Look into the roof void of a tiled house and you will see roof battens: slim timber laths running level across the rafters, with every tile or slate hung from them. They are out of sight and rarely discussed, yet the whole covering depends on them. If a batten is weak, a tile above it sits loose, slips and lets water in. If the spacing is wrong, the lap between courses shrinks and the roof stops shedding water properly. When a roof is re-covered, new battens are one of the least visible items on a quote and one of the most important. This guide explains what graded and treated battens are, how the spacing, known as the gauge, is worked out, which defects mean old battens cannot go back, and how batten choice links to the underlay below and the tiles above. We keep to practical points: what we look for from the loft and what we find on site.
What roof battens do
A batten has three jobs. It holds the tile or slate in the right position, it carries the weight of the covering down into the rafters, and it holds the underlay in place beneath. Each tile hangs on a nib or is nailed through a hole, and the batten takes that load. In a gale it also resists the uplift on the roof.
Battens run horizontally, from one end of the roof to the other, and are fixed to the rafters with nails. On a roof with a ventilated gap above the underlay, a second set of timbers, called counter-battens, may run up the slope over the rafters first, with the tile battens across them. Counter-battens let air and water move freely beneath the battens and help the underlay drain.
Because the batten is under the covering, nobody sees it until the roof is stripped, and by then a fault may have been growing unseen for years. It lives in a damp, shaded place, with the underlay above it on one side and the loft below it on the other. That is why the timber matters, and why a roof that has leaked for a time often shows batten damage before anyone knows there is a problem.
The layer beneath the battens is the underlay, and the two have to be chosen together. Our guide to the roof underlay and sarking covers that layer in detail.
Graded and treated: what the labels mean
Roofing battens are not ordinary timber. They are sawn softwood that has been graded for strength and treated against rot and insect attack. Both words matter.
Graded
Grading means the batten has been checked, by eye or by machine, for knots, slope of grain and other faults that weaken it. A graded batten carries a mark showing it has been inspected to the relevant standard. A batten with a large knot near its centre can snap under the weight of a person walking on the roof, or under the load of the tiles in wind. For the roofer that is a safety matter as well as a quality one, since a batten that gives way underfoot can send a person through the roof. Graded battens are marked so that a roofer, and a building control officer, can check them on site.
Treated
Treatment means preservative has been forced into the timber. Untreated softwood in a damp roof void rots; treated softwood resists it. The treatment is applied before delivery, and any batten cut on site should have the cut ends brushed with preservative so that the open grain does not draw in moisture.
When you read a quote, look for the words graded and treated. If a quote only says new battens, ask what grade and what treatment. A cheaper timber that is unmarked or untreated can fail early, and it is hidden under the covering where nobody will see it until the next strip. Cutting corners here is false economy, because renewing battens later means taking the whole covering off again.
Why spacing matters: gauge and lap
The distance from one batten to the next is called the gauge. It is not chosen by eye. The figure comes from the length of the tile or slate and the amount each course must overlap the one two below it, the headlap. Get the gauge right and each course covers the joints beneath it. Get it wrong and water finds a straight path to the underlay.
The correct gauge depends on three things: the length of the covering, the pitch of the roof, and the manufacturer's instructions for that product. A shallow roof needs a bigger headlap than a steep one, because water runs more slowly and can be pushed back up by wind. A concrete tile has its own stated gauge, a clay plain tile has a different one, and a natural slate has a third. The numbers are published by the maker, and we work to them rather than to habit.
Two specific errors recur. The first is battens fixed at an even spacing that suits the old covering but not the new one, because the replacement tile is a different length. The second is battens set too wide to save timber, which reduces the headlap and risks leaks in driven rain. Neither shows from the street. Both are avoidable on any pitched roof where the battens are set out from the maker's figures.
Eaves and ridge need special attention. The bottom course is usually doubled, with a tilting fillet or an eaves carrier to lift it so that the courses lie correctly, and the top batten is positioned to leave a clean bed for the ridge. The ridge is covered in our guide to why tiles slip, where batten failure is one of the causes.
When battens can be reused and when they cannot
Roof battens are cheap compared with the labour of fixing them. That sounds as though they should always be replaced, and on a full re-cover they nearly always are. But the question comes up on repairs and part re-roofs, so it helps to know what we check.
Battens can usually stay where:
- the timber is dry, firm and free of rot or insect holes;
- they carry a grade mark or are clearly sound softwood;
- the existing gauge suits the covering going back;
- nail holes have not split the timber along its length.
They must be renewed where:
- the timber is soft, crumbly or stained dark from damp, which means rot;
- there are insect exit holes or frass, the fine dust left by wood-boring beetle;
- a batten has sagged between rafters, or been cracked by foot traffic;
- the gauge no longer matches the new tiles or slates;
- the roof is being re-covered with a different or heavier material;
- nails have rusted through, or the batten has been fixed with too few.
Tapping a batten with a screwdriver and listening is a quick test. Sound timber rings and resists the point. Rotten timber sounds dull and the point sinks in.
Old battens that have been exposed to a leak for years are rarely worth saving. Opening up one or two spots along the eaves and in the valleys gives a fair sample of the timber, because those are the places where water collects first and rot starts first.
Battens, clay tiles and slate
Different coverings put different demands on battens. A clay plain tile roof carries a lot of small tiles, each hung from a batten by two nibs, so there are many battens at a close gauge. They must be straight and flat, because a bowed batten lets tiles rock. A natural slate roof uses fewer, wider-spaced battens, but the slates are nailed through, so the nails need solid timber to bite into.
On a plain tile roof the closeness of the courses means that a single failing batten affects a good many tiles. That is one reason we look hard at battens whenever we repair clay roofs. Our guide to clay plain tile roofs describes how those roofs are put together and repaired.
Lighter coverings need no less care, since uplift in wind is a real force. Where a heavier covering replaces a lighter one, the battens, the rafters and the connections have to carry more, and a structural check applies. Under Planning Portal guidance, moving to a heavier covering, by 15 per cent or more, means building control approval is required, with the structure checked. Details are in the Planning Portal re-roofing guidance.
Fixings: nails, counter-battens and ventilation
A good batten is only as secure as its nails. Roofing nails are galvanised or stainless, long enough to pass through the batten and bite into the rafter. Too short and the batten pulls away in wind. Smooth steel nails rust, and rust in a damp void spreads quickly, so corrosion-resistant fixings are the sensible choice, especially close to the eaves.
Each batten is nailed at every rafter it crosses. Joints between battens are made over a rafter, staggered so that no line of joints runs across the roof, and each end is nailed. That staggering matters because a run of joints in a line is a weak line.
Where counter-battens are used, they lift the battens off the underlay and create a drainage path. Water that gets under the tiles runs down the underlay between the counter-battens and out at the eaves, instead of damming behind a batten. In a roof with insulation between and over the rafters, that gap also supports ventilation.
When you have a quote for new battens, it is fair to ask: what grade, what treatment, what gauge, what fixings, and are counter-battens included? A clear answer to each tells you the roofer has thought about the timber. Vague answers usually mean the timber has not been thought about at all, and that is worth knowing before you sign. A written quote can name the grade, the treatment, the gauge and the nails in a single line each, and nothing about that is hard to do for a roofer who has planned the job. When it is missing, ask for it, and compare the answers across quotes before looking at the totals. The comparison often explains a price gap that the totals alone hide. It also tells you what is being compared when two quotes differ.