Level and Mark Post Heights
Use a water level or laser level to mark consistent post heights across all posts. Account for beam thickness and joist depth to ensure your deck frame will be level.
Quick Summary
1-2 hours
Water or laser level
Within 1/8 inch
Deck height - layers
Why Accurate Height Marking Matters
The Foundation for Everything Above
Every component that goes on top of your posts - beams, joists, and decking - depends on the posts being cut to precisely the same height. If posts are uneven by even 1/4 inch, your beam will rock, your joists won't sit flat, and your finished deck will have visible problems. This is your last chance to ensure a level foundation before committing to cuts.
This step requires careful calculation and precise measurement. You're working backward from your desired finished deck height, subtracting each layer (decking, joists, and beam) to determine exactly where each post should be cut. The math isn't complicated, but getting it wrong means starting over with new posts.
The key to success is using a reliable leveling method. A 4-foot carpenter's level isn't accurate enough for spans of 10+ feet. You need either a water level (highly accurate and inexpensive) or a rotary laser level (faster but more expensive) to transfer heights accurately across your entire deck span.
Understanding the Height Calculation
The Formula
Post Top = Deck Surface Height - Decking - Joists - Beam
(All measurements from a common reference point)
Typical Component Depths:
Decking
- 5/4 boards: 1"
- 2x boards: 1.5"
Joists
- 2x8: 7.25"
- 2x10: 9.25"
- 2x12: 11.25"
Beams (if joists rest on top)
- 2x8 doubled: 7.25"
- 2x10 doubled: 9.25"
- 2x12 doubled: 11.25"
Example Calculation
Let's say your ledger top is 48" above grade and you want the deck surface level with the ledger top (standard configuration):
Deck surface height: 48" (level with ledger top)
Minus 5/4 decking: -1"
Minus 2x10 joists: -9.25"
Minus doubled 2x10 beam: -9.25"
Post top height = 28.5" above grade
Important: Beam Configuration Affects Calculation
The calculation above assumes joists rest on top of the beam. If your design has joists attaching to the side of the beam with joist hangers, you don't subtract the beam depth - the beam top would be level with the joist tops. Verify your framing configuration before calculating.
Leveling Methods
Water Level (Recommended for DIY)
A water level is simply a long clear tube filled with water. Since water always seeks its own level, the water surface at both ends of the tube will be at exactly the same elevation regardless of distance or obstacles between them.
Advantages
- - Extremely accurate (within 1/16")
- - Very inexpensive ($15-30)
- - No batteries needed
- - Works at any distance
- - Works around corners and obstacles
Considerations
- - Requires two people
- - Slower than laser level
- - Must avoid air bubbles in tube
- - Water can freeze in cold weather
Rotary Laser Level
A rotary laser level projects a level line around the entire work area. You can work alone, moving from post to post with a detector that beeps when it finds the laser line.
Advantages
- - One-person operation
- - Faster for many posts
- - Useful for many other projects
- - Works in any weather
Considerations
- - Expensive ($200-500 for quality)
- - Requires batteries
- - Bright sun can make line invisible
- - Must be set up on stable surface
What NOT to Use
Don't rely on a carpenter's level and straight board for spans over 8 feet. Even quality levels have slight inaccuracies that compound over distance. A level that's off by just 1/32" per foot creates 3/8" error over 12 feet. Use a water level or laser level for accuracy.
Step-by-Step Process
Calculate Your Target Height
Using the formula above, calculate where the top of your posts should be. Write this number down and double-check your math. A calculation error here means cutting all your posts wrong.
Consider whether you want any slope for drainage. If adding slope (typically 1/8" per foot away from the house), posts farther from the house should be proportionally shorter. Most deck builders skip the slope since deck boards have gaps for drainage.
Establish Your Reference Point
The ledger board is your best reference since it's already installed at a known height. Measure down from the top of the ledger by the joist depth (since joists will hang from the ledger). This gives you the reference height for post tops.
Mark the Reference on First Post
Transfer the reference height to your first post. This is easiest using a long level or by measuring down from a string line stretched from the ledger. Make a clear mark all the way around the post using a speed square.
Double-check this mark by calculating up from the anchor. The distance from anchor to mark should equal your calculated post height. If these don't match, find your error before proceeding.
Transfer Height to All Posts
Using your water level or laser level, transfer the height mark from your reference post to every other post. Take your time - rushing leads to errors. For water levels, wait for the water to stop moving completely before marking.
Verify Before Cutting
Before making any cuts, verify your marks by spot-checking with a different method. For instance, measure from each mark to the ground and confirm the measurements make sense given your site conditions.
Also stretch a string line from the ledger reference point to the far posts and see if it passes at your marks. This is a quick visual check that all posts are at consistent heights.
Cut the Posts
Using a circular saw, cut along your marks on each post. Since a circular saw blade won't cut through a 6x6 in one pass, cut along each face, keeping the saw base flat against the post. The center may need finishing with a reciprocating saw.
Do You Need to Slope Your Deck?
There's ongoing debate about whether decks should be sloped away from the house. Here's what you need to know:
Arguments for Slope
- - Water drains away from house
- - Fewer standing puddles
- - May extend deck life slightly
- - Required by some local codes
Arguments Against Slope
- - Deck boards have gaps for drainage
- - Adds complexity to construction
- - Furniture may roll or sit unevenly
- - Not required by most codes
If You Choose to Add Slope
The standard drainage slope is 1/8" per foot away from the house. For a 12-foot deep deck, the outer posts would be cut 1.5" shorter than calculated for level.
- - Calculate the slope adjustment: (deck depth in feet) x 1/8"
- - Mark outer posts at the adjusted height (standard height minus slope)
- - Middle posts get proportional adjustment
- - Verify with a string line stretched from ledger to outer beam
Common Mistakes to Avoid
Forgetting to Account for Beam Configuration
If your joists sit on top of the beam, subtract the beam depth. If joists attach to the beam side with hangers, don't subtract beam depth. Getting this wrong puts your whole deck at the wrong height.
Using a Standard Level for Long Distances
Even quality carpenter's levels have accuracy limits. Small errors compound over distance, resulting in posts that aren't truly level. Always use a water level or laser level for spans over 8 feet.
Not Verifying Before Cutting
Once posts are cut, there's no going back. A post cut too short must be replaced. Take five extra minutes to verify your marks using a second method before picking up the saw.
Ignoring Actual Lumber Dimensions
A 2x10 is actually 1.5" x 9.25". Using nominal dimensions (2" x 10") in your calculations leads to significant errors. Always use actual dimensions.
Frequently Asked Questions
How do I calculate the correct post height for my deck?
Start with your desired deck surface height. Subtract the decking thickness (typically 1 inch for 5/4 decking or 1.5 inches for 2x), subtract the joist depth (9.25 inches for 2x10 or 11.25 inches for 2x12), and subtract the beam depth if joists rest on top of the beam. The result is your post height from the anchor.
Should I use a water level or laser level for marking post heights?
Both work well. Water levels are inexpensive ($15-30), very accurate, and work at any distance without batteries. Laser levels are faster and easier to use alone, but quality rotary laser levels cost $200+. For most DIY deck projects, a simple water level is the better value and is extremely accurate.
Do deck posts need to be exactly the same height?
Yes, posts supporting the same beam must be cut to exactly the same height (within 1/8 inch) for the beam to sit level. If posts are different heights, the beam will rock or require shimming, which weakens the connection. Take time to mark heights precisely using a reliable leveling method.
Should I slope my deck away from the house?
Most building codes don't require deck slope since deck boards have gaps for drainage. However, a slight slope (1/8 inch per foot) away from the house helps with drainage and prevents puddles. If you add slope, account for it when marking post heights - posts farther from the house should be slightly shorter.