Joint gap on a curve — why a small ring needs a wider mortar joint

Straight block on a circle meets at an angle, not flush — the gap is wedge-shaped and widest on the outer face. A tighter ring needs a bigger angle between blocks, so the gap opens wider. A kit’s precision-cut trapezoids avoid this entirely.

A plan-view (top-down) comparison of two masonry fire pit rings. On the left, a small-diameter ring built from four straight blocks shows a visibly wide, wedge-shaped gap between each block, widest at the outer face, because each block must turn a sharp angle to follow the tight curve. On the right, the same four blocks arranged around a much larger-diameter ring show the gaps almost closed, because a gentler curve needs less angle change between blocks. A note explains that a very wide gap signals the builder should use a bigger ring, a narrower block, or a manufacturer kit with precision-cut trapezoidal blocks that close with no gap regardless of diameter. — Source: Geometry (chord/arc relationship) — companion to the calculator’s own joint-gap warning

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A plan-view (top-down) comparison of two masonry fire pit rings. On the left, a small-diameter ring built from four straight blocks shows a visibly wide, wedge-shaped gap between each block, widest at the outer face, because each block must turn a sharp angle to follow the tight curve. On the right, the same four blocks arranged around a much larger-diameter ring show the gaps almost closed, because a gentler curve needs less angle change between blocks. A note explains that a very wide gap signals the builder should use a bigger ring, a narrower block, or a manufacturer kit with precision-cut trapezoidal blocks that close with no gap regardless of diameter.

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  <a href="https://remodelcalculators.com/construction-diagrams/fire-pit-joint-gap-on-curve" target="_blank" rel="noopener">
    <img src="https://remodelcalculators.com/api/figure-embed/fire-pit-joint-gap-on-curve" alt="A plan-view (top-down) comparison of two masonry fire pit rings. On the left, a small-diameter ring built from four straight blocks shows a visibly wide, wedge-shaped gap between each block, widest at the outer face, because each block must turn a sharp angle to follow the tight curve. On the right, the same four blocks arranged around a much larger-diameter ring show the gaps almost closed, because a gentler curve needs less angle change between blocks. A note explains that a very wide gap signals the builder should use a bigger ring, a narrower block, or a manufacturer kit with precision-cut trapezoidal blocks that close with no gap regardless of diameter." width="800" height="420" style="width:100%;height:auto;border:0" loading="lazy">
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    Diagram: <a href="https://remodelcalculators.com/construction-diagrams/fire-pit-joint-gap-on-curve" target="_blank" rel="noopener">Joint gap on a curve — why a small ring needs a wider mortar joint</a> — courtesy of <a href="https://remodelcalculators.com" target="_blank" rel="noopener">RemodelCalculators.com</a>
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