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Fixing To... Hollow Concrete Block

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Hollow concrete blocks are what it says on the tin – blocks made of concrete with (you guessed it) hollow spaces inside them.

They are popular in construction as they are cost-effective, durable and energy-efficient thanks to the insulating properties of the air pockets in their core – these also help with soundproofing! They are also generally weather- and fire-resistant and their rough surface bonds well to all wet applied materials such as mortar and render. As they are very strong, using them in internal walls means the wall doesn’t have to be as thick – so more floorspace! And thanks to their relative light weight, they’re often used in homes in earthquake-prone areas.

Hollow concrete blocks are generally made of the same material as poured concrete which means water, aggregate, sand and Portland cement, but fly ash is sometimes used to make them even lighter (cinder blocks). As they are pre-cast, they provide consistent finish and accuracy, which means they are ideal for load-bearing structures and partition walls.

pile of concrete blocks in a building site with bags of cement on top of them

How do I know if my wall is hollow block?

If you can see the blocks’ outlines, knocking on them can help you figure out if they are hollow in places or not, however this isn’t aways reliable and won’t help much if the blockwork is covered on both sides.

Hollow concrete blocks tend to be standard sizes, and a common thickness is 100 or 140mm, like these blocks from CEMEX, so the thickness of your wall might give you some clue. Alternatively, drilling a small test hole in a place that’s out of the way is also an option. Obviously if its a ‘new build’ property you’ll have seen what the bricklayer is using. 

What should I know about fixing to hollow concrete?

An issue you may run into when fixing to hollow concrete block is the fact that it’s almost like a cavity wall, but often too thick for actual cavity wall fixings! And if the hollow block is covered with, for example, plasterboard or stucco, you won’t be able to see the outlines of the blocks, which means it’s hard to tell where the cavities are when fixing to it.

With relatively light loads like light fixtures or pictures you can usually just go with light multimaterial fixings like Fischer Duopower and not worry too much about the cavities, as Duopower works in both solid concrete and hollow blocks. Or if the hollow blocks are covered with plasterboard, you can just go with lightweight plasterboard fixings or hooks like Cobra Walldriller.

What if I'm fixing heavier items?

For heavier items, you’ll need to know where the cavities and mortar joints are, as you should avoid fixing anything heavy within 1 inch of the mortar joint. If the outlines of the blocks are visible, you can always drill small test holes in a block that’s out of the way – the positioning of the cavities will be the same in each hollow block.

If the surface is covered and you can’t see the outlines of the blocks, tapping on the block may sound more hollow where the cavities are (depending on the thickness of the material). It’s worth noting that blocks near corners, windows or doors may be filled with concrete or other gap filling materials.

For heavier loads or thick materials with a cavity behind them, we recommend Cobra Fliptoggle as it’s suitable for all wall materials up to 50mm deep. You’ll just need to drill a 12mm hole, insert the fixing which flips and grips the rear of the hollow block, snap the front guide piece to length and then insert your bolt. The M5 FlipToggle fixings can hold 57 kg in 13mm plasterboard and 204kg in hollow concrete each, so they’re ideal for heavy loads where the wall construction may be unsuitable for other cavity fixings.

How do I drill into hollow concrete?

It’s worth remembering that even though you can drill into masonry (including hollow concrete block) with a regular rotary drill and a masonry drill bit, you need to be careful not to damage the drill or burn out the motor, especially if you’re installing lots of fixings. If you want to play it safe, you can always buy or rent a hammer drill instead.

Have you got questions about fixings? Get in touch with us through email, call 01248 295022, live chat or the contact form!