CareHomeDeskCareHomeDeskTwo different things both get called "roofing felt" -- modern breathable underlay and old-style bituminous felt. What each is, how the choice changes roof-void ventilation under BS 5250, and what BS 5534 actually requires of the battens underneath the tiles.
Ask for "roofing felt" today and most merchants and contractors will quote a modern breathable underlay membrane -- a lightweight, spun-bonded synthetic sheet, not the black bituminous felt the word originally described. That older product still exists and is still sold (commonly labelled 'Type 1F' or 'formerly Type 1F', referencing the now-superseded British Standard it was specified to, BS 747), but it has been substantially displaced in new-build and re-roofing work by modern membranes, for reasons that come down to ventilation requirements more than waterproofing performance -- both are covered below. Getting this distinction right matters when reading a quote: 'felt and batten' as a phrase can mean either the traditional product or simply describe the general underlay-plus-batten construction method common to almost all pitched tile and slate roofs, regardless of which underlay is actually specified. If a quote just says 'felt,' ask which product and which standard it's specified to.
Modern roofing underlays are specified to BS EN 13859-1 (the 2010 edition was withdrawn 31 May 2014 and superseded by BS EN 13859-1:2014 -- check which edition a quote references, since the older edition number still circulates informally). The distinction that actually changes how a roof is built is between HR and LR classification. HR (High Resistance) underlay is effectively impermeable to water vapour -- low breathability -- and a roof void built with it needs to be ventilated in essentially the same way as a traditional felt roof, with airflow doing the work of removing moisture vapour that can't pass through the membrane itself. LR (Low Resistance) underlay is vapour-permeable -- 'breathable' -- and allows moisture to diffuse out through the membrane itself, which can reduce, and in some well-sealed-ceiling scenarios remove, the need for additional roof-void ventilation. This is the single most consequential spec decision in a re-roof's underlay choice, because it directly determines how much (if any) eaves and ridge ventilation the roof needs -- see the ventilation section below for the actual figures found in this research.
Traditional black bituminous underslating felt, historically specified as Type 1F to BS 747 ('Reinforced bitumen sheets for roofing'), functions as a non-breathable (effectively HR-equivalent) underlay. BS 747 itself has been superseded -- modern reinforced bitumen membrane products are now specified under the BS EN 13707/13859-2 framework -- but Type 1F-equivalent bituminous felt is still commercially available, sold by roofing merchants as 'Type 1F (BS747)' or 'formerly Type 1F,' indicating a legacy product still in use even though the underlying standard has moved on. It's still specified in some contexts -- this research found retail evidence it's still sold and used for underslating, but could not verify from an authoritative source (BSI, NFRC, Historic England or BRE) a formal policy statement that it's the preferred choice specifically for heritage re-roofs, flat roof verges or valleys, so that should be treated as plausible trade practice rather than a documented rule. What is clear from the ventilation figures below: because it behaves like an HR-classified product, a roof felted with traditional bituminous felt needs the higher level of roof-void ventilation, not the reduced provision available with a modern breathable LR membrane -- this is the practical reason breathable membranes have become the default rather than a performance failing of the traditional product.
BS 5250 ('Management of moisture in buildings') governs roof-void condensation control, and the figures below come from manufacturer technical summaries of the current 2021 edition (Glidevale Protect, IKO, Dreadnought Tiles, Marley -- the BSI standard text itself is paywalled and wasn't accessed directly in this research, so these figures should be cross-checked against the current standard or an NHBC/NFRC technical bulletin before being treated as final for a specific project). For a cold, pitched roof with a large void above horizontal insulation: with an impermeable (HR-type) underlay and a pitch over 10deg, expect roughly 10,000 mm2 of free area per metre run at eaves/low level, plus an additional 5,000 mm2/m at ridge for pitches over 35deg, spans over 10m, or monopitch roofs. With a vapour-permeable (LR) underlay and a well-sealed ceiling, this commonly reduces to around 3,000 mm2/m at eaves with no ridge ventilation required in the base case -- or, with a poorly-sealed ceiling, closer to 7,000 mm2/m at eaves, or 5,000 mm2/m at ridge only as an alternative. Where an LR (breathable) underlay is paired with an air-impermeable outer covering such as profiled metal, at least one manufacturer source specifies additional batten-space ventilation of not less than 25,000 mm2/m at eaves is needed, or the membrane needs to be treated as HR for ventilation purposes regardless of its own classification. One figure worth flagging explicitly as unverified: a commonly quoted rule-of-thumb elsewhere in the industry expresses roof-void ventilation as a ratio of ceiling area (often cited as 1:300) -- this research could not locate that ratio in any primary or independently-citable UK source for pitched-roof soffit ventilation under BS 5250, and it doesn't appear in either of the two manufacturer sources checked, so it should not be used in a UK specification without separately verifying it against the current standard.
Battens compliant with BS 5534 are typically supplied at 25 x 38mm or 25 x 50mm, with a width tolerance of +/-3mm and a minimum thickness that must never fall below 25mm. They must be preservative-treated to Use Class 2 under BS 8417, and are typically slow-grown imported redwood or whitewood, kiln-dried. Compliant battens are graded under BS 5534 and must be indelibly marked with the supplier's name, timber species/origin code, a 'Graded BS 5534' mark, and size -- this marking is a genuine, checkable indicator that the batten on site actually meets the standard, worth asking a contractor to demonstrate rather than assuming. Battens should be fixed to rafters at centres not exceeding 600mm unless a specific structural calculation justifies otherwise, and battens over 72mm deep must be treated as proper structural timber under EN 1995-1-1 rather than standard tiling batten. Where a system is fully supported over sarking boards with a breathable membrane, counter-battens are required underneath the tiling battens to maintain a clear drainage and ventilation path between the membrane and the tiles -- commonly fixed at around 200mm centres with a minimum 25mm airspace, per manufacturer technical guidance for ventilated underlay systems. Batten gauge (the spacing between battens) is set by the specific tile or slate product's headlap requirement, not a single figure across all coverings -- check the manufacturer's own fixing guide for the product being installed.
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