Determine optimal On-Center (OC) spacing and total joist count.
5/4″ Pressure Treated / Cedar (Standard)
2″ Nominal Lumber (2×6)
Composite / PVC (Standard Residential)
Composite (Commercial / Heavy Load)
90° Perpendicular to Joists
45° Diagonal to Joists
This is the dimension perpendicular to the joist run.
Recommended Max Spacing (OC):–
Total Joists Needed:–
First-to-Last Joist Span:–
*OC = On Center (center of one joist to center of the next). Calculations account for a starter and finisher joist.
Understanding Joist Spacing for Decks
Building a safe and durable deck requires strict adherence to structural standards, specifically regarding joist spacing. The spacing of your framing joists is determined primarily by the strength and material of the decking boards that span across them. If joists are spaced too far apart, the decking boards may sag, bounce, or even break under load.
What is "On-Center" (OC)?
Construction spacing is measured "On-Center". This means the measurement is taken from the center of one joist to the center of the next, rather than the gap between them.
Common Joist Spacing Standards
The maximum distance allowed between joists depends on the type of decking board you intend to install. Here are the general rules for residential applications:
Decking Material
90° Placement (Perpendicular)
45° Placement (Diagonal)
5/4″ Wood (Cedar/Pressure Treated)
16″ OC
12″ OC
2″ Nominal Wood (2×4, 2×6)
24″ OC
16″ OC
Composite / PVC
16″ OC (check manufacturer)
12″ OC
Why Diagonal Decking Changes Spacing
When you install decking boards diagonally (usually at a 45-degree angle to the joists) for aesthetic appeal, the actual distance the board must span between joists increases. To compensate for this longer span and prevent sagging, the joist spacing must be tightened.
A standard 16″ OC spacing typically reduces to 12″ OC for diagonal installation.
A stronger 24″ OC spacing typically reduces to 16″ OC for diagonal installation.
Composite vs. Wood Decking
While wood has natural rigidity, composite decking (made of plastic and wood fibers) is often more flexible. While many modern composites are rated for 16″ on-center spacing, they can feel "bouncy" in hot weather if spaced at the maximum limit. Many professional deck builders prefer 12″ OC for composites to ensure a solid, concrete-like feel underfoot, even if the manufacturer allows 16″.
Calculating the Number of Joists
To calculate how many joists you need, divide the total width of the deck (in inches) by the spacing (in inches) and add one (for the end joist). However, layout is rarely perfect. You will often need to adjust the spacing slightly inward to ensure equal gaps or to accommodate railing posts.
function calculateJoistSpacing() {
// Get Inputs
var material = document.getElementById('deckingMaterial').value;
var angle = document.getElementById('boardOrientation').value;
var widthFeet = parseFloat(document.getElementById('deckWidth').value);
// Validation
if (isNaN(widthFeet) || widthFeet round up to 8 spaces.
// Actual spacing = 120 / 8 = 15 inches.
var actualSpacing = widthInches / numberOfSpaces;
var totalJoists = numberOfSpaces + 1;
// Display Results
var spacingDisplay = maxSpacing + '" (Maximum)';
// If the math results in a convenient even spacing, show that too, but keep it simple
// We will output the Max allowed, and the count required to stay UNDER that max.
document.getElementById('resSpacing').innerHTML = maxSpacing + '" On-Center';
document.getElementById('resCount').innerHTML = totalJoists + ' Joists';
document.getElementById('resSpan').innerHTML = widthFeet + ' ft (' + widthInches.toFixed(0) + '")';
document.getElementById('results').style.display = 'block';
}