Reserve a fixed height for every slot based on the size you serve most often, style it so an empty slot looks deliberate, and never insert an advert above content that is already being read.

Why adverts are the difficult case

Reserving space for an image is easy, because you know the image and its proportions before it loads.

An advertising slot is different in three ways, and each one breaks the usual fix.

The content arrives from a third party, late, after an auction you do not control.

It may come in any of several permitted sizes, so there is no single height to reserve.

And sometimes nothing is returned at all, leaving a space that either collapses or sits empty.

Which means the general advice to reserve the space is correct and insufficient, and it needs a decision about which space.

Pick the size you actually serve

The practical resolution, and it requires looking at your own data rather than at the list of permitted sizes.

Your ad platform reports which sizes actually filled each slot over the last month, and the distribution is usually far narrower than the permitted list suggests.

If one size fills a slot most of the time, reserve that height and accept a small shift on the minority.

The alternative, reserving the largest permitted size, produces a permanent gap on every page view to prevent an occasional shift, which is a bad trade on a page anybody scrolls.

Where two sizes dominate roughly equally, reserving the larger of the two is usually the lesser cost, since a gap below an advert reads better than the page moving.

Restrict what a slot may serve

Frequently the better answer, and it is a settings change rather than a technical one.

If a slot can return four heights, reduce it to one or two.

You lose a small amount of auction competition and gain a slot that behaves predictably, and for most small publishers the revenue difference is smaller than expected.

This is particularly worth doing above the fold, where a shift is most damaging, and worth leaving flexible further down where nobody is reading yet.

Decide it per slot rather than site-wide, since the position determines how much the unpredictability costs.

Never place a slot above the content

The first two are the ones that generate complaints and the ones publishers are most reluctant to move, because those positions earn the most.

A reserved slot in those positions is workable. An unreserved one is the single worst thing you can do to a reading experience.

Style the empty state

An unfilled slot is common and it is usually handled badly.

A reserved box that collapses when nothing is returned produces exactly the shift you were preventing, just in the opposite direction.

Keep the space, and make it look intentional: a subtle background, or your own content in the space, such as a link to a related page or your own offer.

Filling unsold inventory with your own material is standard practice, costs nothing, and turns dead space into something useful.

What does not work is a blank white rectangle with a border, which reads as a broken page.

A worked example

A regional publisher had a stability measurement in the poor range across their article pages and could not identify a single cause.

A recorded page load, played back slowly, showed three separate movements: a leaderboard above the headline expanding from nothing, a mid-article slot appearing, and a third slot collapsing when it went unfilled.

Their platform data showed one size filled the leaderboard about eighty percent of the time.

They reserved that height, restricted the slot to two sizes rather than five, moved the leaderboard below the headline, and filled unsold slots with links to their own popular articles.

The measurement moved into the good range within the usual reporting delay.

Advertising revenue fell by a small amount from the reduced size options and was more than recovered by the traffic improvement over the following two quarters.

Lazy-loaded adverts need the same treatment

Loading adverts only as they approach the visible area is good practice, and it introduces the shift problem further down the page.

A slot that has no height until the advert loads will push everything below it at the exact moment somebody is scrolling toward it, which is worse than a shift during initial load because the reader is actively moving.

Reserve the height for lazy-loaded slots as well, from the beginning, even though they will be empty for most of the page's life.

Load them slightly before they enter the viewport rather than exactly at the boundary, so the fill happens off-screen.

Sticky and interstitial formats

These do not cause layout shift, because they float above the page rather than sitting in it, and they carry a different cost.

A sticky footer advert occupies part of a small screen permanently, which on a phone is a meaningful proportion of what the reader can see.

Interstitials that cover the content on arrival have been treated as a ranking problem on mobile for several years, separately from any stability measurement.

So the trade is real: floating formats protect your stability score and can damage the experience more than the shift did.

Judge them on what they do to a reader on a phone rather than on what they do to a number.

The counter-case

Stability is not the only thing that matters on a monetised page, and optimising for it alone will cost you.

Reserving generous space for every slot on a page with six of them produces a page that is mostly gaps, which reads as an advertising site with some content attached.

Restricting sizes reduces competition in the auction, and on a site where advertising is the business model that is real revenue rather than a rounding error.

The honest position is that there is a genuine tension here between revenue and reading experience, and the resolution is per slot rather than a principle applied everywhere.

Fix the positions above the fold properly, since those carry the most damage, and accept more variability further down where the cost is lower.

What to do

  1. Record a page load on mobile and watch it slowly.
  2. Check which sizes actually fill each slot.
  3. Reserve that height rather than the largest permitted.
  4. Restrict sizes on slots above the fold.
  5. Move any slot that sits above the headline.
  6. Fill unsold space with your own content.
  7. Reserve lazy-loaded slots from the start.

Step two is the one that makes the rest proportionate, since reserving for a size you rarely serve is its own cost.

The general version of this problem is covered in the layout that jumps while loading.

Frequently asked questions

Why are adverts harder to reserve space for?

Because the content arrives late from a third party, may come in several sizes, and sometimes does not arrive at all. There is no single known height to reserve in advance.

Which height should I reserve?

The size that actually fills the slot most often, from your platform's own reporting. Reserving the largest permitted size leaves a permanent gap to prevent an occasional shift.

Should I restrict what sizes a slot can serve?

Above the fold, usually yes. You lose a little auction competition and gain predictable behaviour, and for most small publishers the revenue difference is smaller than expected.

Where should adverts not go?

Above the headline, or between the headline and the first paragraph, unless the space is fully reserved. Those positions earn most and cause the most damage.

What happens when a slot goes unfilled?

A collapsing box produces the same shift in reverse. Keep the space and fill it with your own content, such as links to related pages, rather than leaving a blank rectangle.

Do sticky adverts solve this?

They avoid layout shift by floating above the page, and they occupy part of a small screen permanently. Judge them on the reading experience rather than the measurement.

West Coast Media Solutions Inc. provides web design, web development, hosting, digital marketing, and business consulting to organisations across Canada, drawing on more than twenty-five years in the field.

Monetised pages jumping around?

Check which ad sizes actually fill each slot, then reserve that height. Reserving the largest is its own cost.

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