Concrete screw length and embedment
The length is the material plus the embedment — and the embedment has a maximum as well as a minimum, which is the part most often got wrong by applying wood instincts to masonry.
= 1.5"
= 1.5"
Buy
3 1/4"× 1/4" screw
Carbide bit
3/16"
Drill this deep
1 3/4"
It will embed
1 3/4"
Where to buy this
- 3 1/4" × 1/4" concrete screwsHome DepotLowe'sAmazon
- 3/16" carbide bitHome DepotLowe'sAmazon
It has to be a carbide-tipped hammer-drill bit — a twist drill of the same marking will not cut concrete. Many screws come with the matching bit in the box.
Plain search links, nothing sponsored. Prices and stock are the shop's, and a local lumberyard will usually beat both on solid stock.
Checks
- Embedment is in range — passes1.5", between the 1" minimum and the 1.75" maximum.
- A stocked length reaches — passes3.25" covers 3".
- That screw embeds correctly too — passes3.25" embeds 1.75".
The third check exists because rounding up to a stocked length pushes the screw deeper than you asked for, and deeper has a limit here.
Deeper is not stronger
Embedment has a maximum as well as a minimum: 1" so the threads engage enough concrete, and 1.75" because past that the screw cuts more concrete than its flutes can clear. The debris packs and the screw stops turning or shears off in the hole. This is the opposite of the rule for wood, and it is the part most often got wrong by applying wood instincts to masonry.
Drill about a quarter inch past the embedment and blow or vacuum the hole out. A screw bottoming on packed dust feels tight while holding almost nothing, and that is the usual reason one that "went in fine" lets go later.
What the concrete is doing
The load a concrete screw holds is a property of the concrete far more than of the screw. Block, brick and mortar joints all hold well under sound poured concrete, and a crumbling edge holds nothing at all. Nothing here covers a base material that is not sound.
Anything structural — a ledger carrying a deck above all — has its fasteners, spacing and embedment set by the code and an engineer, and often wants a wedge anchor or a through-bolt rather than a screw.
Screw and bit
- 3/16" screwFurring strips, electrical boxes, light brackets — the common small one.
- 5/32" bit
- 1/4" screwSleepers, plates and anything carrying real load into sound concrete.
- 3/16" bit
The pairings are published by the screw makers, and the bit is not the screw's own size — which catches people out. It has to be a carbide-tipped hammer-drill bit: a twist drill of the same marking will not cut concrete and will not survive trying.
About fastening to concrete
- How long should a concrete screw be?
- The thickness of what you are fastening plus the embedment. A 1 1/2" sleeper with 1 1/2" of embedment wants 3", so you buy the 3 1/4" and get 1 3/4" in the concrete. That is the whole calculation — the part worth checking is that the screw you end up buying still embeds within range, because rounding up to a stocked length pushes it deeper.
- Is deeper always stronger?
- No, and this is where wood instincts mislead. Embedment has a maximum as well as a minimum: 1" minimum so the threads engage enough concrete, and about 1 3/4" maximum because past that the screw is cutting more concrete than its flutes can clear. The debris packs, and the screw either stops turning or shears off in the hole. Both ends of that range are checked here.
- Why does the hole have to be deeper than the screw?
- Because concrete dust has nowhere else to go. A screw bottoming out on packed dust feels tight while holding very little, and it is the most common reason a concrete screw that "went in fine" pulls out later. Drill about a quarter inch past the embedment and blow or vacuum the hole out — the cleaning matters as much as the depth.
- Which bit goes with which screw?
- A 3/16" screw takes a 5/32" carbide bit and a 1/4" screw takes a 3/16". Those pairings are published by the screw makers and they are not the same as the screw's own size, which catches people out. It has to be a carbide-tipped hammer-drill bit, too — a twist drill of the same marking will not cut concrete and will not survive trying.
- Can I hang a deck ledger with these?
- Not off this page. A ledger is a structural connection carrying live load over a drop, and its fasteners, spacing and embedment come from the code and an engineer — and often from a wedge anchor or through-bolt rather than a screw. Base material matters too: block, brick and mortar joints all hold far less than sound poured concrete, and the load a concrete screw carries is a property of what it is in more than of the screw.
You might also need
- Concrete footings and slabsCubic yards for a run of post footings or a slab, and the bags of ready-mix that buys.
- Bolt and lag spacingEdge, end and row distances for bolts through a timber, in the multiples of diameter the NDS sets.
- Wall framing stud countField studs at 16" or 24" on centre, corner studs, and the king and jack studs every opening adds.