# I-joists or sawn lumber: 5 constraints in a floor retrofit

- Link: https://www.qtywise.com/blog/i-joists-sawn-lumber-floor-retrofit/
- Published: 2026-11-04T09:00:00-05:00
- Author: Jamie C.

## Floor levels constrain the whole assembly depth

The I-joists vs sawn lumber comparison should start with the existing building and
the approved replacement floor system. Record finished levels, available depth, 
clear spans, loads, service routes and supports. A member that fits the visible 
gap may still need different bearings, connections or assembly details. Obtain the
responsible designer’s assessment before selecting or ordering replacement structural
members.

Use the [joist quantity calculator](https://www.qtywise.com/framing-lumber-stairs/floor-joist-calculator/)
after the layout, maximum spacing, bearing and stock-length assumptions have been
specified. The tool counts from those inputs; it does not establish a safe span 
or choose between structural products. Keep the approved drawing revision with the
takeoff so a later quantity change can be traced to its design basis.

Measure the distance between required finished levels, then account for the actual
floor and ceiling build-up. Doors, stair landings and adjoining rooms can expose
a level conflict that a joist-depth comparison misses. Give the designer a section
through the full assembly, including finishes and any service zone, instead of only
a nominal timber depth.

For a hypothetical retrofit, a preferred assembly that is 25 millimetres deeper 
than the available zone creates a coordination problem before quantities are ordered.
The 25 millimetres is an illustrative conflict, not an acceptable tolerance. Resolve
the levels and assembly with the relevant designers; shaving a structural member
to make it fit is not a quantity adjustment.

## The load and movement requirements need a defined design basis

Sawn-lumber selection needs the actual species, grade, dimensions and applicable
design values. Engineered I-joists need their identified manufacturer, series and
product guidance. Similar depth and spacing do not establish equivalent capacity
or floor behavior. Include concentrated loads and proposed partitions in the design
information rather than assuming the previous floor arrangement proves the new one
acceptable.

The American Wood Council’s [span-table tutorial](https://awc.org/resource-hub/span-table-tutorial/)
explains a sequence based on loads, deflection, span, species and grade. Its worked
example considers a 15-foot-1-inch design span at 16-inch spacing and explicitly
checks the relevant table and material values. That is a documented teaching example,
not a replacement-member recommendation for a different building or jurisdiction.

Keep the distinction between a quantity comparison and a design comparison in each
quotation. Ask bidders to price the same accepted load and performance requirements,
then identify any design changes they propose. A shorter material list can reflect
a changed system rather than a saving on an equivalent system. Have the responsible
designer review that change before comparing totals.

### A floor-area figure leaves important design inputs unstated

Two rooms with the same area can have different clear spans, support locations and
concentrated loads. A bathroom conversion may also change finish weight or service
needs. Record those proposed changes explicitly. The installer should not discover
them from a revised shopping list after the structural system has already been selected.

## Service routes must fit the actual opening rules

Weyerhaeuser’s [TJI guidance](https://www.techsupport.weyerhaeuser.com/hc/en-us/articles/201758840-Where-Can-I-Put-Holes-in-a-TJI-Joist)
directs designers to product-specific hole charts and states that its flanges must
not be cut or notched. The permitted web opening depends on the identified joist
and the chart’s conditions. Do not transfer a detail from another I-joist series
or treat a solid-sawn drilling convention as a generic I-joist rule.

Draw the pipe or duct route against the joist layout, including fitting space and
the position of supports. A service centerline alone does not describe the required
opening. Ask who checks opening dimensions before cutting and who approves a revised
route if an existing beam, hanger or connection occupies the intended space.

For sawn lumber, have the responsible designer identify the applicable hole and 
notch restrictions for the actual member and location. Avoid choosing a member solely
because it appears easier to cut. A permitted opening must belong to the accepted
design, and an unapproved alteration can invalidate the basis of a supplier’s comparison.

Weyerhaeuser’s published [support note](https://www.techsupport.weyerhaeuser.com/hc/en-us/articles/201758810-Repair-Detail-for-TJI-Joists-with-Holes-Too-Close-to-Bearing)
on holes too close to bearing documents a specific recurring coordination problem.
It refers to defined chart conditions and sends applications outside its technical-
bulletin parameters to technical support. The useful lesson is to identify the product
and loading before seeking a repair; the note is not a universal site repair detail.

## Supports and connections can control the replacement choice

Survey the actual bearing surfaces, their condition and the available connection
space. Record where the joists meet walls, beams, rims and other members. Ask the
designer to confirm the full load path and the required supports. A span dimension
alone cannot reveal damaged masonry, inadequate bearing or a connection that cannot
be installed in the available space.

Weyerhaeuser’s TJI [Specifier’s Guide](https://www.weyerhaeuser.com/application/files/5617/5451/8519/TJ-4000.pdf)
contains product-specific bearing and connection details alongside its opening charts.
Use the accepted details for the identified product and installation. A hanger, 
rim or blocking component should be selected and counted as part of the system rather
than replaced by whatever material happens to be left on site.

Separate joist quantities from connectors, specified blocking, rim components and
temporary works in the procurement schedule. For both systems, ask who supplies 
each item and who inspects concealed work. Keep the price of removing and reinstating
finishes visible; a direct comparison of member prices omits work that can dominate
an occupied-building retrofit.

| Retrofit constraint | Evidence for either option | Decision needed before ordering | 
| Available depth | Full floor and ceiling section | Accepted levels and assembly | 
| Loads and movement | Project design basis and identified members | Approved structural system | 
| Services | Coordinated routes and product opening rules | Approved opening locations | 
| Supports | Surveyed bearings and connection details | Complete load path | 
| Access | Member route and installation sequence | Feasible delivery and temporary works |

## Delivery and installation access affect the complete scope

Check how full-length members will reach the work area. Doors, stairs, scaffold,
lifting routes and existing ceilings can change the installation method. Compare
the longest approved stock piece with the actual access route. Do not assume a shorter-
piece splice is acceptable because it would make delivery easier; any structural
splice needs an expressly approved detail.

For an occupied house, identify the order of removal, temporary support, installation
and reinstatement with the responsible team. Decide which rooms can remain in use
and how services are isolated and restored. A light or convenient member can still
require substantial preparatory work. Use the actual contractor sequence rather 
than a generic installation-speed claim.

### Check a quantity example only after fixing the layout

Consider a hypothetical bay with 16 feet between the first and last joist centerlines
and an already approved maximum spacing of 16 inches. The interval count is ceiling(
16 × 12 ÷ 16) = 12; including both boundary centerlines gives 13 joists. Two independent
identical bays therefore require 26 before any separately justified spare allowance.

If a separate approved detail specifies a 12-foot clear span plus 3 inches bearing
at each end, the required member length is 12.5 feet. A hypothetical 14-foot purchased
stock length must be cut only as the accepted product detail permits. Offcuts are
not automatically reusable structural lengths. These stated dimensions are a takeoff
example, not a design for a safe floor.

Reproduce the count in the [joist quantity calculator](https://www.qtywise.com/framing-lumber-stairs/floor-joist-calculator/)
using the same centerline width, specified spacing and independent-bay count. Keep
the bearing-to-bearing span separate from the distribution width. Swapping those
dimensions can produce a plausible count for the wrong direction while leaving the
operator unaware that the layout has changed.

## Compare complete approved packages before placing the order

Use a shared scope to compare the two options: identified members, connectors, blocking,
delivery, temporary support, service coordination and finish reinstatement. Request
current local quotations and identify exclusions. The benefit is a comparison tied
to a buildable approved layout, with substitutions visible before the owner pays
for materials.

 * Survey finished levels, clear spans and bearing conditions.
 * Give the designer the actual loads and proposed changes.
 * Coordinate service openings with the exact member guidance.
 * Count connections and specified secondary components separately.
 * Confirm full-length access and the accepted installation sequence.

Retain the [joist quantity calculator](https://www.qtywise.com/framing-lumber-stairs/floor-joist-calculator/)
worksheet with the approved member schedule and the supplier’s product documents.
Use it to check counts and stock-length assumptions when the layout changes. Structural
adequacy, opening approval, temporary support and compliance remain decisions for
the responsible design and construction team.