Intumescent mastic
A mastic that swells in the heat of a fire and closes the gap it is sitting in.
Get a priceSqueeze a bead of ordinary sealant into a gap and hold a lighter under it. It goes soft and slides out.
Do the same with intumescent mastic and it puffs up and fills the gap instead.
The tubes are grey, pink or white, depending on the maker. You put it on with a gun. Buckets of it go on with a trowel. Check the label, because some types are made for joints and others for pipes.
What it is good for
It suits the small gap round a pipe or conduit where it passes through a wall, once the main opening has been filled. It suits the joint where a wall meets the floor slab above, the gaps round the frames of service hatches, and the edges of a batt or board seal where it meets the blockwork. It also suits a narrow hole cored through a wall for one cable.
What it is not good for
Big openings. A large hole is filled with a batt first, and mastic goes on after as a finish.
Anywhere the movement is large. A joint that opens and closes with the building needs a flexible product, and it has to be specified for the movement.
Surfaces that are damp, dusty or oily. Mastic does not bond to those, so the surface is cleaned first.
How we decide the depth
The maker's tested detail says how deep the mastic has to go and how wide the gap can be. A thin skin over a deep hole does not count. We use a backing material to hold the mastic at the right depth, and the finished bead is smoothed so it is easy to inspect.
The label, not the colour
Pink, red, grey and white mastics all exist. The colour tells you nothing. The label does, because it names the product and the date it went in.
The photo
A skim of mastic over an empty void looks the same from the corridor as a full bead. So we photograph each one before the last coat goes on, and keep the photos with the record.
Questions we get asked
Is intumescent mastic the same as fire rated foam?
No. They are different products with different tested uses. See expanding foam and fire stopping.
Can I paint over it?
Some can be painted and some cannot. Check the data sheet first. Painting over a mastic that is not meant to be painted can change how it behaves.
How long does it last?
Manufacturers state a working life for each product. We keep the data sheet with the job record, so you can check it.
Send us the postcode and a photo or two. We will tell you what it needs and give you a price.
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