Battens And Laths, And Why Old Roofs Sag

Key takeaways

• Battens are a graded structural product with defined dimensions, not any convenient timber.
• Their spacing, called the gauge, is set by the covering and the lap required, not chosen on site.
• They rot from water that has already got past the covering, so failure follows a leak rather than causing one.
• Relaying a covering onto old battens is the most consequential shortcut in roofing.
• Their condition can be checked from the loft by pressing on them where visible.

What a batten is doing

Battens are horizontal timbers nailed across the rafters, over the underlay, and the covering hangs from or is nailed to them. That makes them structural in the ordinary sense: they carry the dead load of every tile on the roof plus whatever wind and snow add to it, spanning between rafters, and they take the withdrawal load on every nail when wind tries to lift a tile. A roof covering is only ever as secure as the timber its fixings are in, which is why the specification of this apparently minor component is defined rather than left to whatever is on the van.

The gauge, which is arithmetic rather than judgement

The spacing between battens is called the gauge, and it is derived rather than chosen. It follows from the length of the tile or slate and the headlap required, which is the amount each course must overlap the one below to keep water out. Change the gauge and you change the lap, and a covering laid at too great a gauge has insufficient overlap and will admit driven rain while looking entirely conventional from the ground. This is one of the reasons a covering cannot simply be swapped for a different size of tile without setting the battens out again.

Graded, and why it is stamped

Battens used for roofing are supplied to a specification covering dimensions, species, permitted knots and moisture content, and graded material is marked along its length so it can be identified after installation. That marking matters because once the roof is finished nobody can see the timber, and it is the only way to establish afterwards that the correct product was used. Ungraded timber of the same nominal size may be weaker, more prone to splitting when nailed, and less dimensionally consistent, which affects the line of the courses as well as the strength.

How they fail

Battens do not usually fail from load. They fail from water. Where the covering has been letting water past for years, or where the underlay has perished so water reaches the timber, the batten stays damp and decays, losing the strength to hold a nail. The pattern is telling: decay concentrates near the eaves where water accumulates, beneath valleys and around chimneys where junctions leak, and in bays where the underlay sags and holds water. It is the consequence of a long standing defect rather than the cause of one, which is why finding rotten battens usually means the roof has been letting water in for a considerable time.

The symptom you actually see

Failing battens announce themselves through the covering rather than directly. A course line that has developed a wave in it, rather than running straight, indicates the timber behind has dropped or the fixings have lost their hold. Tiles slipping in a cluster in one area, rather than scattered across the roof, points at a local batten problem rather than a general fixing one. And a tile that will not stay put after being refixed usually means the nail has nothing sound to bite into. That distinction between clustered and scattered failure is the most useful diagnostic a homeowner can apply.

Checking them yourself

From the loft hatch or from boarded flooring, the battens are visible between the rafters wherever there is no underlay obscuring them, and on older roofs with perished felt you can often see a good deal of them. Where you can safely reach one, press it firmly with a thumb: sound timber resists completely and softened timber gives. Look for dark staining, which marks where water has been running, and for splitting along the grain around nail positions. Do not walk out across the joists to reach further ones, and treat what you can see near the hatch as a sample rather than a survey.

The shortcut that matters most

When a roof is recovered, stripping and replacing the battens is a significant part of the labour, and reusing the existing ones is the single most consequential corner that gets cut. The result looks identical: a new covering, correctly laid, on timber that is at the end of its life. Fixings driven into softened battens hold for a while and then let go, and the roof begins losing tiles within a few years of work that was supposed to last decades. Because it is entirely invisible once finished, it is also the omission least likely to be discovered until it fails.

Counter battens, and when they appear

On some roofs you will find a second set of timbers running vertically up the slope beneath the horizontal battens. These are counter battens, and they create a drainage and ventilation gap between the underlay and the covering so that water on the membrane can run freely to the eaves and air can move. They are standard on some constructions and on roofs at lower pitches, and they are one of the details that distinguishes a considered build up from a minimal one. Their absence is not a fault on a conventional roof, and their presence is a reasonable sign of a properly specified one.

What to ask when a roof is being recovered

Three questions. Are the battens being renewed, or reused, and on what basis was that decided. What grade and size will be used. And what happens if, once the covering is off, the rafters are found to be uneven or the existing gauge is wrong for the new tiles. The answers are quick for anybody who has planned the job and vague for anybody who has priced it by rule of thumb. It is also worth asking for a photograph of the battens in place before the covering goes back on, since that is the only opportunity anybody will ever have to see them.

Why it is worth the extra

New battens are inexpensive as a material; the cost is in the labour of stripping and fixing them, which is happening anyway if the covering is coming off. Set against that, the consequence of reusing failed timber is the whole job again within a decade, including the access, which is usually the largest single item. It is the clearest example in roofing of a saving that is really a deferral, and it is the reason a quotation that does not mention battens should be assumed not to include them until somebody confirms otherwise.

The connection to everything else

Batten condition sits at the centre of most roofing decisions without being visible in any of them. It determines whether refixing a slipped tile will hold. It is the difference between a covering that can be relaid and one that cannot. It is what makes an old roof with sound slates still worth stripping and relaying rather than simply repairing. And it is the reason a proper assessment involves somebody in the loft rather than only on a ladder, because there is simply no way to judge from outside whether the timber that holds the entire covering on is still sound enough to keep doing it, and that judgement changes what every other finding on the roof actually means.

Fixing them, and the detail nobody sees

How a batten is nailed matters as much as the timber itself. Fixings go into the rafter rather than merely into the underlay, at every crossing rather than intermittently, and joints between lengths must be made over a rafter with adjacent joints staggered rather than falling in the same bay. A batten jointed mid span has nothing supporting it at the join and will deflect under load, producing the wavy course line that indicates trouble from the ground. All of this is standard practice and all of it is invisible once the covering is on, which is another argument for photographs taken during a recover while the battens are exposed.

Why the covering cannot always go back on the same gauge

One situation catches people out during a recover. If the new covering is a different size from the old one, the batten spacing has to change, which means the existing battens cannot simply be reused even if they are sound. It also means the number of courses across the slope changes, and the setting out at the eaves and the ridge has to be worked out again so the courses land correctly at both ends rather than leaving an awkward part course at the top. This is ordinary work for somebody who does it regularly and it is a real constraint, and it is another reason changing tile type is a bigger decision than choosing a colour.

Frequently Asked Questions

What are roof battens?

Horizontal timbers nailed across the rafters, over the underlay, from which every tile or slate hangs and into which every fixing is driven. They carry the whole weight of the covering and resist wind uplift on each unit, which makes them structural despite their modest size.

What is the gauge?

The spacing between battens, derived from the length of the tile or slate and the headlap required rather than chosen on site. Change the gauge and you change the overlap, and insufficient overlap produces a roof that looks conventional and admits driven rain.

Why do battens rot?

From water that has already got past the covering, or that reaches them because the underlay has perished. Decay concentrates near the eaves, beneath valleys and around chimneys where junctions leak. It is a consequence of a long standing defect rather than a cause of one.

How do I know if my battens have failed?

The covering tells you. A course line that has developed a wave, or tiles slipping in a cluster in one area rather than scattered across the roof, both point at the timber behind. A tile that will not stay put after refixing usually means the nail has nothing sound to bite into.

Can old battens be reused when a roof is recovered?

They should not be where they have decayed, and reusing them is the most consequential shortcut in roofing. The finished job looks identical, the fixings hold for a while, and the roof starts losing tiles within a few years of work that was meant to last decades. It is entirely invisible once complete.

Does it matter what timber is used?

Yes. Roofing battens are supplied to a specification covering dimensions, species, knots and moisture content, and graded material is marked along its length. That marking is the only way to establish after installation that the correct product was used, since nothing is visible once the covering is on.

What are counter battens?

A second set of timbers running vertically up the slope beneath the horizontal battens, creating a drainage and ventilation gap between the underlay and the covering. They are standard on some constructions and at lower pitches, and their presence indicates a considered build up.

Can I check the battens myself?

Partly, from the loft hatch. Where they are visible and safely reachable, press firmly with a thumb: sound timber resists and softened timber gives. Look for dark staining and splitting around nail positions. Treat what you can see near the hatch as a sample rather than a survey, and do not crawl out across the joists.