Rooftop antenna poles: structure and fixing to the roof

How a roof differs from the ground

In a tower on the ground the load is taken by the foundation and, below it, the soil. The foundation is sized for the pole: the wind moment, vertical force and shear are known, and the soil has been studied by a survey (see Telecom tower design).

On a roof the building plays the part of the foundation. It already stands, its structures are designed for their own weight, snow, roof equipment and people, and there may be no reserve for a new pole. So the order is reversed: first find out what the roof can carry, then choose the pole to suit, not the other way round.

There is one more difference. Wind pressure grows with height above the ground, and the top of a pole on a tall building stands in a stronger flow than the same pole at ground level. At the edge and corner of a roof the flow is in places accelerated as well. So the moment at the base of a rooftop pole can be larger than its own height suggests.

What a rooftop pole consists of

The make-up is the same as for a tower on the ground (see Types of telecom towers): a shaft of sections, a base plate at the bottom, a platform, a ladder, antenna brackets and a lightning rod. The difference is that every element weighs on the roof, so no extra steel is added.

Which loads go into the calculation

The loads are the same as for a tower on the ground: wind on the shaft and on the antennas, ice, the weight of the pole and equipment, plus people on the platform and erection. Wind acts from any side, and ice adds weight on the antennas, cables and brackets and enlarges the area the wind presses on. For antenna and mast structures DBN requires ice-and-wind loading to be taken into account, with more in Height of a mast or telecom tower.

The calculation needs input data: the wind and ice regions, the terrain type, the height of the top of the equipment above the ground (not above the roof), the wind area of the equipment and its mass.

The result of the calculation is three values at the base of the pole for each load combination. The vertical force compresses the roof under the plate. The shear tends to slide the plate along the roof. The wind bending moment compresses the roof under one edge of the plate and pulls the anchors out at the opposite one, and since the wind changes direction, the anchors are calculated all the way round. These three values are what is passed to the building designer.

How the pole is fixed to the roof

The method is chosen by the roof construction and by whether the slab may be drilled. Below are four main options; in a particular project they may be combined.

Where the pole passes through the roofing

On light roofs of steel profiled deck the load is not passed to the deck itself: it is brought to the purlins, trusses and columns. If the base plate lies on the deck, an auxiliary frame is put under it and that frame rests on the structural members.

Where the shaft or frame crosses the waterproofing, the detail is agreed with the roof design: the plate is raised on a curb or a stand and the waterproofing is turned up onto it. The detail has to be made so that the waterproofing can be restored without removing the pole.

What the building designer checks

The support reactions of the pole are input data for checking the building, not its result. The building designer looks at whether the structures will carry the new load, and usually checks the following:

• the roof slab: overall bending from the new point load and local effects under the base plate, that is, anchor pull-out and punching;
• the columns and walls that carry the roof: the increase in axial forces from the moment passed down;
• the foundations and the soil under them: extra pressure and uneven settlement, especially in old buildings;
• rotation and deflection of the roof: the displacement of the pole tip is made up of its own deflection and the rotation of its base, and the permissible value is set by the antenna maker;
• the actual condition of the structures: corrosion of the reinforcement, cracks, wear, because the calculation follows what is there and not what the original design said.

We prepare the KM and KMD drawings of the pole and its base detail: KM and KMD design, and we make the pole, plates and frames: steel fabrication. Checking and, if needed, strengthening the structures of the building is done by the building designer, and we pass the loads at the base for all combinations. The condition of the roof and the supporting structures is assessed by a technical survey, which is a separate service. Erection is done by our partner construction company.

Lightning protection and earthing

A rooftop pole is usually the highest point of the building, so lightning strikes it first. The building may already have lightning protection, and the new pole changes the picture: its shaft or a separate down conductor is connected to the building lightning protection system and its earth termination. The path the current takes from the tip to the ground, and how potentials are equalised between the pole, the building structures and the screens of the antenna cables, is set by the lightning protection design.

What the pole structure itself does, the shaft as a down conductor, the section joints and the rod at the top, is described in Lightning protection of a telecom tower; earthing is covered in Pole earthing, and a steel mast as a lightning rod in Steel mast as a lightning rod and down conductor.

An anchor fixing on an uneven slab, under a layer of zinc or paint, should not be taken as a reliable electrical connection without a check. It is better to provide a separate bond with a resistance test.

Galvanizing and corrosion protection

The pole, base plate, frame and fixings on a roof are usually protected by hot-dip galvanizing: hot-dip galvanizing. On a roof it is harder to look after the coating later, so the zinc goes on before erection.

There are two roof-specific points. Water stands near the base plate, so the detail is designed to let it run off and not lie on the flange. And where galvanised steel touches the roofing or other metals, the galvanic couple is checked: for example, zinc in contact with copper is a corrosion risk. A place where the zinc has been damaged by drilling or welding is repaired on site.

What to send for the calculation

To select the pole and the fixing you will need:

• the building address (for the wind and ice regions), the building height and the height of the pole top above the roof;
• a list of antennas: number, sizes, mass, mounting height;
• drawings or sketches of the roof: the type of slab or deck, thickness, column and purlin spacing, the place where the pole can stand;
• the results of a roof survey, if one was done;
• the way sections can be lifted onto the roof, and whether the roof may be drilled.

Leave blank what you do not have: we will clarify it as we go.

Standards and validity

Decisions are checked against the current editions of these documents (as amended): loads and actions under DBN V.1.2-2:2006 «Loads and actions. Design standards», steel structures under DBN V.2.6-198:2014 «Steel structures. Design standards», the reinforced concrete structures of the roof and the anchor detail in concrete under DBN V.2.6-98:2009 «Concrete and reinforced concrete structures. Basic provisions», reliability and structural safety under DBN V.1.2-14:2018 «General principles of reliability and structural safety of buildings and structures», lightning protection under DSTU EN 62305-3:2021.