Fire Mastic Calculator

Coverage depends on the joint width and depth. A standard 310ml cartridge of intumescent sealant covers approximately 3.1 metres of a 10mm x 10mm joint, 1.5 metres of a 10mm x 20mm joint, or 1.0 metres of a 10mm x 30mm joint. Larger joints use significantly more sealant. Always allow 10-15% extra for waste, cut-off and tooling. The calculator provides exact coverage based on your joint dimensions.

Fire-rated sealant (intumescent mastic) contains materials that expand when exposed to fire, sealing gaps and preventing fire and smoke spread. Standard silicone or acrylic sealant will burn, melt or fall away in a fire, providing no fire resistance. Fire-rated sealants are tested to BS EN 1366-3 (penetration seals) or BS EN 1366-4 (linear joints) and carry specific fire ratings (30, 60, 120 minutes). Only fire-rated products should be used in fire-resisting construction.

Fire mastic (intumescent sealant) is preferred for joints that may experience movement, around service penetrations where flexibility is needed, and for finishing exposed seal faces. Fire cement is better for rigid applications in masonry and concrete walls, large void filling and where a hard finish is required. Fire cement typically costs less but cannot accommodate movement. For door frames, intumescent sealant is standard. For pipe penetrations through concrete, either product works.

The required depth depends on the fire rating and the specific product. Typical minimum depths are around 10-15mm for a 30-minute rating, 15-25mm for 60 minutes and 25-50mm for 120 minutes, but the depth must always match the manufacturer tested detail — installing less than the tested depth invalidates the fire rating. Intumescent acrylic is often only tested to seal narrow gaps (frequently up to about 5mm) without backing; for wider gaps a mineral-wool backer rod or fire batt is fitted first to support the sealant and set the correct depth, with many products tested up to roughly 25mm gap that way. Always follow the gap and depth limits in the product datasheet rather than a rule of thumb.

Yes — fire stopping including fire mastic seals should be inspected before being concealed, as once it is covered the installation can no longer be checked. Photographic evidence of each seal is best practice and is required under third-party installer certification schemes (FIRAS, IFC, LPCB) and increasingly expected under the Building Safety Act 2022 golden-thread regime; the responsible person also has record-keeping duties under the Regulatory Reform (Fire Safety) Order 2005, though that order does not itself prescribe photographing every seal. Building control may request to inspect fire stopping during construction. Seals should also be checked during fire risk assessments — deteriorated, cracked or missing sealant must be replaced to maintain the fire resistance of the construction.