How to Price Floor Joist Replacement: Labour, Materials and Margin Guide

Summary

This guide is written for the carpenter or builder pricing a joist replacement or repair job, not the homeowner trying to understand a quote. Floor joist replacement sits in an awkward pricing spot: it's structural work governed by Approved Document A and BS 5268-7 span requirements (see joist installation for the full installation standard), but the actual cost on any given job is driven far more by access and cause than by the timber itself. Two jobs replacing the same number of joists at the same span can differ by several hundred pounds depending on whether it's a ground-floor void with a hatch already in place, or a first-floor bedroom needing the entire floor lifted and the ceiling below protected.

Joist replacement almost always starts as a diagnosis job before it's a carpentry job — a bouncy floor, a soft spot underfoot, or visible sag usually means rot, insect damage, or historic over-notching by a plumber or electrician, and the underlying cause has to be identified and dealt with (often meaning a separate damp specialist or a change to how services are run through the new joists) or the replacement timber will eventually fail the same way. See bouncy floor for the diagnostic process and rising damp / condensation for common damp causes that need addressing alongside the structural repair.

For contractors, this is a job that rewards a proper site survey before quoting — the difference between a straightforward sistering job and a full multi-joist replacement with ceiling-below disruption is easily a factor of three or four in price, and getting this wrong at quote stage either loses the job on price or loses money on delivery.

Key Facts

• Standard specification — C16 47×195mm sawn softwood at 400mm centres remains the default for domestic replacement joists up to around 3.7–3.9m clear span (table-edition dependent); match the existing structural specification unless it's demonstrably undersized, in which case upsize and note this on the job — see joist span calculator for span verification
• Sistering vs full replacement — sistering (fixing a new joist alongside a damaged one, sharing the load) is viable where the original joist retains reasonable bearing and isn't so rotten it can't provide any residual support; full replacement is required where the joist is failed at the bearing, extensively rotten, or insect-damaged beyond the point sistering can address
• Timber cost — C16 sawn softwood 47×195mm runs roughly £8–£15 per linear metre at time of purchase; C24 or larger sections cost more; always check current supplier pricing rather than relying on a fixed figure, as timber prices have been volatile in recent years
• Joist hangers — galvanised pressed steel hangers to BS EN 14545, roughly £3–£10 each depending on size, plus sherardised square-twist nails (never round-wire nails) for every hole
• Preservative treatment — pre-treated timber or on-site preservative application for joist ends bearing on or near masonry, particularly in older solid-wall properties with a history of damp
• Access driver: ground floor vs upper floor — ground-floor suspended timber floor work (crawl space/limited headroom, often via an existing or new access hatch) is typically the cheapest access type; upper-floor work needs floorboards or chipboard decking lifted and usually protection or reinstatement of the ceiling below
• Access driver: ceiling below — if removing/disturbing joists risks the ceiling below (common on older lath-and-plaster ceilings), factor in either careful working to avoid disturbance or a ceiling repair/redecoration allowance as a separate line item — don't absorb this into the joist price
• Disposal — old rotten or insect-damaged timber typically goes via skip or licensed waste removal; treated timber (older CCA/tanalised sections) may need to go to an appropriate waste stream rather than general skip — check current guidance if in doubt
• Cause remediation is separate scope — damp-proofing, leak repair, or insect treatment is usually priced and carried out separately (often by a specialist) from the joist carpentry itself, but must be scheduled to happen before or alongside the joist work, not left until afterward
• Notching and drilling on replacement joists — new joists must still comply with the notching/drilling rules (0.07L–0.25L notch zone, max 12.5% depth) if services are being re-routed through them at the same time — see joist installation for the full rule set
• Building Control — replacement of structural floor joists, particularly where span, spacing or loading changes from the original, is generally best notified to Building Control even for repair-type work; minor like-for-like replacement in an existing dwelling is sometimes treated as non-notifiable repair, but this is locally variable — ****
• Party wall consideration — where joists bear on or near a shared party wall (common in Victorian/Edwardian terraces), replacement work may trigger a notice requirement under the Party Wall etc. Act 1996 — check before starting on terraced properties
• VAT — standard rate 20% for most domestic joist replacement work

Quick Reference Table

Detailed Guidance

Diagnose Before Pricing

Almost no joist replacement job should be quoted purely from a description over the phone — "the floor feels bouncy" or "there's a soft patch by the bathroom" could mean anything from a single undersized joist to a widespread rot problem from a long-standing leak. A site visit to lift a floorboard or access the void, assess the extent of damage, identify the likely cause, and check for any related issues (adjacent joists, wall plate condition, ventilation to a suspended timber floor) is the only reliable basis for a firm price. See bouncy floor for the diagnostic decision tree.

Two things commonly discovered on survey that change the scope significantly: rot or insect damage that's spread further than the visible symptom suggested (check adjacent joists, not just the one showing the problem), and a lack of subfloor ventilation on a suspended timber ground floor, which is often the actual root cause and needs addressing (airbricks cleared or added) or the replacement timber will eventually suffer the same fate.

Sistering vs Full Replacement — The Core Pricing Decision

Sistering — fixing a new joist of matching or greater depth alongside the damaged one, so the new timber carries the load — is the lower-cost option where the existing joist retains enough sound timber and bearing to be worth leaving in place rather than removing. It's fast, causes less disruption (often no need to disturb the ceiling below on an upper floor), and is a legitimate, commonly used repair method rather than a compromise.

Full replacement is necessary where the joist has failed or is failing at the bearing (the most structurally critical point), where rot or insect damage has compromised too much of the timber's cross-section for sistering to provide adequate support, or where the joist needs to be removed entirely to deal with damp at the bearing point (masonry repair, DPC work, ventilation improvement). Full replacement costs more because the old joist must be safely removed (often while temporarily supporting the floor load either side) before the new one goes in.

Pricing Ground Floor vs Upper Floor Access

A ground-floor suspended timber floor with a hatch already in place, or where one can be created without major disruption, is the cheapest access scenario — the carpenter works in the void from below or through the hatch, and no finishes need lifting or reinstating. Budget the lower end of the ranges above for this access type.

Upper-floor work always means lifting floorboards or chipboard decking, which adds time for lifting/storing/relaying (or replacing, if boards are damaged or chipboard that doesn't lift cleanly), and introduces risk to the ceiling below — particularly on older properties with lath-and-plaster ceilings, which are far more likely to crack or partially collapse when the floor above is disturbed than a modern plasterboard ceiling. Where the ceiling below is a genuine risk, either price in careful, slower working to minimise disturbance, or price the ceiling repair as an allowance within the same quote so the customer isn't surprised by a second job appearing mid-way through.

Multi-Joist Jobs: Price by Day Rate, Not Per Joist

Once a job involves three or more joists — typically leak damage that's spread along a run, or a whole bay affected by long-term damp — per-joist pricing breaks down because much of the cost (access setup, temporary support, ceiling protection, waste removal) is shared across all the joists rather than repeated for each one. Price these jobs as carpenter day rate (typically £220–£350/day) plus a labourer if needed (£150–£200/day) plus materials (timber, hangers, treatment, fixings) plus disposal, rather than trying to multiply a per-joist figure — a multiplied per-joist price on a multi-joist job usually either overcharges (double-counting shared setup cost) or undercharges (missing the genuinely larger scope of temporary support and access needed for a wider area).

Worked Example 1: Single Joist Sistering, Ground Floor, Easy Access

Property: 1930s semi, suspended timber ground floor, one joist showing early-stage wet rot at the outer wall bearing (source: historic minor damp, now resolved with improved airbrick ventilation), existing hatch access, no other joists affected.

Worked Example 2: Multi-Joist Replacement, Upper Floor, Historic Leak Damage

Property: Victorian terrace, first-floor bathroom, 4 joists rotted from a long-standing (now repaired) waste pipe leak, floorboards to be lifted across the bathroom and adjoining landing, lath-and-plaster ceiling below the bathroom to be protected and made good after.

Note the damp/leak repair itself is not included in this figure — it's priced and scheduled separately, ideally completed and verified dry before the joist work starts.

Materials: Matching Existing vs Upgrading

Where the original joists were undersized for their actual span (common in older properties, or where a previous alteration changed the loading without re-checking the structure), replacing like-for-like perpetuates the problem. Check the span against current guidance (see joist span calculator) before specifying replacement timber, and price an upgrade to a larger section or C24 grade where the original was genuinely inadequate — this is worth flagging to the customer as a structural improvement, not just a repair, and priced accordingly rather than absorbed into a like-for-like figure.

Building Control and Party Wall Considerations

Structural floor joist replacement — particularly where span, spacing, or loading changes — generally warrants a conversation with Building Control before starting, even where the work might be classed as repair rather than new structure. ****, as practice varies between authorities on what counts as notifiable repair versus routine maintenance. On terraced or semi-detached properties where joists bear on or near a party wall, check whether the Party Wall etc. Act 1996 notice requirement applies before starting — this is easy to miss on what looks like a straightforward internal repair job, and getting it wrong can delay or complicate the job significantly if a neighbour objects after work has started.

Frequently Asked Questions

How do I know whether to sister or fully replace a joist?

Assess how much sound timber remains, particularly at the bearing ends where load transfers to the wall. If the joist still has solid, un-rotted timber at both bearings and the damage is confined to a section that a sistered joist can bridge, sistering is usually the more cost-effective and less disruptive repair. If the bearing itself has failed, or rot/insect damage runs through more than a limited section, full replacement is the safer specification — a sistered joist next to a joist that's failed at the bearing doesn't fully resolve the structural risk.

Why does upper-floor joist replacement cost so much more than ground floor?

Almost entirely down to access and risk to the ceiling below. Ground-floor suspended timber work is done from a void or crawl space with no finishes to lift or reinstate. Upper-floor work requires lifting floor decking (and often relaying or replacing it), and carries real risk to the ceiling of the room below — particularly older lath-and-plaster ceilings, which can crack or fail when the floor structure above is disturbed. Both of these add labour time and risk that ground-floor work simply doesn't carry.

Should I always investigate the cause before quoting the repair?

Yes — quoting joist replacement without identifying why the joist failed risks the same failure recurring in the new timber, and it's a conversation worth having explicitly with the customer even if the cause remediation is a separate job by a different specialist. A joist replaced without fixing an active leak or a blocked subfloor ventilation path is a repair that's likely to need repeating.

Can I use engineered joists (I-joists) to replace a damaged sawn timber joist?

Sometimes, but it needs careful specification — I-joists have different bearing, notching and drilling rules to sawn timber (see joist installation and engineered timber) and matching them into an existing sawn-timber floor structure needs the connection details worked out properly, not just swapped in. For a single joist replacement within an otherwise sawn-timber floor, matching like-for-like with sawn C16/C24 timber is usually simpler and avoids compatibility issues at the hangers and bearings.

Regulations & Standards

• Approved Document A — Structure; governs joist sizing, span and loading for replacement as much as new installation

• BS 5268-7 — Structural use of timber; span tables for joists

• BS EN 1995-1-1 (Eurocode 5) — Design of timber structures; current European standard underlying modern span calculations

• BS EN 338 — Structural timber strength classes (C16, C24, etc.)

• BS EN 14545 — Timber structures — joist hangers; steel grade and load capacity

• BS 8103-3 — Code of practice for timber floors and roofs for housing

• Party Wall etc. Act 1996 — notice requirement where structural work affects or is near a party wall

• Wood Protection Association guidance — preservative treatment specification for timber at risk of damp exposure

• Approved Document A Structure

• TRADA Span Tables for Domestic Floor Joists

• BS 5268-7 Structural Use of Timber

• Party Wall etc. Act 1996 Guidance

• Wood Campus Joist Repair and Installation Guidance

• joist installation — full joist sizing, spacing and installation standard

• bouncy floor — diagnosing joist deflection, rot and failure before quoting repair

• joist span calculator — verifying replacement joist span/size against current tables

• engineered timber — I-joist and engineered timber options and their different notching/drilling rules

• rising damp — common damp cause requiring remediation alongside joist repair

• timber quantities — timber quantity and cost calculation for larger joist replacement jobs