizontal plane (that is, to the horizon).
And when a plumb object meets a level object, they form a 90-degree angle, which is the basis of “square.”
Fortunately, each of these properties can be checked easily with the techniques described here. By monitoring your deck building for plumb, level, and square at each step, you can ensure that succeeding steps start off on the right foot.
How to Establish Plumb - Perfectly Vertical Posts, Walls, Other Structures
When you start installing the posts for your deck, you will need to make sure that each one is plumb. To do so, hold a carpenter’s level securely against the post, then move the post until the bubbles are centered in the two end vials. Check and plumb two adjacent sides.
Another tool that can be used is a plumb bob. On a still day, a plumb bob will hang perfectly straight up and down from a piece of string.
How to Find Out if a Framing Member or Structure Level
Beams, joists, and decking should all be level. Check them with a carpenter’s level placed in the horizontal position. When the bubble is centered in the middle vial, the object is level.
When you are using a level to check a long board or post, place the level at several spots along the entire length. If the bubble changes resting spots from one location to another, the lumber is probably bowed and may need to be replaced.
Squaring up an irregularly-shaped deck (left) still requires that its surface be properly leveled and its posts properly plumb.
Levels are some-what delicate tools. To see if yours is accurate, place it on a flat, solid surface that is nearly plumb or level. With the level in position, take a close look at the location of the bubble. Then, without lifting the level, rotate it on its axis 180 degrees, and let it come to rest in the same position on the surface. If the bubble 4s in the same position as before, the level is accurate.
Sometimes you need to establish a level between a reference point on one surface, such as a ledger, and a point on another, such as a post, without having an installed board between them on which to set a level.
There are several methods to accomplish this chore easily and reliably.
The first is to set the level at the center of a long, straight board, then move the board up and down until the bubble is centered This effectively allows you to turn a 2- or 4-foot level into an 8-, 10-, or 12-foot level.
For those times when a carpenter’s level will not reach, such as when you are establishing ledger locations on two sides of a house or are determining post heights for a large deck, a water level is indispensable. It operates on the principle that water in a flexible tube will seek the same level on each end. You can make your own using any length of 3/8-inch or 5/i6-inch clear vinyl tubing.
Fill it with water and a little food coloring for visibility. Make sure there are no air bubbles or kinks in the tubing and keep the tubing ends open. Set one end of the tubing so that the water level aligns with your reference mark.
Then place the other end of the tubing in the desired location. Once the water stabilizes, it will be level with the other end.
An even simpler approach is to buy a commercial water level that beeps when level has been established in the tube.
Buy, rent, or borrow a dumpy level, transit, or builder's transit or level and tripod. This approach can be very accurate but requires some learning and skill to make proper use of the instrument.
How to Check the Deck Floor Frame or any Wall or Floor Framing Installation for Squareness
Most decks are rectangular or made up of several adjoining rectangles. If the corners of the frame are not square, the decking will be harder to install and the results may look odd or worse. There are two methods used by carpenters to check for square, depending on the situation.
Measure Diagonals to Determine if a Frame is Square
To quickly determine if a rectangle is square, measure both diagonals. If they are identical, the corners are square. Be sure in using this method to confirm you are working with a true rectangle; check that parallel sides are exactly the same length.
Use the 6-8-10 rule to Determine square or right angles in deck, porch, floor, wall, or roof framing or any other buildng framing or construction task
To check a single corner for square, use the “3-4-5” technique. Measure along one side 3 feet, along a perpendicular side 4 feet, then measure the diagonal formed between these two spots.
If it equals 5 feet, the corner is square.
This approach can be even more accurate if you use a set of larger numbers that have the same ratio, such as 6-8-10 or 9-12-15.
Everything you ever wanted to know about the 6-8-10 rule for squaring up anything you're building can be read and the mathematical underpinnings of angles, slopes, and tangents can be read at FRAMING TRIANGLES & CALCULATIONS.
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 Steve Bliss's Building Advisor at buildingadvisor.com helps homeowners & contractors plan & complete successful building & remodeling projects: buying land, site work, building design, cost estimating, materials & components, & project management through complete construction. Email: firstname.lastname@example.org
Steven Bliss served as editorial director and co-publisher of The Journal of Light Construction for 16 years and previously as building technology editor for Progressive Builder and Solar Age magazines. He worked in the building trades as a carpenter and design/build contractor for more than ten years and holds a masters degree from the Harvard Graduate School of Education.
Excerpts from his recent book, Best Practices Guide to Residential Construction, Wiley (November 18, 2005) ISBN-10: 0471648361, ISBN-13: 978-0471648369, appear throughout this website, with permission and courtesy of Wiley & Sons. Best Practices Guide is available from the publisher, J. Wiley & Sons, and also at Amazon.com
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