Installing counter battens on a roof

Counter battens have three jobs. The first and most important is ventilation. There has to be a gap between the membrane and the roof covering so that air can flow from the eaves up to the ridge. This air layer carries away moisture that passes through the membrane from the loft, and it dries out any water that gets under the covering in driving rain or snow. The height of the counter batten is therefore also the depth of the ventilation gap.

The second job is mechanical. The counter batten presses the membrane against the rafter along its whole length, so wind cannot lift it and it does not tear around the nails. The tiling battens that carry the covering are then fixed on top of the counter battens.

The third job is creating a true plane. Rafters on older roofs, and sometimes on new roofs built from lower-grade timber, are rarely in one line. Counter battens can be packed or trimmed so that the tiling battens, and later the covering, lie straight. If this step is skipped, a wavy roof cannot be corrected later without stripping it.

Why not simply fix the tiling battens straight onto the rafters? Because they would sit directly on the membrane. Water running down the membrane would be stopped by every batten, the membrane under each batten could never dry, and air would have no path to flow. Counter battens lift the tiling battens off the membrane, so water can run freely down to the gutter.

The NUMIQ price list has three related items. Each has its own page, so it helps to know the difference:

• Counter batten installation (this page): only the strips running down the slope along the rafters, fixed through the membrane or through existing boarding. The area is measured in m² of roof. Tiling battens are not included.
• Installing tiling battens for tiles and slates: the horizontal battens fixed across the counter battens, spaced to suit the specific tile, slate or shingle. This covers batten gauge, the eaves and ridge battens and the extra battens at valleys.
• Battening and counter battening of roofs (combined): both layers done by one crew in one continuous run.

Counter battening on its own makes sense in these situations:

• You want the roof closed temporarily with membrane and counter battens as soon as possible, and the covering arrives weeks later, for example because of tile delivery times.
• Full boarding is planned under a metal roof, and the counter battens provide ventilation between membrane and boarding or carry the boarding itself.
• The roofer will fit the tiling battens together with the covering, because they want to set the gauge themselves to suit the exact tile.
• Insulation boards are laid over the rafters (a so-called warm roof), and the counter battens are fixed with long screws through the boards. This is a separate and more demanding job.
• During a renovation the original timber boarding or sarking stays, a new membrane goes on top, and the counter battens are fixed onto it.

If one company is doing your roof from membrane to covering with no waiting time in between, the combined item is usually more practical. The crew does not have to come back, set up and dismantle safety equipment twice, and one person is responsible for the quality of both layers.

Counter battens go on at the end of a particular stage of work. If what lies underneath is not finished, the crew either waits or makes compromises that cause problems later.

What must be ready before the carpenters arrive

• Roof structure checked and repaired. Damaged, rotten or sagging rafters must be replaced or strengthened before the membrane goes on. Once the counter batten is fixed, nobody will see them again.
• Timber treatment. If old roof timbers are being treated against woodworm or fungal decay, the treatment must be finished and dry.
• Breathable membrane. Either it is already laid, or the same crew lays it just before the counter battens. It runs horizontally from the eaves upwards with laps as specified by the manufacturer and should not be left uncovered for long. Most membranes tolerate sunlight only for a limited time, which the manufacturer states.
• Eaves detail. The eaves flashing or eaves support tray that the membrane laps onto should be fitted, so water from the membrane runs into the gutter and not onto the wall.
• Penetrations. Chimneys, soil vent pipes, roof windows and access hatches should at least be set out, so the counter battens can be interrupted around them correctly.
• Scaffold and safety. Scaffolding at the eaves or another fall protection system, access for materials and a dry place to store the timber.

On a block of flats, the work has to be agreed with the building management or owners' association. On a house, check with your local building authority whether replacing the covering, membrane and battens needs notification or approval in your case. Requirements depend on the scope of the work and on local building regulations, and adding insulation in particular can bring extra requirements, so do not assume in advance that nothing needs to be arranged.

How counter battening fits between other trades

Counter battening sits in the middle of the roof build-up. Before it come the carpenters with the roof structure, possibly the insulation fitters with over-rafter boards, and the roofer or leadworker with the eaves detail. After it come the tiling battens, the roof covering, the chimney flashings and the roof windows.

• Counter batten height with the roofer. It changes the height of the eaves, the connection to the gutter and the finished ridge height.
• Roof window positions. Window manufacturers have their own fitting instructions for connecting the membrane and diverting water above the window.
• Loft insulation. If mineral wool is fitted between the rafters from below, it must not push the membrane up into the ventilation gap. The membrane then bulges between the counter battens and reduces the airflow.
• Lightning protection and aerials. Their brackets are often fixed through the covering into the battens and counter battens. It helps to know about them in advance.

Checking the line of the rafters

The carpenter first runs a string line or lays a long straightedge across several rafters to find which ones stand proud and which ones dip. With new planed timber the differences are small. On old roofs you can find rafters that sag by several centimetres. This is where the crew decides which counter battens need packing and which need trimming.

Placing the counter batten and sealing tape

The counter batten goes exactly on the centre line of the rafter, not beside it. On low pitches, when the roof will stay temporarily covered for a longer time, or when the membrane manufacturer requires it, a nail sealing tape is applied under the counter batten or to its underside. When a nail goes through, the tape closes around the shank, so water cannot seep through the holes into the insulation.

Fixing

The counter batten is nailed or screwed into the rafter at regular centres, usually roughly every 30 to 50 cm, depending on the design and the expected loads. The fixing must go deep enough into the rafter. Nails are galvanised, often ring-shank, which hold better. On warm roofs with insulation over the rafters, system screws of the specified length are driven at the angle the manufacturer prescribes.

Joints and levelling

Counter battens are butt-jointed, always over a rafter. Where rafters dip, packers go under the counter batten, or a deeper section is used and trimmed where needed. The carpenter keeps checking the plane with a string line stretched between the outer counter battens.

Eaves, ridge and penetrations

At the eaves the counter battens stop so that the air inlet between membrane and covering stays open. The opening is later protected by an eaves vent strip or comb against birds and insects. At the ridge the counter battens are cut so that air can escape through a ventilated ridge. Around roof windows, chimneys and dormers the counter battens are interrupted with enough of a gap for air to flow round the obstacle, so no closed pocket without ventilation is created.

Valleys and hips

On hipped and complex roofs the counter battens are cut at an angle at hips and valleys. In a valley an extra counter batten or a board is often added to support the valley lining. This is where the crew's experience shows: every cut has to fit, and the membrane in the valley must not be stretched tight like a drum skin.

Timber

Counter battens are made from softwood, typically spruce or pine, usually treated against insects and decay. The timber should be dry, roughly 20 % moisture content or less, straight and free of large loose knots. Wet timber twists and shrinks on the roof and loosens the nails. If the counter battens arrive damp, store them under cover with spacers between the layers so they can dry.

Height and cross-section

On houses, counter battens are usually roughly 40 to 60 mm deep and about 50 to 60 mm wide. A deeper counter batten means a bigger ventilation gap. You need that mainly with long rafters, low pitches, complex roofs with many penetrations and habitable lofts. What counts is what the design and the manufacturers of the covering and membrane specify. The width has to be enough to fix the tiling battens securely.

Fixings and sealing

• Nails: galvanised, plain or ring-shank, of sufficient length. With thin counter battens on slender rafters, screws are often the better choice.
• Timber screws: for deeper counter battens, where packing is used, and in renovations where hammering would loosen old timber or the plaster in the loft below.
• System screws for over-rafter insulation: long screws with certified fixing performance, installed to the manufacturer's layout.
• Nail sealing tape under counter battens: foam or butyl, where the membrane stays uncovered for longer or the pitch is low.

Counter battens on boarding and under metal

On full boarding with a bitumen felt or membrane, counter battens are fixed through the boarding into the rafters, not just into the boards. Under a metal roof on boarding, the counter battens create a ventilated gap. Without it, condensation forms on the underside of the sheets and drips into the loft.

The price per m² on this page is based on an ordinary roof. A specific roof can be simpler or considerably more demanding. These things make the work easier:

• A simple gable roof with long, plain slopes
• New, straight rafters in planed timber
• Safe access with the scaffold already up
• Few penetrations and roof windows
• Membrane already laid and materials ready on site

These things make the work longer and harder:

• Hips, valleys, dormers and other junctions where every counter batten has to be cut
• A steep pitch and a high roof, which need more thorough fall protection
• Old, uneven rafters where packing is needed almost every metre
• Over-rafter insulation with long screws and precise spacing
• Fixing through existing boarding, where the rafter positions have to be found first
• Many penetrations, chimneys and windows
• Bad weather, wind and rain, when work on the roof is impossible or unsafe

Most counter battening mistakes are not visible straight after installation. They show up after the first winter, sometimes only after several years.

• Counter battens too shallow. The ventilation gap is not enough, moisture builds up under the covering, battens go mouldy and stains appear on the plasterboard in the loft.
• Blocked air inlet or outlet. At the eaves the gap gets filled with mortar, metal or a vent strip that is too dense; at the ridge there is no ventilated ridge. Air has nowhere to flow, even if the counter battens are deep enough.
• Counter batten off the rafter line. The nail passes only through membrane and insulation. Over time the counter batten loosens and the membrane is left with holes.
• Short or ungalvanised nails. They rust and work loose, and the covering moves in strong winds.
• Missing sealing tape where it was needed. During a temporary cover, water seeps through the nail holes into the insulation, and nobody notices until a stain appears on the ceiling.
• Continuous counter battens below a roof window. A closed space without ventilation forms above the window, moisture lingers and the timber suffers.
• Damp timber. The counter battens twist, the plane is lost and the tiles show visible waves.
• Rafters not levelled. Looking along the roof at an angle you see dips and humps. The only fix is relaying the covering.
• Membrane damaged during work. Walking on the membrane between rafters, dropped tools, cuts while trimming. Small tears should be taped at once with a tape approved for that membrane.

Fixing counter battens is work at height on a sloping surface, often on a membrane that is slippery when damp. It needs scaffolding or other fall protection, harnesses where appropriate and experience of moving around a roof structure. That is why this page does not describe it as a DIY job. If you want to make things easier for the crew, you can clear the area around the house, provide dry storage for the timber, empty the loft so the work can be checked from below, and agree the dates so the membrane is not left uncovered for long.

How long does it take? On a simple gable roof of a house, a crew of two or three carpenters usually completes the counter battening in one to two days. A hipped roof with dormers, an uneven structure or over-rafter insulation can take several times as long. There is no curing or drying time involved. Weather can still cause delays, because working on a wet membrane is not safe.

Before the carpenters leave and the tiling battens and covering go on, it is worth walking the roof from the scaffold and checking from inside the loft:

• Every counter batten sits on the rafter centre line and is firm, with no movement when pushed
• Nails or screws are at regular centres and are galvanised
• Joints are butt joints over a rafter, without large gaps
• Counter batten height matches the design or the agreement with the roofer
• The roof plane shows no visible waves when checked against a string line
• The air inlet at the eaves is open and the membrane drains into the gutter
• Space for ventilation is left at the ridge
• Counter battens around roof windows, chimneys and dormers are interrupted with a gap for airflow
• Sealing tape is actually fitted wherever it was agreed
• The membrane is not torn, punctured or pushed up into the ventilation gap
• No nails that missed a rafter are visible from the loft
• The timber is dry, treated and not twisting
• Waste, offcuts and nails are cleared from the roof and around the house