uPVC Fascia Boards: Capping vs Full Replacement, Sizes, and What to Buy

The roofer fixes the tiles, the guttering goes up, and a year later the gutter is sagging at one end with a tide mark down the wall behind it. The fascia behind that gutter was a thin capping board screwed over rotten timber, or it had no support where the bracket needed to bite, and the weight of a full gutter in heavy rain pulled it loose. The fascia is the one roofline board that carries a real load, and getting the spec wrong is the difference between a roofline you forget about and one you revisit with a ladder every winter.

What it is and what it's for

The fascia is the board fixed across the ends of the roof rafters along the eaves (the lower edge of the roof, where it overhangs the wall). It does two jobs. It closes off the cut ends of the rafters so weather, birds, and insects can't get into the roof void, and it provides the flat vertical face that the guttering screws to. Everything the gutter collects, every litre of rain off the roof, hangs off the fascia.

On a new extension the fascia is almost always white or anthracite (a dark near-black grey) uPVC, the same rigid plastic used for window frames. uPVC replaced timber on roofline work because it doesn't rot, doesn't need painting, and shrugs off thirty years of UK weather with a wipe-down. The trade name you'll hear is "roofline", which covers fascia, soffit, and bargeboard as a set.

A fascia board is not a structural roof component in the way a rafter or a wall plate is. It carries the gutter and closes the eaves, nothing more. But that gutter load is real, and the way the board is fixed and supported decides whether it holds.

Fascia, sub-fascia, and tilt fillet

Three things sit at the rafter feet and homeowners often muddle them. The fascia is the visible front board. A sub-fascia is a timber board fixed to the rafter ends behind the uPVC, giving a continuous solid line for the fascia and gutter brackets to fix into rather than relying on each rafter end individually. Many full-replacement jobs add a treated timber sub-fascia precisely so the gutter brackets always land on something solid.

The tilt fillet is a thin angled batten along the bottom edge of the roof. It lifts the first course of tiles or the felt slightly so water runs cleanly off the roof edge into the gutter rather than tracking back behind it. On a felt roof the tilt fillet also kicks the felt or membrane up over the back edge of the fascia so rain is thrown forward into the gutter, not down the back of the board. None of this is visible once the gutter is on, which is why it gets skipped on cheap jobs.

How the fascia meets the roof felt and the eaves drip

The fascia is where the roof covering hands its water over to the guttering, and that handover has to be detailed properly or rain runs behind the gutter and down the wall. The roof underlay, whether old bitumen felt or a modern breathable roof membrane, is dressed over the back of the fascia and into the gutter. On its own, soft membrane sagging into a gutter holds water and rots, so most jobs now include an eaves drip or an eaves support tray, a rigid plastic tray that sits over the fascia top edge and carries the membrane out over the gutter lip.

Get this wrong and you get a slow leak that never shows on the roof but stains the wall behind the gutter every time it rains hard. When you inspect a finished eaves from the ground, the membrane or drip should clearly overhang into the gutter, not stop short behind it. If you can see daylight or felt tucked behind the gutter back rather than draped into it, the detail is wrong no matter how tidy the uPVC looks.

Capping over timber vs full replacement

There are two completely different ways uPVC fascia is fitted, and they are not interchangeable. Knowing which one your job needs is the single most useful thing on this page.

Capping board (9mm). A thinner board fixed over the existing timber fascia, which stays in place. The timber does the structural work of holding the gutter; the uPVC is a maintenance-free face that hides the old paint-flaking timber. This is the cheaper, faster option and it's common on retrofit work where the original timber is sound. The catch: if the timber behind it is soft or rotten, the capping hides the problem and the gutter fixings have nothing solid to grip.

Full replacement board (16mm). A thicker, self-supporting board that replaces the timber fascia entirely. The old timber comes off, the rafter ends are checked, and the 16mm uPVC is fixed straight to the rafter feet. This is the correct spec for a new extension where there's no existing timber, and the honest choice on a re-fascia job where the old timber is past saving. The board is rigid enough to span between rafters and take the gutter load on its own.

What to do with old timber found at strip-off

On a re-fascia job the surprises appear once the old board comes off. Rafter ends that have sat behind a leaking gutter for years are often soft, splayed, or rotted back several inches. A good roofer will not just bolt new uPVC over the damage. The rotten ends get cut back to sound timber and a new timber splice or a fresh sub-fascia is fixed to give the uPVC and gutter brackets a solid line to bite into. Expect this to add timber and a little labour to a quote, and treat a roofer who refuses to look behind the old board as a warning sign. The whole point of replacing timber fascia with uPVC is to stop the rot cycle, and that only works if the timber it fixes to is sound.

Sizes, thicknesses, and colour

uPVC fascia comes in standard 5m lengths. The two variables that matter are thickness and depth.

Thickness is 9mm for capping and 16mm for full replacement, as above. There is also an 18mm and 20mm option from some manufacturers for deeper boards or higher gutter loads, but 16mm covers virtually all domestic extension work.

Depth (the height of the board, eaves to bottom edge) runs from 150mm up to 300mm in 25mm steps, with 175mm, 200mm, 225mm, and 250mm being the common sizes. The depth you need is whatever matches the existing roofline on the house, or whatever covers the rafter ends plus the gutter on a new build. Measure the old board before you order. A board that's too shallow leaves a gap above the gutter; too deep and it looks heavy and won't sit right against the wall.

Colour. White is the default and the cheapest. Anthracite grey (RAL 7016) is now hugely popular on extensions to match modern grey window frames, and woodgrain foils (rosewood, golden oak, black ash) exist for period or timber-effect schemes. The coloured and foiled boards are a printed or coloured layer over the white core, and they cost roughly 30 to 50% more than plain white.

How it's fixed, and why support matters

You won't be fitting this yourself unless you're confident on a scaffold or tower at eaves height, but understanding how it goes on lets you check the roofer's work.

The board is fixed to the rafter feet (the cut ends of the rafters) or, on a replacement, to a continuous timber sub-fascia or tilt fillet. Fixings are polytop nails or stainless steel screws in a matching colour, driven through the board into solid timber at every rafter, typically at 500-600mm centres, never more than 600mm apart. uPVC expands and contracts with temperature, so fixings sit in the centre of the board, not hard at the ends, to let it move without buckling.

The critical point: the gutter brackets must land on solid backing. Fixing a gutter bracket into a 9mm capping board with no timber behind it, or into the gap between two rafters on a 16mm board, gives the bracket nothing to bite. Under a full gutter the screw works loose and the run sags.

Thermal movement and joint detail

A 5m length of white uPVC can grow several millimetres on a hot day and shrink back on a cold night. That sounds trivial until it has nowhere to go. Pinned hard at both ends with no allowance, a board bows out in summer and the joints gap in winter. This is why proper jointing uses a joint trim, a slim cover strip that bridges the gap between two board ends and lets each board slide a little inside it. The board ends sit with a small expansion gap behind the trim rather than butted tight.

Dark boards move more than white because they get hotter in the sun, so anthracite and black-ash foils make this detail more important, not less. A run of fascia that pops its joints or shows a wavy bottom edge after the first warm summer was fitted with no expansion allowance. From the ground, a good run looks dead straight along the bottom edge with neat covered joints; a bad one ripples.

Corners, box ends, and the bargeboard junction

Fascia rarely runs in a single straight line. Where it turns a corner or stops at a gable, dedicated trims keep it weatherproof and tidy.

Box ends close the open end of a fascia run, for example where a flat-roof extension fascia meets a wall. A box end caps the cut end so weather and wildlife can't get behind the board.

External and internal corners handle changes of direction where the eaves wrap around the building. External corners turn outward around a projecting corner; internal corners tuck into a re-entrant angle. Both come as moulded uPVC trims that match the board colour and avoid a raw mitred joint that would gap and let water in.

The bargeboard junction is the corner at the gable where the level eaves fascia meets the sloping bargeboard running up the gable verge. This is a fiddly mitre and a common spot for a sloppy finish: a wide gap, a smear of sealant, or a bargeboard that doesn't line up with the fascia depth. On an extension with a gable end, look hard at this junction. A clean job has the bargeboard and fascia meeting in a tight mitre with matching colour and depth, no daylight, and no fat bead of silicone doing the job a proper cut should do.

Eaves ventilation

The fascia, soffit, and gutter together close off the eaves, and that closed eaves needs a path for air to move into the roof void to stop condensation building up in the loft or rafter zone. That ventilation is handled at the soffit, usually with a ventilated soffit board or a continuous vent strip, and sometimes with an over-fascia vent that sits behind the top edge of the fascia.

The over-fascia vent matters most where the roof has no eaves soffit overhang to ventilate through, or where the membrane is dressed tight over the wall plate and would otherwise choke the air path. It is a slotted plastic strip fixed along the top of the fascia, under the first tile course, drawing air in over the top of the fascia and into the roof at the eaves. On a cold-roof loft this airflow is what keeps the timbers dry. The fascia itself doesn't ventilate, but if it's set too high or packed out wrongly it can block the air path, and an over-fascia vent restores it. The detail to get right lives on the soffit side, covered in the ventilated soffit page.

How much do you need

Measure the total length of eaves the fascia runs along (both sides of a typical extension), plus any bargeboard runs up the gable if you're matching them. Divide by 5m to get the number of standard lengths, and round up.

Worked example: a single-storey rear extension 5m wide with eaves on the front and back gives two 5m runs, so 10m of fascia. That's two 5m lengths exactly, but joints in the middle of a long run look poor, so for a clean front face you'd want each run in one length where the span allows. A gable end with the roof pitched at, say, 35 degrees over a 5m span adds two sloping bargeboard runs of roughly 3m each, so a further 6m, call it two more lengths once cutting waste is allowed. That takes a modest extension from two lengths to four or five before fittings.

Then count the fittings against the layout: a box end for each open end, an external or internal corner at each change of direction, a joint trim wherever two board ends meet, and one bargeboard-to-fascia mitre per gable corner. Allow 10% on the board count for cuts and the odd damaged length. Order all fittings (joint trims, corners, ventilated soffit, fixings) at the same time so the whole roofline arrives together.

Cost and where to buy

White uPVC fascia is cheap. A 5m length of 9mm capping and a 16mm full-replacement length both sit in the modest per-length ranges shown in the table above, and anthracite and woodgrain foils add 30 to 50%. The bigger cost on a re-fascia job is the labour and access, not the board: scaffold or tower hire and a roofer's day rate dwarf the material cost.

Roofing Superstore, Roofline Direct, and JJ Roofing Supplies are the online specialists with the full range of depths, colours, and matching fittings, usually beating the sheds on foiled boards. Wickes and Travis Perkins carry white 9mm and 16mm board plus basic fittings; Screwfix and Toolstation are better for the fixings (polytop nails, stainless screws) and trims than for the long boards themselves. Buy the boards, soffit, and all the trims from one supplier in one order so the whites and foils come from a matched batch.

What a good quote includes

A re-fascia quote that only lists "supply and fit new uPVC fascia and gutter" hides where the corners get cut. A quote worth trusting names the board thickness (16mm, not just "uPVC"), confirms the old timber comes off and rafter ends are checked, allows for new timber where ends are rotten, includes the eaves membrane or drip detail, lists the access (scaffold or tower) as a separate line, and itemises the fittings and matching colour. A quote that promises to cap over existing timber on a tired roofline, with no mention of inspecting what's behind it, is the cheap number that comes back to bite.

Lifespan, cleaning, and when capping is false economy

Good uPVC fascia lasts twenty-five to thirty years with no maintenance beyond an occasional wash. Cleaning is a soft cloth, warm water, and mild detergent; abrasive pads and solvent cleaners scuff the surface and dull the finish, and on foiled boards they can lift the foil. White boards yellow slightly over decades but rarely fail before the fixings or the timber behind them.

That timber is the catch. Capping over sound timber is sensible value. Capping over tired timber is false economy: the uPVC outlives the rotten board it hides, the gutter brackets loosen, and within a few winters the job comes back as a full strip-off and replacement that costs more than doing it properly the first time. If there is any doubt about the timber, the money spent on full replacement is cheaper over the life of the roofline than capping twice.

Where you'll need this

• Roof covering - the fascia is fixed to the rafter ends before the guttering goes up, closing the eaves and carrying the gutter run

Fascia boards appear on every pitched-roof project at the eaves, whether that's a new extension, a re-roof, or a roofline refresh on an existing house. They pair with the soffit underneath, carry the guttering and feed the downpipe, and run up the gable as bargeboards. Get the thickness, depth, and colour right at order stage, and the detail behind the board sound, and the roofline is a wipe-down-once-a-year job for the next thirty years.