GMAT Weaken CR: Alternative Cause | OG | Northtown Roof

The Official GMAT question that begins “The roof of Northtown’s municipal equipment-storage building collapsed” shows when an alternative cause weakens a causal argument, and when it does not.

An alternative cause is one of the most common types of weakeners in causal arguments. So when a choice seems to offer another cause, it is natural to select it. But sometimes you select a choice that looks like an alternative cause and still get the question wrong.

If that has happened to you, this post will help you work out whether an alternative cause can weaken the conclusion, before you commit to it.

The Question

Here is an easy Official question that shows the checks in action.

A town’s recently built equipment-storage building lost its roof under a heavy snowfall. The building met the local safety codes in every respect except one: the nails attaching the roof supports to the columns were smaller than the codes specify. An editorial concludes that the collapse shows how even one small, apparently insignificant departure from safety standards can have severe consequences.

Step 1: Confirm that the argument is causal

An alternative cause can weaken only a causal argument, so start here.

This conclusion never uses the word “cause.” It says the collapse exemplifies how a small departure can have severe consequences. But think about what that requires. The collapse can be an example of a small departure leading to a severe consequence only if the small departure is what led to the collapse. If the roof came down for some other reason, it is not an example of anything the author is describing.

So underneath the wording, the author is claiming that the undersized nails caused the roof to collapse.

The reasoning behind that claim runs by elimination. The building met the codes in every respect except the nails. So, the author reasons, the nails must be what went wrong.

Step 2: See how an alternative cause weakens it

The author’s reasoning by elimination has a gap. It checks the building against the codes, but it never asks whether something else could have caused the roof to collapse.

The correct choice fills exactly that gap. It brings in a new factor - weight of snow - and tells you the snow on this roof was heavier than the maximum the roof was designed to handle.

To understand it better, you can put numbers on it. Say the codes were written for roofs carrying at most 100 units of snow, and this roof was carrying 120. A roof built entirely to code, with the correct nails, was never designed to hold that load. It might have come down anyway. Until now, the snow was just the situation, a load the roof was supposed to handle. This choice turns it into a cause.

Now the collapse has two possible explanations: the undersized nails, or snow beyond what any code-compliant roof was built for. The author’s evidence no longer points to the nails alone, so your belief in the conclusion drops. That is what an alternative cause does.

Notice that this choice never mentions the nails. It does not need to. An alternative cause weakens the argument by offering another explanation for the outcome, not by discussing the author’s explanation.

When an Alternative Cause Does Not Weaken

This is where students who select an alternative cause can still go wrong. Before selecting one, run two checks.

Check 1: Does the conclusion need the author’s cause to be the only reason?

The snow choice works because the original conclusion needs the nails to be the only reason the roof collapsed. The snow is treated as the situation the roof should have handled, not as a cause. However, what if the conclusion said that the nails were one of the reasons behind the roof collapse? Let’s check this conclusion:

The undersized nails were one of the factors behind the collapse.

Does the snow choice weaken this version? No. The roof could have been carrying more snow than the codes allowed for and held up by weaker nails than the codes required. Both could have played a part in bringing it down. Excess snow and weak nails can be true at the same time, so learning about the snow does not make you doubt that the nails were one of the factors.

So an alternative cause weakens a conclusion that needs the author’s cause to be the only reason. It does not weaken a conclusion that only claims the cause played a part.

Can you think of a choice that would weaken the modified conclusion?

Check 2: Can the alternative actually produce the outcome?

Another choice in this question tells you that, because the building was not meant for people, some safety provisions that apply to office buildings did not apply to it.

This can look like an alternative cause. If some provisions did not apply, maybe the building was weaker somewhere else, and maybe that weakness brought the roof down.

But look at what the choice actually says. It tells you some provisions did not apply. It does not say those provisions concern roof strength, snow load, or anything else that could make a roof collapse. A different set of rules is a difference, not a cause.

The snow choice is different. Excess weight on a roof is exactly the kind of thing that can bring a roof down. That is what makes it a real alternative cause.

What the correct choice does not need to do

One last point. The snow choice does not prove that the nails played no role. They may still have been the reason behind the collapse. The choice only makes you less confident that the nails were the reason, and that is all a weakener has to do.

Quick Checklist

Before you select or reject an alternative cause in a Weaken question, ask yourself:

Is the conclusion a causal claim?
Does the conclusion need the author’s cause to be the only reason, or does it only claim that the cause played a part?
Can the factor in this choice actually produce the outcome?

About the Weaken Beginner Series

This question is part of the Weaken Beginner Series, in which easy Official GMAT questions are used not to test what you know, but to reveal how you think. Each question targets a specific reasoning habit, one that shows up in a simple form on easy questions and a complex one on hard questions. If you are getting started with Weaken questions or if Weaken questions feel inconsistent for you, this series is built to show you exactly where the process is breaking down.

When Can an Alternative Cause Actually Weaken the Argument?