§ 15

Mechanical fastening: fasteners per m² in a wind zone

How tight should I screw the tracks in the edge zone?

Answer: A mechanically fastened roof felt or membrane roof is held down by fasteners in the seams. Each fastener can take a certain calculated force, and the wind uplift in the zone must be distributed to enough of them. The calculator divides the wind uplift by the fastener's load capacity and gives the number per m², the greatest distance between fasteners in the seam at your runner's cover width and the total in the zone. The wind uplift per zone (middle, edge, corner) and load capacity must come from the wind load calculation and the fastener's ETA or pull-off test. The calculator invents none of them.

Formula

Antal pr. m² = vindsug (kN/m²) ÷ bæreevne pr. befæstigelse (kN)

Hver søm bærer en stribe tag på bredde med dækbredden, så antal pr. m søm = antal pr. m² × dækbredde

Største afstand i sømmen = 1 ÷ (antal pr. m søm) = bæreevne ÷ (vindsug × dækbredde)

Work each zone separately: corner and edge zones get the biggest wind suction and thus tightest fastenings

Symbols and units in the formula
Symbol	Meaning	Unit
vindsug	Design wind suction in the zone (from the wind load calculation)	kN/m²
f_rd	Fastening's calculated load-bearing capacity (ETA/manufacturer)	kN/stk
daekbredde	Track coverage width (distance between seams)	m
zoneareal	Zone area	m²
=	Fasteners per m²	stk/m²
=	Greatest distance between fastenings in seam	m
=	Fasteners in the zone	stk
=	Fasteners per running metre nail	stk/m

Validity and source

Source: DS/EN 1991-1-4 (wind suction per zone) + fastening's ETA/manufacturer data

Does not apply when

Both wind suction and load capacity must be CALCULATED values (with partial coefficients), otherwise safety is lacking. The supplier's fastening plan and their requirements for minimum number and maximum distance always take precedence. The calculator does not replace wind load calculation according to DS/EN 1991-1-4.

Worked example

The calculator's own default values inserted. The figures are calculated with exactly the formula the tool uses.

Substituted

Design wind suction in the zone (from the wind load calculation)1.80 kN/m²
Fastening's calculated load-bearing capacity (ETA/manufacturer)0.40 kN/stk
Track coverage width (distance between seams)0.90 m
Zone area40 m²

Giver

Fasteners per m²4.50 stk/m²
Greatest distance between fastenings in seam0.25 m
Fasteners in the zone180 stk
Fasteners per running metre nail4.05 stk/m

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Sådan bruger du den

På et mekanisk fastgjort tag er det befæstigelserne i sømmene, der holder taget nede, og de skal sidde tættest i rand og hjørner. Beregneren regner antal pr. m², største afstand i sømmen og antal i zonen ud fra vindsuget og befæstigelsens bæreevne. Begge tal skal komme fra vindlastberegningen og befæstigelsens ETA.

Step by step

Find det regningsmæssige vindsug for zonen i vindlastberegningen.
Find befæstigelsens regningsmæssige bæreevne i ETA'en eller fra en udtræksprøve på stedet.
Skriv banens dækbredde mellem sømmene og zonens areal.
Læs største afstand i sømmen, og sammenlign med leverandørens fastgørelsesplan.

Typical errors

At bruge karakteristiske værdier i stedet for regningsmæssige. Så mangler sikkerheden.
At bruge midterzonens afstand ud til kanten.
At glemme, at bæreevnen afhænger af underlaget. En skrue i gammelt træ eller tynd stålplade kan holde meget mindre.

Examples from everyday life

Randzone på et erhvervstag

Randzonen har et regningsmæssigt vindsug på 1,8 kN/m², befæstigelsen kan tage 0,4 kN, banerne har 0,9 m dækbredde, og randzonen er 40 m².

Design wind suction in the zone (from the wind load calculation): 1.8 kN/m²
Fastening's calculated load-bearing capacity (ETA/manufacturer): 0.4 kN/stk
Track coverage width (distance between seams): 0.9 m
Zone area: 40 m²

It means: Du skal bruge 4.5 pcs/m², med højst 0.25 m mellem befæstigelserne i sømmen, og 180 pcs i hele randzonen.

Midterzonen på samme tag

I midterzonen er vindsuget 0,9 kN/m², og zonen er 600 m².

Design wind suction in the zone (from the wind load calculation): 0.9 kN/m²
Fastening's calculated load-bearing capacity (ETA/manufacturer): 0.4 kN/stk
Track coverage width (distance between seams): 0.9 m
Zone area: 600 m²

It means: Her er kravet 2.25 pcs/m², og afstanden i sømmen må være 0.49 m. Hele midterzonen kræver 1,350 pcs.

Rule of thumb

Dobbelt vindsug betyder halv afstand i sømmen.

How to check the answer

Antal pr. m² gange bæreevne skal give vindsuget: 4,5 × 0,4 = 1,8 kN/m².