How to install steel sheets.
- What is a steel deck?
- How do you fix steel sheets to the roof at the top of a wave?
- How to calculate the number of fixings on a steel deck roof?
- How to calculate the number of fasteners on a steel deck roof
- Seaming at overlaps
What is a steel pan?
Steel sheets are widely used for roofing and cladding. Their description, precise application, and conditions of use are described in the DTU40.35 . The aim here is to explain how a steel deck roof can be fixed.
Steel sheets are ribbed sheets made of coated steel. There are also aluminium sheets.
They are also called “ribbed profiles” or “ribbed steel sheets” (TAN).
– Ribs or waves: trapezoids consisting of two webs and a rib top. We have complete main ribs and two edge ribs
– The pitch or modulus: distance between two ribs
– Ranges: parts between two ribs. It is in the beaches that the majority of the water flow is located during rainy weather.
– The useful width of the pan (1m)
– Rib height: height of the trapezoids (h)
– Longitudinal deck stiffeners
The steel sheets are generally fixed at the top of the rib (sum of the waves), and this on all types of support (wood, steel…).
However, it is possible to fix them to the deck under certain conditions, and only in the case of steel purlins. In any case, it is more delicate to fix the steel panels on the deck, since this is where we have the greatest amount of water run-off during rainy weather. For deck installation, some experience is required and there will always be a greater risk of seepage (leakage) with deck fixings than with ridge fixings.
The top of wave fixing method (at the top of the rib) is therefore the safest way to avoid possible infiltration, which is why it is the most widespread fixing method in France. We will therefore only talk here about fixing at the top of the rib.
Shape of a rib of a steel sheet :
On the figure above we can see :
- the top of the rib
- the small base (a) at the top of the rib
- the large base (b)
- the height of the rib (h)
- The core
These dimensions are specific to a steel pan reference. Thus, if one knows the type of steel sheet and its brand, one can find the dimensions (a), (b) and (h) in the profiler catalogs. Conversely, if you do not know the type of a panel, you can find its designation with the measured (a), (b) and (h) dimensions.
How are steel sheets fixed to the roof at the top of the corrugation?
The steel sheets are placed on supports, which we will specify here:
At the top of the roof we have the “ridge”, with the “ridge purlins”. See green on the diagram
At the bottom of the roof (bottom of the slope), we have the eaves (eaves). See in light blue on the diagram
On the sides of the roof (sides of the slope) we have the eaves. See orange on the diagram
On the slope of the roof we have the “purlins”, which are arranged perpendicular to the slope of the roof. See red on the diagram. The purlins are the “supports” for the steel sheets. The distance between purlins is the “span” of the steel sheets.
In roofing, the steel sheets are positioned on the “purlins”, in the direction of the roof slope (the steel sheets are laid lengthwise according to the roof slope).
The rib lines of the panels are in the direction of the slope. It is known that water will run off the roof following the slope. See white arrow on the purple steel pan in the diagram
There are therefore different types of purlins, supporting the steel sheets:
- Wooden purlins (e.g. pine rafters)
- Thick steel purlins (beams, IPE merchant bars, IPN…)
- Steel purlins with a thickness of 1.5 mm to 5 mm.
- Concrete purlins with steel insert.
It should be noted that the minimum thickness of steel purlins supporting steel roofing sheets is 1.5 mm.
There are several ways to fix the steel sheets:
- by screwing: using self-drilling or self-tapping screws (roofing screws)
- by screwing: using screw-in lag screws (roof lag screws)
- by hammering and screwing: use of tapping screws,
- by using roof hooks.
These fixings must comply with DTU 40
The use of hooks (also called bolt-hooks) is a traditional method that is simple to understand. It is necessary to pre-drill the top of the rib of the steel sheets. The hook must be shaped to fit the purlin and must protrude from the sheet (D=70mm above the purlin, thus allowing it to protrude above the rib of the pan). The hooks are tightened with a nut. The hooks should be hot-dip galvanised for durability.
There are two types of roof lag screws, screw-in lag screws and tapping lag screws.
Screw-in lag bolts are screwed into the wood with a rotating tool (screwdriver or spanner or crankshaft).
Hammer-in lag bolts are mainly driven in with a hammer (by hammering), and the final tightening is done with a spanner (screwing in the last centimetres).
The use of self-drilling screws is the most modern method, and saves time by direct installation without a pilot hole, especially for professionals. For good durability, self-drilling screws have an aluminium head (TETALU screws), which is a stainless material (normal atmosphere). The head of the screws can also be painted (lacquered) to match the colour of the steel sheet (usually in colours following the RAL code). These screws should be installed with suitable electric screwdrivers, equipped with a depth stop.
The type of self-drilling screw depends on the support (type and material of the purlin):
- for wooden purlins, the screw to use is the TETALU P1 screw,
- for steel purlins from 5 mm to 13 mm thick, the TETALU P13 screw,
- for steel purlins with a thickness of 1.5 mm to 5 mm, the TETALU P5 screw
Fixing on steel purlins with a thickness of 1.5 mm to 5 mm:
These purlins are of the cold-formed type (Z, C or omega purlins made by profiling or bending). The appropriate screw is the TETALU P5 6.3×75 screw.
Fixing on steel purlins of thickness 5 mm to 13 mm:
These purlins are IPN, IPE, HEA… which are called hot rolled or beams. The appropriate screw is the TETALU P13 5.5×80
In certain more corrosive environments (seaside, polluted atmosphere, or chemical environment…), stainless steel fasteners should be used (austenitic stainless steel A2 or A4 depending on the case). Thus, it is advisable to choose stainless steel hooks, lag bolts and self-drilling screws.
STAINLESS STEEL SCREWS
A rider adapted to the shape of the rib (trapezoidal shape) and an EPDM neoprene washer, placed under the rider, should be associated with these fixings.
The riders can be made of galvanised steel for galvanised steel sheets, or of pre-painted steel for pre-painted coloured steel sheets. In this case, they have the same colour as the steel sheets.
The shape of the bracket must be adapted to the rib of the steel sheet for a good hold. When tightening the fastener, the use of a rider will avoid deformation or punching of the pan thanks to the distribution of pressure over the entire surface in contact between the underside of the rider and the top of the rib. They are mandatory according to the DTU 40.35.
See the extract from the DTU for 63/100 and 75/100 trays.
The straddles are therefore linked to the type of steel sheet as they adapt to the dimensions (a), (b) and (h) of the steel sheet ribs. If you do not know the type of pan, you can measure (a), (b) and (h) and find the appropriate rider. If the type of steel pan is known, a jumper is taken for this pan.
At the level of the ridge, the steel pan is covered by shaped finishing accessories (ridge strips, single slope ridge, double slope ridge with cut edges, hinged half-ridge with flange…). These finishing accessories are flat folded sheets, which are no longer compatible with the riders. The same applies to the edge strips. In this case, it is necessary to replace the straddles with bump washers.
Sealing at the fastener is achieved with EPDM neoprene sealing washers. These washers have an external diameter of 20 mm and a thickness of 3 mm. The diameter of the hole is chosen according to the shank of the fastener so that the mounted washer is tight on the smooth part of the screw. Thus, for a 6.3 mm screw, the hole for the washer will be equal to 5 mm to tighten on the screw shank.
How to calculate the number of fixings on a steel roof?
Here is the minimum distribution of fasteners at the top of the wave according to DTU 40.35
Simplified method and example:
We have seen that at the ridge there are humpback washers instead of straddles (ridge fasteners made from flat sheets). On the part shown in green, we will have bump washers at the ridge purlins. For the other purlins in red on the rest of the roof, the fixings will be with jumpers.
1/ see the number of purlins on the slope and on the building.
2/ calculate the cumulative length in linear metres of the purlins.
3/ see the number of ribs in the pan.
4/ calculate the number of screws
5/ calculate the number of boss washers (ridge)
6/ calculate the number of straddles
Example: Building of 1732 m² with a 3x333x39 steel sheet roof
Building 100 m long with two identical slopes. Width 17.32 m and 30° angle
10 m slope, purlins every 1.5 m
1/ number of purlins per slope :
10 m/1.5 m = 6.666 or 7 purlins.
Therefore, if two identical sides, there are 14 purlins.
2/ there are 14 purlins and the building is 100 metres long.
There are therefore a total of : 14 purlins x 100 metres = 1400 mL
3/ steel pan 3x333x39: there are 3 ribs / mL
4/ number of screws = cumulative length of purlins x number of ribs / mL
i.e. 1,400 mL x 3 = 4,200 screws
5/ number of screws at the ridge = 2 purlins x 3 ribs/mL x 100 mL = 600 bump washers
6 / number of straddles = 4200 – 600 = 3600 straddles
Conclusion: 4,200 screws, 3,600 straddles, 600 bump washers and 4,200 neoprene washers are required.
Sewing to the overlaps
An essential accessory for the correct installation of steel sheets is the seam screw, especially at the longitudinal overlaps.
The special feature of the seam screws for steel sheets is that they have small drill points, which provide good resistance to tearing in thin profiles (63/100 or 75/100 sheets, etc.)
Position of the seam screws on the rib according to DTU 40.35:
The main fastener (screw + jumper + washer) is considered as a seam screw.
For slopes > 10%, seam screws are required for purlin spacings greater than 2 metres, and if the purlin spacing exceeds 3.5 m, they are required every metre.
For small slopes (<10%), seam screws should always be used.
A seam screw should be placed in the middle of two purlins if the space between purlins is less than 2 metres and every metre above 2 metres.
Example: building with a 10% slope / 3x333x39 steel sheet / 1.7 m between purlins and 5 purlins per side
Building length: 50 metres with 2 slopes.
– The table says L/2 (you have to sew between two purlins)
– 5 purlins: there are 4 spaces between the purlins.
– One sheet to be laid every meter
– Number of sewing screws per side: 4 x 50 metres = 200 screws per side
– There are two sides, i.e. 2 x 200 = 400 seam screws