What do the F and S ratings on a brick mean?
Two ratings that decide whether a brick survives a UK winter and whether it damages the mortar around it — and the positions where only the top rating will do.
- Difficulty
- beginner
- Time
- 8 min
- Read
- 3 min
Short answer
F is freeze-thaw durability: F2 is frost resistant, F1 is moderately frost resistant, F0 is not frost resistant. S is soluble salt content: S2 is low, S1 is normal. For anything exposed — a garden wall, a parapet, a chimney, below the damp proof course, anywhere that stays wet — you want F2 and, in aggressive conditions, S2.
These two letters on a data sheet decide whether a brick lasts a lifetime or blows its face off after five winters, and they are routinely ignored because a brick with a bad rating looks exactly like one with a good rating on the day it is delivered. The failure appears years later, and by then it is a rebuild rather than a return.
Step by step
- Read the F rating as a wetness question.Frost damage happens when a brick is saturated and then freezes. F2 bricks are made and fired to survive repeated saturation and freezing. F1 will survive moderate exposure where the brick can dry between wettings. F0 is for internal or fully protected work only.
- Identify the positions that need F2.Free-standing garden and boundary walls, parapets, copings, chimneys, retaining walls, brickwork below the damp proof course, anything at ground level catching splash, and any wall on a severely exposed elevation. All of these stay wet.
- Read the S rating as a mortar question.Soluble salts in the brick migrate into the mortar with moisture and can attack it, and can crystallise at the surface. S2 has a low active soluble salt content and is specified where the brickwork will be persistently wet — the same positions that need F2.
- Do not confuse the ratings with hardness.A hard, dense brick is not automatically frost resistant and a soft one is not automatically vulnerable. It depends on the pore structure — whether water in the brick has room to expand when it freezes. This is why some very soft handmade bricks last for centuries.
- Ask for the data sheet.The ratings are on the manufacturer's technical data sheet and often not on the pack. Any merchant can produce it. If nobody can tell you the rating, treat the brick as unrated and do not use it anywhere exposed.
- Apply the same thinking to the mortar.A frost-resistant brick in a weak, saturated, unprotected mortar still fails, at the joint. Mortar in exposed positions wants a suitable mix, plasticiser for the entrained air, and protection while it cures.
- Fix the wetness as well as choosing the brick.Ratings are a durability specification, not a substitute for a coping that throws water clear, a damp proof course, a working gutter and a ground level below the damp proof course. Most spalling is caused by a wall being wetter than it should ever have been.
Tips
- Old bricks carry no rating at all. What they have instead is a track record — a brick that has been in a wall for a hundred and twenty years and is still sound has proved itself more thoroughly than any test.
- Reclaimed bricks vary enormously within a single load, because they come from different parts of a building with different exposures. Sort them and put the best ones where the weather is.
- A coping stone with a drip, projecting on both faces, does more for the life of a garden wall than any upgrade in brick specification.
Questions people ask
Can I use F1 bricks for a garden wall?
Not sensibly. A free-standing wall is wetted on both faces and on top and dries slowly, which is exactly the condition F2 exists for. Use F2 and cap the wall properly.
What does F0 mean?
Not frost resistant. Internal work, or fully protected positions only. Many cheap commons are F0 and they will spall within a few winters if used outside.
Do I need S2 for a normal house wall?
Usually not. S1 is the norm for general above-ground brickwork in a wall that can dry. S2 is specified for persistently wet positions — below ground, in parapets and chimneys, and in retaining situations.