How do I fix into a concrete floor?
Choosing between concrete screws, expansion anchors and resin, how deep to go without breaching the membrane, and finding out what is in the slab first.
- Difficulty
- intermediate
- Time
- 40 min
- Read
- 4 min
- Safety
- warning
Short answer
For light and medium fixings, a concrete screw into an accurately drilled SDS hole is quick, strong and removable. For heavy loads use a throughbolt or resin. Keep holes as shallow as the fixing allows, because the damp-proof membrane and any underfloor heating sit within the slab build-up.
A concrete ground floor is not just concrete. It is typically a slab, a membrane, sometimes insulation, and a screed on top, and every one of those layers changes what a fixing does and what damage it can cause.
Safety
What you'll need
- SDS drill and bits
- concrete screws or throughbolts
- vacuum or blower
- cable, pipe and heating detector
- eye protection
Step by step
- Find out what the floor build-up is.Age of the house, presence of underfloor heating, and whether the surface is a screed or the structural slab. If there is wet underfloor heating, do not drill without a thermal camera or the original layout drawing.
- Scan before drilling.A detector will find heating pipes and conduit near the surface. Warm the heating first and use a thermal camera if you have access to one — pipes show clearly.
- Decide the depth you actually need.Match it to the fixing's stated embedment and go no deeper. A shallow accurate hole is safer in every respect than a generous one.
- Use an SDS drill.A combi drill in a concrete slab is a losing battle. An SDS-plus with a good bit will drill a clean accurate hole in seconds.
- Choose a concrete screw for most jobs.They cut their own thread in a precisely sized hole, need no plug, can be removed and refitted, and are strong enough for battens, frames, floor plates and brackets.
- Choose a throughbolt or resin for heavy loads.Machinery, structural posts, gym equipment. Throughbolts need sound concrete and edge distance; resin suits anything less than perfect.
- Clean the hole.Vacuum it out. Concrete dust in the hole is the difference between a concrete screw cutting a full thread and one that jams on debris and snaps.
- Do not over-torque a concrete screw.They cut a thread in brittle material. Driven too hard they strip the hole and go loose without warning. Firm and stop.
- Seal the hole if the membrane may be breached.Where a hole has gone through into a suspect layer, bedding the fixing in resin or sealant limits the moisture path.
- Consider a non-drilling alternative.Heavy furniture, gym flooring and even some frames can be held with construction adhesive or ballast rather than a fixing. In a floor with underfloor heating, that is often the right answer.
Tips
- Concrete screws are the modern default for floors — no plug, removable, and much easier to install correctly than a throughbolt.
- Where there is underfloor heating, run it hot for an hour then look at the floor with a thermal camera. The pipe layout becomes obvious.
- In a screed rather than a structural slab, keep fixings shallow. Screed is much weaker than the slab under it.
Common mistakes
- Drilling deeper than necessary — Depth is the only thing standing between the bit and the damp-proof membrane, insulation or a heating pipe. There is no benefit to a hole deeper than the fixing needs.
- Using an ordinary plug and screw — A nylon plug in a floor is loaded in a way it handles poorly, and floor fixings usually see shock loads. Concrete screws or anchors are the right family.
If it doesn't work
Concrete screw snapped while driving
Cause: Hole not cleaned, or too shallow, or driven with an impact wrench — Fix: Vacuum the hole, drill slightly deeper than the screw, and drive with controlled torque rather than an impact wrench.
Fixing loose in the hole
Cause: Hole oversize, or the screed crumbled — Fix: Move, or drill through the screed into the slab beneath and use a longer fixing. Resin is the reliable recovery.
Damp patch appearing around a fixing
Cause: Damp-proof membrane breached — Fix: Remove the fixing, fill the hole with a suitable resin or sealant, and relocate the fixing. Persistent damp may need professional assessment.
Bit stops dead a short way in
Cause: Reinforcement mesh or bar — Fix: Move at least 75mm. Do not drill through reinforcement in a structural slab.
Questions people ask
Can I fix a stud wall to a concrete floor?
Yes — a sole plate fixed with concrete screws at regular centres is standard practice. Use a DPC strip under the plate on a ground floor to keep the timber off the concrete.
What if I hit underfloor heating?
Turn the system off and isolate it immediately, and get a heating engineer. A pierced pipe in a screed is a repairable but urgent job.
Do I need to drill at all for a shed base or a frame?
Often not. Weight, ballast or adhesive can hold a great deal, and avoiding holes in a floor with a membrane and heating is worth some inconvenience.