The structural integrity and safety of your deck heavily rely on proper support posts and their spacing. Incorrect spacing can lead to sagging, instability, and potentially catastrophic failure. This calculator helps you determine the optimal placement of your support posts based on your deck's width and the maximum allowable spacing dictated by building codes and the strength of your ledger board (if applicable).
The Math Behind the Calculation
The fundamental principle is to divide the deck's width into segments that do not exceed the maximum allowed post spacing. The calculation is as follows:
Number of Spans: The deck width is divided by the maximum post spacing. We then take the ceiling of this number to ensure no span exceeds the maximum.
Number of Posts Required: The number of posts is typically one more than the number of spans. However, if a ledger board is used, the outermost edge posts might be supported by the house framing and adjacent posts, potentially reducing the need for a post directly under the ledger at the edge of the deck span.
Actual Spacing: The deck width is divided by the total number of spans (calculated from the required number of posts minus one) to determine the actual, consistent spacing between posts for optimal support.
Formulae (Simplified):
Number of Spans = ceil(Deck Width / Max Post Spacing)
Total Posts = Number of Spans + 1 (Adjusted for ledger board use)
Actual Spacing = Deck Width / Number of Spans
When to Use a Ledger Board
A ledger board is a horizontal board attached to the house framing, to which the deck joists are fastened. Using a ledger board is a common and structurally sound method for attaching a deck to a home. It provides support for the joists directly adjacent to the house, influencing the number of posts required for the outer edge of the deck. If a ledger board is used, the outermost posts might be placed at the edge of the deck, with the joists spanning from the ledger to these outer posts.
Interpreting the Results
The calculator will provide you with:
The recommended number of support posts needed.
The optimal, consistent spacing between these posts to ensure even load distribution and structural stability.
Always consult local building codes and a qualified structural engineer or contractor for specific project requirements and safety standards.
Example Calculation
Let's consider a deck that is 16 feet wide and you want to maintain a maximum post spacing of 6 feet. You are using a ledger board attached to the house.
Deck Width: 16 ft
Maximum Post Spacing: 6 ft
Using Ledger Board?: yes
Calculation:
Number of Spans = ceil(16 ft / 6 ft) = ceil(2.67) = 3 spans.
With 3 spans, you need 3 + 1 = 4 posts. However, with a ledger board, the first "post" position is at the house. So, we need posts at 0ft (house ledger), and then spaced for 3 spans. This means posts are needed at the 0ft mark (ledger), and then after 3 spans.
Actual Spacing = 16 ft / 3 spans = 5.33 feet.
Therefore, you would need support posts (or connections to the house framing) at approximately 0 ft (ledger), 5.33 ft, 10.67 ft, and 16 ft from the house wall.
function calculateDeckPosts() {
var deckWidth = parseFloat(document.getElementById("deckWidth").value);
var postSpacing = parseFloat(document.getElementById("postSpacing").value);
var ledgerBoardInput = document.getElementById("ledgerBoard").value.toLowerCase();
var resultDiv = document.getElementById("result");
resultDiv.innerHTML = ""; // Clear previous results
// Input validation
if (isNaN(deckWidth) || deckWidth <= 0) {
resultDiv.innerHTML = "Please enter a valid deck width greater than 0.";
return;
}
if (isNaN(postSpacing) || postSpacing <= 0) {
resultDiv.innerHTML = "Please enter a valid maximum post spacing greater than 0.";
return;
}
if (ledgerBoardInput !== 'yes' && ledgerBoardInput !== 'no') {
resultDiv.innerHTML = "Please enter 'yes' or 'no' for using a ledger board.";
return;
}
var numSpans = Math.ceil(deckWidth / postSpacing);
var totalPostsNeeded = numSpans + 1;
var actualSpacing = deckWidth / numSpans;
var resultHTML = "
Calculation Results:
";
resultHTML += "Based on a deck width of " + deckWidth + " ft and a maximum post spacing of " + postSpacing + " ft:";
resultHTML += "You will need approximately " + numSpans + " spans.";
if (ledgerBoardInput === 'yes') {
resultHTML += "With a ledger board, the primary support is at the house. You will need approximately " + (numSpans) + " additional support posts (positions: after each span).";
resultHTML += "The suggested spacing for these posts is " + actualSpacing.toFixed(2) + " ft.";
resultHTML += "Post Locations (from house): 0 ft (ledger), " + actualSpacing.toFixed(2) + " ft, " + (actualSpacing * 2).toFixed(2) + " ft, …, " + deckWidth + " ft.";
} else {
resultHTML += "You will need approximately " + totalPostsNeeded + " support posts (including the outer edges).";
resultHTML += "The suggested spacing for these posts is " + actualSpacing.toFixed(2) + " ft.";
resultHTML += "Post Locations: 0 ft, " + actualSpacing.toFixed(2) + " ft, " + (actualSpacing * 2).toFixed(2) + " ft, …, " + deckWidth + " ft.";
}
resultHTML += "Ensure compliance with local building codes and consult a structural professional.";
resultDiv.innerHTML = resultHTML;
}