Choosing ground screw length and diameter for a deck

Most ground screw problems come from the wrong length, not the wrong product. A screw that stops in loose topsoil or in the frost layer will move, however good the steel is. Work through the decision in four steps: soil, frost, load, head.

Step 1: Read the soil

The screw gets its grip from the soil around the helix. Different ground behaves very differently:

• Firm clay and dense sand. Good bearing. Standard lengths usually reach enough firm material.
• Gravel and stony ground. Good bearing once the screw is in, but large stones can block it. Probe first and be ready to shift a position.
• Made ground and fill. Common on new-build plots. The top layer may be rubble, topsoil or loose fill and gives little useful bearing. Plan for longer screws or extensions.
• Organic soils, peat and waterlogged ground. Weak near the surface. This is engineer territory; do not guess.
• Rock or old foundations. A screw may not go in at all. A pilot hole, a shifted layout or a different foundation may be needed.

The simplest test costs one screw: drive a test screw at two or three positions before committing to the layout. Resistance, wobble and refusal tell you more than any chart. On larger projects, the driving torque is a useful indicator of capacity.

Step 2: Check the frost depth

In cold climates, wet soil freezes, expands and lifts anything sitting in it. The thread must anchor below the frost-affected layer, or the deck will heave out of level in winter. Your local building regulations give the design frost depth.

That one figure explains the range. Standard deck screws are 1–2 m long because they need to reach below the frost layer plus a margin of firm soil. In regions without ground frost, a short screw with a wide head can do the job instead; see deck foundations in regions without ground frost.

Step 3: Match diameter and wall to the load

Longer and wider screws engage more soil. Thicker walls resist bending and twisting during driving and in service. As general practice:

• Light, ground-level garden decks: 68 or 76 mm shafts.
• Softer soil, raised decks, longer extensions: larger diameters (89 or 101 mm) or 3–3.5 mm walls.
• Heavier structures such as hot tubs, pergolas or roofs over the deck: get a design check.

The Fastshifter range in numbers, from the product data:

Type	Head	Diameters	Lengths	Wall
M	Flat plate, M12 to M24 stud	68, 76 mm	750–2050 mm	1.8–3.5 mm
U	U-head for timber post or beam	81, 101 mm	685–1600 mm	1.8–3 mm
N (connection series)	Plate with M20 stud, couples to N extensions	76, 89 mm	1200 mm	3.5 mm
GM	GM adapter head; 220 mm round adapter available	76 mm	1200 mm	3 mm
Extensions	Couple to the base screw	68, 76, 89 mm	500 or 1000 mm	1.8–3.5 mm

Several M and U sizes also come as EXTRA versions, with a reinforced head section and a longer helix weld for higher loads or softer ground.

Step 4: Pick the head for the frame

• Type M suits bolted connections: post shoes, beam brackets, levelling nuts and steel base plates. The 76 × 1600 mm M18 is a typical deck size.
• Type U takes a 90–100 mm timber post or beam directly: drop it in, drill through and bolt, as with the 101 × 1000 mm U.
• N series is built around extensions. The 76 × 1200 mm N couples to N extensions when firm soil is deeper than expected.
• GM series takes a round adapter with a 220 mm plate for round posts, steel columns and post shoes.

When to add an extension

Drive the base screw until only the coupling is above ground, join the extension, tighten the joint fully and keep driving. Typical cases:

• the first metre gives no useful bearing;
• downhill supports on a slope stand proud of the ground;
• decks over water or wetland;
• raised terraces well above grade.

Extensions can be stacked where very deep anchorage is needed. Remember that a long unsupported length above ground behaves differently from a buried one; bracing may be needed on tall supports.

When to stop and get advice

• Peat, organic or permanently waterlogged soil.
• Unknown fill on a recently built plot.
• Heavy or concentrated loads.
• Steep slopes or decks well above ground.
• Results from your test screws that do not make sense.

In these cases a geotechnical check or an engineer’s design costs far less than relevelling a deck. Compare options in ground screws vs concrete footings.

Read next: how many pedestals per m².