Double threaded screw for insulation — DGZ

DGZ is the Rothoblaas double threaded insulation screw, certified under ETA-11/0030, fixing counter battens and façade battens through the insulation and holding them clear of the structure behind.

The screw carries the batten load back to the rafter, joist or panel through the insulation, without a bracket or secondary joist crossing the layer. ETA-11/0030 covers it in solid timber, glulam, CLT, solid wood panel and LVL, in hard, soft and façade insulation, at 60° or 90°. Design is to EN 1995-1-1 and the UK National Annex.

Key features

• Certified in non-pre-drilled LVL, the highest withdrawal case on the sheet, at fax,k 15.0 N/mm², ρa 500 kg/m³, characteristic density 410 to 550 kg/m³
• Also covered in solid timber, glulam, CLT and solid wood panel
• Double thread with a 3 THORNS tip, 100 mm point side and 60 mm head side, which is what allows the tight minimum spacings below
• Thirteen lengths, Ø7 from 220 to 380 mm and Ø9 from 240 to 520 mm
• Cylindrical head, Ø9.50 mm at Ø7 and Ø11.50 mm at Ø9, seated inside the batten
• Certified for hard, soft and façade insulation, fixed at 60° or 90°, both tabulated in the ETA
• TX30 on Ø7 and TX40 on Ø9, bit included in every box

Thermal bridging

Fixing the insulation continuously, rather than interrupting it with timber counter-joists, was calculated to improve the U-value of the roof build-up by 10 to 15%. The calculation was carried out by EURAC Research as part of the MEZeroE project, funded by the European Union's Horizon 2020 programme under grant agreement No 953157. It compares a continuous insulation package against one interrupted by secondary joists; it is a calculation, not a site measurement, and the result for any specific roof depends on that roof's build-up.

That benefit depends on restraint. The datasheet is explicit that excessive anchoring or incorrect arrangements should be avoided in order not to compromise the thermal performance of the package. More fixings than the design calls for works against the reason for choosing this detail.

Minimum spacings

The 3 THORNS tip is what buys the tight spacings. They are independent of insertion angle and of grain angle.

• Between screws, a1 and a2: 5·d, so 35 mm at Ø7 and 45 mm at Ø9
• To end grain, a1,CG: 10·d, so 70 mm at Ø7 and 90 mm at Ø9
• To the edge, a2,CG: 4·d, so 28 mm at Ø7 and 36 mm at Ø9

These hold only in members of at least 12·d thickness, 84 mm at Ø7 and 108 mm at Ø9, and at least 8·d width or 60 mm, whichever is greater. The datasheet does not print that precondition next to the spacings, so check the member before you rely on them.

Before you fix

• Check the tip cannot break through the underside of the member. This is the most common error on long insulation screws
• Batten minimums are not optional: 50/30 mm base and height at Ø7, 60/40 mm at Ø9
• Pre-drilling is required above 550 kg/m³ characteristic density. Note that pre-drilling LVL puts you outside the 15.0 N/mm² case above
• Electrogalvanised zinc at minimum 5 µm, so service class 1 and 2 only, C2 atmospheric and T2 wood corrosivity. Not for exterior or coastal exposure

The same screw, two code ranges. DGZ and DWZ are the same Rothoblaas screw under two codes: DGZ is the designation used in ETA-11/0030, DWZ is the Holz Technic code. Same manufacturer, same geometry, same approvals. The range here is the wider one, running to 520 mm at Ø9 where DWZ stops at 400 mm, and the DWZ selection table is this same table narrowed to the stocked lengths, so some deeper cells are blank there. For deeper build-ups, this is the page. The screw is also certified in flange-head (DGT) and countersunk-head (DGS) versions, which we do not stock. An EVO version with a Zn-Al flake coating is available on request for service class 3 applications.

Design and specification support

Rothoblaas MyProject is a free calculation tool that produces a fastening calculation report for this screw. It is a design aid for the specifier. It does not replace the structural design of the roof, and the connection must still be checked and signed off by the project's structural engineer. See also our guide to Eurocode 5 timber design, our structural engineering support, and the full structural timber screw range.

Common questions

Does a larger diameter mean fewer fixings? Rothoblaas's own worked example on the datasheet runs both diameters over the same roof: GL24h 120x160 joists at 0.70 m centres, 20 mm planking, 160 mm soft insulation at σ(10%) 0.03 N/mm², C24 60x40 battens, 30% pitch, 1.70 kN/m² snow. It resolves to 324 screws at Ø7 or 252 at Ø9. That is an illustration from one build-up, not a sizing rule. The number for your roof is your engineer's.

Why does the ETA give two different fax,k values? Because withdrawal depends on the timber. In solid timber, glulam, CLT and solid wood panel, fax,k is 11.7 N/mm² at ρa 350 kg/m³ with characteristic density up to 440 kg/m³. In non-pre-drilled LVL at ρa 500 kg/m³ and characteristic density 410 to 550 kg/m³ it rises to 15.0 N/mm². Pre-drill the LVL and the higher figure no longer applies.

Price shown is per box of 50 pieces. See the table below for codes, lengths and pack prices.