Every portable building sits on runners, the skids that carry the floor and later carry the building down the road. Most sheds roll out of the shop on plain lumber, but a growing number of builders order runners with notches cut into the top edge, spaced to hold each floor joist. The change looks small on paper and shows up fast on the jobsite.
Whether a building moves on skids, trailers, or trucks, the floor system starts with the runners underneath. This article compares the three floor systems builders actually use, explains how notched runners are made, walks through a floor build, and lays out the cost math so you can decide when the switch pays.
Notched vs. Blank Skids: What Changes
Builders choose among three floor systems: 4 by 4 runners with no notches, 4 by 6 runners with no notches, and 4 by 6 runners with notches cut on 12 inch or 16 inch centers. Each step up adds strength and precision, and each one changes how the crew works.
| Skid system | Floor strength | Build speed | Precision | Relative cost |
|---|---|---|---|---|
| 4×4 blank | Good | Slowest | Depends on crew | Lowest |
| 4×6 blank | Better | Medium | Depends on crew | Mid |
| 4×6 notched | Best | Fastest | Built into the cut | Highest |
A builder who switched from 4 by 4s to notched 4 by 6s described the difference in two places: the floor and the road. The bigger runner adds strength to the whole floor system, which matters most on large portable buildings, and the notches lock each joist in place so the floor stays square through assembly and transport.
Strength is not just a feel-good claim. The runner carries the full weight of the building, and on a unit that will be lifted, dragged, and bounced down a highway, the floor is the part that takes the strain. Builders who haul big buildings say the difference between a 4 by 4 and a notched 4 by 6 shows up the first time the trailer hits a rough grade.
Why the notches matter
With blank runners, a crew has to mark every joist location, measure twice, and toe-nail each joist into place. One misread tape means a floor that is off, and an out-of-square floor makes the whole building crooked: walls that lean, doors that bind. A notched runner removes the measuring step entirely. The joist drops into the slot, sits at the right spacing, and gets nailed through the notch.
How Notched Runners Are Made
Lumber suppliers cut notched runners with a range of machines. Some use a dado saw, which is fast but leaves more variation from cut to cut, while others use CNC routing, which is slower but repeats the same cut every time. One wholesale supplier described it plainly: the robot can only go so fast, and you cannot speed it up or slow it down, so quality stays consistent even when volume does not.
The notch itself is a structural change, not just a convenience. Structural engineers have written in detail about strengthening notched floor joists, because a notch changes how a joist carries load, and the same logic applies to the runner the joist sits in. The load path runs from the joist through the notch into the runner, so the notch depth, spacing, and cut quality all affect how the floor performs under load and in transport.
Supply matters too. A stocking wholesaler can warehouse runners straight from the mill, which shortens lead times for builders who order by the truckload. Builders who wait for custom cutting pay more per piece and schedule their floor framing around the supplier’s queue, so it pays to ask how a supplier produces runners before you commit.
What to check when you order runners
- Notch spacing matches your joist layout: 12 inch or 16 inch centers
- Notch depth matches the joist depth so the top edges align
- End angle is cut for easy dragging and road travel
- Species, grade, and treatment match your climate and ground contact
- Moisture content is low enough that the runner will not twist as it dries
Building a Floor System on Notched Runners, Step by Step
The build sequence changes when the runners arrive pre-notched. The crew still sets the foundation, but the floor framing becomes a placement job instead of a layout job. The steps below match what production builders report after the switch, and most of them take less time than the equivalent work on blank lumber.
- Set and level the blocks or pads, then lay the runners with the notches facing up
- Check the runners for square and for consistent height across the pair
- Drop each floor joist into its notch at 12 or 16 inch centers
- Fasten each joist through the notch with the specified nails or screws
- Square the frame by comparing diagonals before you sheath the floor
- Install the sheathing, then re-check for twist before framing the walls
The tool list is short: a level, a framing square, a nail gun or drill, and a tape for the final check. Crews still verify the diagonal measurements, because no pre-cut runner removes the need to confirm the frame is square before sheathing. What disappears is the marking and re-measuring that eats time on blank runners.
Suppliers point out that the system only works if the runners themselves are precise. If the notch spacing drifts, the joists sit uneven, the floor goes out of square, and the builder has to go back and fix it, which is exactly the rework the product is supposed to remove. That is why the cutting method matters as much as the lumber grade.
Costs, Labor, and When to Switch
Notched runners cost more than blank lumber because the manufacturer puts labor into every cut. Builders weigh that premium against three savings: less measuring time, fewer nails, and less rework. One supplier estimated that builders save labor on every floor because joists pop into place, and they cut down on supplies because notched runners use fewer nails than toe-nailing into blank 4 bys.
- Material grade and treatment affect price more than the notches
- Notch count scales with spacing: 12 inch spacing means more cuts and a higher price
- End profiling adds a little cost and a lot of ease on the road
- Delivery cost per mile matters when you buy by the truckload
When the switch pays for itself
The math favors notched runners most for big buildings, production builders, and anyone who moves units regularly. A 12 by 20 foot floor with joists on 16 inch centers holds a dozen or more joists; marking, measuring, and toe-nailing each one takes a crew a real share of the morning. For a builder doing several floors a week, the time savings add up faster than the price premium.
Nail count is a small line item that still matters. Toe-nailing a joist into a blank runner takes two or three nails per side, while a notched joint takes one clean fastener through the slot. Across a production year, the difference runs into boxes of nails, and the cleaner joint is stronger than a row of toe nails angled by eye.
Haulers notice the difference on the road too. Angled ends let a building slide onto a trailer and across the ground without the skid digging in, and the stronger runner carries the load without flexing. For portable buildings, especially big ones, the runner is the part that takes the strain in transport, and that is where notched 4 by 6s prove their value.
Transport, Placement, and Long-Term Performance
The same features that speed up the floor build carry over to delivery day. Notched runners with angled ends pull easier when a building slides on the ground, and they do not dig into soft earth the way a blunt end will. On the trailer, the runner distributes the load across the frame so the building rides square and arrives without racking.
Ground contact deserves attention over the long term. Runners sit on blocks or directly on soil, so treated lumber, a moisture barrier, and good drainage keep them sound for decades. A building placed on wet ground with untreated runners will rot from the bottom up no matter how well the floor was built.
A yearly inspection takes minutes: look for cracked notches, split ends, and soft spots where the runner meets the ground, and replace any runner that has lost its bearing before it fails under a loaded floor. Notched skids will not fix a floor that sits on bad ground, but they take the measuring error out of the framing and put strength under the load where it belongs.