From 15 September 2026, some Canadian steel and aluminum entering the United States can face two extra duties on the same shipment. One is the Section 338 surcharge. The other is the Section 232 national-security duty. On listed lines where both are 50%, the combined additional duty can reach 100%.
The change is real. It is also narrower than the version travelling online.
Washington did not put every Canadian steel coil, slab or aluminum ingot at 100%. The Harmonized Tariff Schedule line decides. If a product is not on the stacked list, the 100% claim does not apply.
Status note: Checked 24 September 2026. Proclamations signed 8 September 2026. Scope changes took effect at 12:01 a.m. ET on 15 September 2026. No later pause of the stack was on the record at publication. A separate set of import exclusions, also signed on 8 September, is written to start on 29 September 2026. That is a ban on listed products, not the same thing as the 100% stack.
What actually changed
On 8 September 2026, President Donald Trump signed a package of five proclamations under Section 338 of the Tariff Act of 1930. The White House cast it as a response to Canada's latest retaliation after talks broke off in August.
Two dates matter.
The product additions and removals, including the stacking language, apply to goods entered for consumption, or withdrawn from warehouse for consumption, at or after 12:01 a.m. eastern time on 15 September 2026. The White House fact sheet and the motor-vehicle scope proclamation both use that time.
A second set of measures excludes certain Canadian products from importation from 29 September 2026. Those are bans. They should not be folded into a single "100% on all steel" headline.
The motor-vehicle scope proclamation is the clearest official statement of the stack. After listing the products added to and removed from the 50% Section 338 list, it says the duties imposed under Proclamation 11048, as modified, "shall apply in addition to duties imposed pursuant to section 232 of the Trade Expansion Act of 1962."
That one line is the whole legal point.
Until mid-September, goods already hit by Section 232 in several categories were exempt from the Section 338 surcharge. Trade lawyers and logistics houses read the new text as lifting that exemption for the motor-vehicle and alcohol baskets. The dairy basket is the exception that reporting still treats as keeping the old shield. Anyone classifying a live shipment should check the current HTS notes, not a social-media summary.
Two statutes, one invoice
Section 232 of the Trade Expansion Act of 1962 is the national-security tariff tool. The United States has used it on steel, aluminum and other products. In this dispute, the relevant Section 232 rate on the listed metal lines is 50%.
Section 338 of the Tariff Act of 1930 is older and less familiar. It lets the president impose additional duties, up to 50% ad valorem, if he finds that a foreign country is discriminating against US commerce. The July 2026 proclamations used that power against Canada over motor vehicles and dairy. The 8 September package changed which Canadian products sit inside those baskets.
USMCA origin does not get a product off the Section 338 list. The White House fact sheet is explicit: the Section 338 tariffs apply to covered goods whether or not they qualify under the US-Mexico-Canada Agreement, and they apply on top of Section 232.
So the importer is not choosing between one extra duty or the other. On a stacked line, both can appear on the same entry.
That is why 50% plus 50% can become 100% additional duty. It is arithmetic on listed classifications, not a new 100% statute.
The 100% figure is real. The "all steel" version is not.
Universal Logistics, writing for importers after the proclamations, listed specific steel and aluminum classifications that would now carry both 50% duties, for a combined 100%, from 15 September. The examples included certain aluminum non-alloyed profiles, bars and rods. Those are HTS lines, not a sector-wide decree.
Global Trade Alert, which modelled the September package, reached the same legal reading: from 15 September, Section 338 can sit on top of Section 232 for the modified motor-vehicle and alcohol baskets. It also warned against treating the change as a uniform tax on every Canadian metal shipment.
CBP classification practice points the same way. The duty follows the line. A product that looks like "steel" in ordinary speech may sit on a line that is stacked, a line that still carries only one of the two duties, or a line that was removed from the Section 338 list when other products were added.
The 8 September package both added and deleted products. The White House named rock salt and cement among the removals, and ATVs and extra dairy among the additions. Steel and aluminum appear in the added set for some classifications. They do not swallow the whole Canadian metals trade.
Say this: some listed Canadian steel and aluminum lines can now stack to 100% additional duty. Do not say: Canadian steel is now at 100%.
Why Washington stacked the duties
The proclamations sit inside the wider Canada-US fight that opened this summer and then broke down in late August. Canada announced counter-tariffs. Washington answered with Section 338 scope changes and, from 29 September, selected import bans.
The administration's public case is retaliation for retaliation. The legal case inside the proclamation is narrower: the president says Canada is still discriminating against US motor-vehicle commerce, and that changing the product list will offset that burden.
Those are political claims. They do not change the customs mechanics.
For a Canadian mill, the relevant question is whether its exported line is in Annex I, Part A, and whether it already pays Section 232. For a US buyer, the question is the landed cost on that same line. For a politician, the temptation is to collapse both into a round number.
The round number is carrying more weight than the annexes support.
What this means in the plant, not on the podium
A stacked 100% additional duty on a listed line is large enough to reroute orders, delay entries, or push a buyer to another supplier. It can also push more volume onto lines that were not stacked, if those substitutes are close enough. That is how tariff stacks usually work. They do not have to cover an entire industry to change behaviour at the margin.
Canada remains far more exposed to the US goods market than the other way around. That does not mean every Canadian metals job now faces a 100% wall. It means the listed lines are where the new cost lands first.
US downstream users of Canadian slab, plate, extrusion or wire can absorb the duty, pass it on, switch source, or slow purchases. None of those outcomes is proven in the proclamations. The proclamations set the rate. They do not forecast employment.
This article does not invent job losses or price jumps. Those numbers, if they come, will show up in company filings, Statistics Canada export data and US import entries. They are not in the 8 September texts.
The 29 September bans are a different instrument
It is easy to merge the 15 September stack with the 29 September exclusions. They were signed on the same day. They are not the same tool.
A stacked duty still lets the goods in, at a higher cost. An exclusion from importation is a ban on listed products. Section 338 allows both. Mixing them in one sentence makes the metals story sound larger than the HTS annexes support.
Anyone writing about "Canadian metal shut out of the US" needs to name the excluded lines and the 29 September date. Anyone writing about "100% tariffs" needs to name the stacked lines and the 15 September date. Both claims can be true on their own lists. Neither is true of Canadian metal as a whole.
What would change this piece
Three facts would force a rewrite.
First, a later proclamation or CBP notice that pauses or unwinds the stack. At publication, that pause was not on the record.
Second, a published HTS annex showing that the stacked 100% treatment now covers all, or almost all, Canadian steel and aluminum exports. The current texts do not say that.
Third, official entry data showing that the listed lines are so large that the "some lines" caveat becomes misleading in practice. That would be an evidence change, not a slogan change.
Until then, the accurate story is the mechanism. Washington can now charge Section 338 and Section 232 on the same listed Canadian goods. On some of those goods, the extra duties add to 100%. The exact line still decides.
Sources: White House proclamation, 8 September 2026; White House fact sheet, 8 September 2026; Federal Register presidential documents; Universal Logistics trade alert; Global Trade Alert, September package.