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What Are Lag Screws and Why Use Them?
Lag screws (also called lag bolts) are heavy-duty wood screws with a hex head and coarse thread, designed to create extremely strong wood-to-wood or wood-to-metal connections. Unlike ordinary wood screws, lag screws require pre-drilling and provide shear and withdrawal resistance comparable to bolted connections. For African timber construction — from trusses to deck subframes — lag screws are the preferred alternative to through-bolts where access is limited.
Sizes and Specifications
Lag screws range from 1/4" to 1" diameter and 1" to 16" length. The larger the diameter and length, the higher the load capacity. For structural timber connections in African construction, M8-M12 (5/16"-1/2") diameter is most common. Length selection depends on: at least 2x the side member thickness should penetrate the receiving member, plus adequate thread engagement in the main member.
| Diameter | Length Range | Driver Size | Common Application |
|---|---|---|---|
| 1/4" (M6) | 1"-3" | 3/8" hex | Light timber frames |
| 5/16" (M8) | 1.5"-6" | 1/2" hex | Decking, light framing |
| 3/8" (M10) | 2"-8" | 9/16" hex | Structural timber frames |
| 1/2" (M12) | 3"-12" | 3/4" hex | Heavy structural connections |
| 5/8" (M16) | 4"-16" | 15/16" hex | Marine, heavy load applications |
| 3/4" (M20) | 6"-16" | 1-1/8" hex | Bridge timber, special structures |
Materials and Coatings
For African environments, material selection is critical: Hot-dip galvanized (HDG) is the standard for outdoor and treated timber applications — the zinc coating protects against corrosion and is compatible with ACQ-treated lumber. Stainless steel 316 provides maximum corrosion resistance for coastal applications but is expensive. Plain (uncoated) steel is only suitable for indoor dry environments and must be protected from moisture. Coach screws (lag screws with a square head) are preferred in some African markets for manual installation without power tools.
| Material/Coating | Corrosion Resistance | Cost | Best Environment |
|---|---|---|---|
| Hot-dip galvanized | High | $$ | Outdoor, treated timber |
| Stainless steel 316 | Maximum | $$$$ | Coastal marine |
| Stainless steel 304 | High | $$$ | Inland outdoor |
| Plain steel | Low | $ | Indoor dry only |
| Zinc electroplated | Moderate | $ | Indoor/protected outdoor |
Proper Installation Technique
Installation errors account for most lag screw failures. Critical steps: Pre-drill the pilot hole — use a bit 1/64" smaller than the screw's minor diameter for the shank portion, and a smaller bit (接近 TAP drill) for the threaded portion. The pilot hole must be deep enough to accommodate the full screw length. Use a washer — always use a flat washer under the hex head to distribute load and prevent embedment into the wood. Apply lubricant sparingly — parafin wax or soap reduces torque and prevents thread stripping. Never over-torque — stop when the head is flush with the washer; over-torqueing causes thread stripping in the wood.
Withdrawal and Shear Load Capacity
Lag screw load capacity depends on wood species, screw size, and penetration depth. As a general reference for Douglas Fir-Larch (common in African structural timber imports): a 3/8"×4" lag screw with 2.5" thread engagement provides approximately 400-500 kg withdrawal resistance and 600-800 kg shear resistance. For pine species, reduce by 20-30%. Always use manufacturer load tables or have a structural engineer calculate for critical applications.
| Screw Size | Min Penetration | Withdrawal (kg) | Shear (kg) |
|---|---|---|---|
| M8×50mm | 35mm | 200-250 | 300-400 |
| M10×75mm | 50mm | 400-500 | 600-800 |
| M12×100mm | 65mm | 600-800 | 900-1200 |
| M16×125mm | 80mm | 900-1200 | 1400-1800 |
African Construction Applications
For African timber projects, lag screws are widely used in: Roof truss connections — joining purlins to rafters using galvanized M10 lag screws; Decking subframes — attaching deck boards to bearers, using stainless or HDG M10×75mm; Fence construction — structural fence posts to rails; Formwork supports — temporary timber frame for concrete casting; Solar panel mounting — timber rail to steel frame connections (inland only, use HDG). For treated timber (kiln-dried, ACQ-treated), always use hot-dip galvanized lag screws — zinc-plated will corrode rapidly due to chemical reaction with the treatment.
Frequently Asked Questions
What's the difference between a lag screw and a structural wood screw?
Lag screws have a tapered shank and coarse thread designed for wood, with a hex head requiring a wrench. Structural wood screws (like GRK, SPAX) are newer designs with reamer threads and no pre-drilling required, but are more expensive. For heavy timber connections, lag screws remain the most cost-effective choice.
Can I use lag screws instead of bolts for timber connections?
Yes, for many applications lag screws are a valid alternative to through-bolts. They provide comparable load capacity without requiring access to both sides of the connection. However, for critical structural connections requiring inspection or where the joint may be disassembled, traditional through-bolts with plates are still preferred.
Why do lag screws break during installation?
The most common cause is incorrect pilot hole sizing — too small causes high torque and snapping; too large prevents thread engagement. Always pre-drill with the correct bit size for shank and threaded portions. Also ensure the screw is perpendicular to the wood surface; angled installation creates bending stress.
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