Flush Beam (Joists inside hangers)
Dropped Beam (Joists on top of beam)
Height of the beam supporting the joists from underneath.
Understanding Deck Ground Clearance
When building a deck, the "clearance" refers to the unobstructed air space between the ground surface (grade) and the lowest structural member of the deck (usually the joists or the beam). Maintaining proper clearance is vital for the longevity of your structure.
Why Clearance Matters
Ventilation: Airflow prevents moisture buildup under the deck, which reduces the risk of mold, mildew, and wood rot.
Thermal Expansion: In composite decking, heat dissipation is necessary to prevent warping.
Code Compliance: Many local building codes require a minimum clearance of 6 inches for standard pressure-treated lumber that is not "Ground Contact" rated.
Low Clearance Solutions
If your calculation results in less than 6 inches of clearance, consider the following:
Use Ground Contact (UC4A) rated lumber for all framing members.
Install a vapor barrier and gravel under the deck to minimize rising ground moisture.
Increase the gap between deck boards to 1/4 inch to improve vertical airflow.
Consider a "sleeper" system if the deck is being built directly over concrete.
Standard Joist Dimensions
Nominal Size
Actual Height
2×6
5.5 inches
2×8
7.25 inches
2×10
9.25 inches
document.getElementById('beamSetup').onchange = function() {
var droppedBeamSection = document.getElementById('droppedBeamSection');
if (this.value === 'dropped') {
droppedBeamSection.style.display = 'block';
} else {
droppedBeamSection.style.display = 'none';
}
};
function calculateDeckClearance() {
var targetHeight = parseFloat(document.getElementById('targetHeight').value);
var deckingThickness = parseFloat(document.getElementById('deckingThickness').value);
var joistDepth = parseFloat(document.getElementById('joistDepth').value);
var beamSetup = document.getElementById('beamSetup').value;
var beamDepth = 0;
if (beamSetup === 'dropped') {
beamDepth = parseFloat(document.getElementById('beamDepth').value);
}
if (isNaN(targetHeight) || isNaN(deckingThickness)) {
alert('Please enter valid numbers for Height and Thickness.');
return;
}
// Calculation: Total Height – Boards – Joists – Beam (if dropped)
var netClearance = targetHeight – deckingThickness – joistDepth – beamDepth;
var resultDiv = document.getElementById('deckResult');
var clearanceValue = document.getElementById('clearanceValue');
var clearanceStatus = document.getElementById('clearanceStatus');
var framingBreakdown = document.getElementById('framingBreakdown');
resultDiv.style.display = 'block';
if (netClearance < 0) {
resultDiv.style.backgroundColor = '#f8d7da';
resultDiv.style.color = '#721c24';
clearanceValue.innerHTML = "Clearance: " + netClearance.toFixed(2) + " inches";
clearanceStatus.innerHTML = "⚠️ WARNING: Negative Clearance. Your framing is thicker than your total height. You may need to excavate or use a lower profile framing system.";
} else if (netClearance < 6) {
resultDiv.style.backgroundColor = '#fff3cd';
resultDiv.style.color = '#856404';
clearanceValue.innerHTML = "Clearance: " + netClearance.toFixed(2) + " inches";
clearanceStatus.innerHTML = "⚠️ LOW CLEARANCE: Ventilation may be restricted. Ensure you use 'Ground Contact' rated lumber and allow for adequate board spacing.";
} else {
resultDiv.style.backgroundColor = '#d4edda';
resultDiv.style.color = '#155724';
clearanceValue.innerHTML = "Clearance: " + netClearance.toFixed(2) + " inches";
clearanceStatus.innerHTML = "✅ ADEQUATE CLEARANCE: This setup provides sufficient airflow for standard deck construction.";
}
framingBreakdown.innerHTML = "Construction Breakdown:" +
"- Surface Height: " + targetHeight + "\"" +
"- Decking Board: " + deckingThickness + "\"" +
"- Joist Profile: " + joistDepth + "\"" +
(beamDepth > 0 ? "- Dropped Beam: " + beamDepth + "\"" : "- Flush Beam: Included in joist plane") +
"= Air Space: " + netClearance.toFixed(2) + "\"";
}