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The member's clear span between supports.
Clear span, support face to support face. On a roof this figure usually gets checked twice rather than once: the member has a structural limit of its own, and anything hanging below it — a plastered or boarded ceiling — has a tighter one. Two checks, one span, and the finish normally wins.
The code deflection limit that applies to this member, expressed as a fraction of the span.
The limit comes from what the member SUPPORTS, not from what the member is. A rafter over an open carport and an identical rafter over a plastered ceiling are given different limits, because the plaster cracks long before the timber is anywhere near distressed. Live-load and total-load limits are also separate numbers checked separately — passing one says nothing about the other.
Allowable deflection
0.65 in
They open the calculator with your figures already in it
Roof/Floor Deflection Limit Calculator: 0.65 in — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- Standard code deflection limits are expressed as a span fraction (e.g. L/180, L/240, L/360); allowable deflection = span ÷ ratio
Inputs used
- Span
- 19.5 ft
- Deflection Limit Ratio
- L/360 (floor, plaster ceiling)
What this calculation does not cover
- The number returned is the allowance, not the deflection. How far the member actually moves depends on the load, the modulus of elasticity and the second moment of area, none of which this page asks for, so a 6 m span at L/360 gives 16.7 mm of permitted sag with no indication of whether the joist under it moves 8 mm or 40 mm.
- Codes normally impose two limits on the same member at once, one on live load alone and a looser one on dead plus live, and this page returns a single ratio at a time, so an L/360 allowance is the right yardstick only for the live-load half of that pair. The three options offered are the common generic set; brittle finishes, supported masonry and glazing attract tighter fractions in many codes. Some codes also pair the ratio with an absolute cap in millimetres that a long span reaches before the fraction does, and this page only ever divides.
- Span is used exactly as entered, which is the wrong input for a cantilever. Several codes define L for a cantilevered member as twice the projection, so a 1.2 m (4 ft) balcony joist entered as 1.2 m (4 ft) returns roughly half the allowance intended; how L is defined at a cantilever, and whether it is measured clear or centre to centre of bearings, is jurisdictional.
- The allowance has no time in it. Timber and concrete keep deflecting under sustained load, and codes deal with creep by multiplying the long-term component of the calculated deflection before it is compared with a limit like this one, commonly by a factor of the order of 1.5 for dry timber and 2.0 for green material, with the exact figure set by the governing code. A member that clears the limit on the day it is installed can exceed it a year later under the same dead load.
- Clearing a deflection limit is a serviceability result, not a structural pass: bending stress, shear near the supports, bearing crushing on the wall plate and buckling of an unrestrained compression edge are separate checks a member can fail while sitting comfortably inside its L/240 allowance. On a low-slope roof there is also ponding, where water collects in the deflected shape and the added weight deepens it further, which is a stiffness and drainage question no span fraction answers.
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This result is a specification — 0.65 in — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-02 · in the site-wide review of 2026-09-06 · v1.0.1
Regulatory standards & verification citations1
- Standard code deflection limits are expressed as a span fraction (e.g. L/180, L/240, L/360); allowable deflection = span ÷ ratio
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