WINDOW SILL EXTENSIONS & OVERSILLS

Twelve profiles cover the window and roof-edge junctions on a rendered or insulated wall, and this audit found the depth guidance on the old page contradicting the guidance on its own product pages. Everything below is either a fetched figure or a stated omission, within the wider rendering materials range.

How Deep a Profile the Build Needs

Window sill extensions here run in four depths on a 2.5 m profile from £23.90 as at August 2026, and the depth is chosen against the finished wall so the drip edge clears the new render face rather than stopping short of it.

The method is straightforward and does not need a table: measure the total added build outward from the original masonry face, counting insulation, adhesive, basecoat and finish, then choose a depth that projects past that finished face rather than level with it. What this page will not do is convert that into a depth-per-insulation-thickness chart.

• Why no chart appears here: the previously published mapping on this page and the mappings on the individual product pages disagree — one had the 140 mm profile covering builds to 120 mm while its own product page stopped it at 90 mm, and the 170 mm profile shifted band on every page it appeared on.
• Why it is not derivable: a depth-to-build mapping depends on a manufacturer's stated minimum drip projection, and no manufacturer document for these profiles was retrievable this session by pointer, page or search.
• How to settle it: send the build-up — board type, thickness, and the render system going over it — to the Renders World technical desk, which confirms the depth against current profile geometry before you order.
• What is certain from the range itself: four depths at 110 mm, 140 mm, 170 mm and 220 mm, each on a 2.5 m length, in white and anthracite, with matched verge trims and end caps.

Depths, Colours and What Each Profile Costs

White and anthracite are priced identically depth for depth: £23.90 at 140 mm and £30.40 at 220 mm in both colours, so colour is a design decision with no cost consequence. Across the priced depths the range spans £30.40 − £23.90 = £6.50, and the step from 140 mm to 170 mm is £27.90 − £23.90 = £4.00 — small enough that stepping up a depth where the build is borderline costs little. All figures were read from the Renders World product endpoints in August 2026, are material cost only, and carried no displayed VAT.

The colour pairing matters more than the price. Each end cap belongs to one trim by colour and is not usable without it, so an anthracite run takes the anthracite end cap pair and a white run the white pair, ordered per profile run rather than per window.

Fitting Oversills, Verge Trims and End Caps in Sequence

The sequence is what makes these profiles read as one line across an elevation, and it sits inside the render programme rather than before or after it. Two of the steps below are the ones most often reordered on site.

• Fit after the boards, before the reinforced layer: the profile goes on once insulation is bonded and fixed, so the basecoat and mesh can return up to its edge and lock it into the system.
• Measure every opening separately: reveal widths on older housing vary along the same elevation, and one template carried across a wall fits poorly somewhere.
• Cut with a fine-toothed metal blade and deburr the cut edge, so an end cap seats without being forced onto a rough finish.
• Bed the rear upstand in one continuous run of sealant rather than in intermittent dabs — a single uninterrupted line is the detail that keeps driven rain out at the frame junction.
• Where a profile arrives with a protective film, leave it on through basecoat and topcoat and peel it once the render has cured, which keeps cementitious splash off the visible face.

Fixing centres, fall angle and overlap depths are all real specification questions and all belong to a manufacturer document this session could not obtain, so they are confirmed on ordering rather than stated here. The sill extension fitting guide works through measuring and cutting in order, and the reveal and opening detailing guide places these profiles alongside the beads in the same sequence. The finish going over them comes from the thin-coat silicone render range.

Which Figures Are Documented and Which Are Confirmed on Ordering

This audit removed a substantial number of specifications that had nothing behind them, and naming the gap serves a specifier better than a confident figure would. Twelve products in this collection have no manufacturer document on file, and none could be found this session.

• Documented: the six prices above, from the store's own endpoints, with their fetch month.
• Name-derived, so certain: the four depths, the 2.5 m length, the two colours, and the pairing of each end cap to its trim.
• Removed for want of a source: material and coating type, the RAL reference for anthracite, aluminium gauge, film UV performance, fixing centres, fall angle, overlap behind a fascia, minimum drip projection, render-layer allowance, end clearance for caps, service-life and design-horizon figures, and a U-value improvement range.
• Confirmed on ordering: profile geometry against your build-up, which the technical desk checks against current stock.

Oversill, Verge Trim or Bead — Matching the Profile to the Junction

• Below an opening, take an oversill: it sits over the existing sill and carries the drip edge outward past the new finish, in the depth your build needs.
• At the top of the wall, take a verge trim upstand: its upward return terminates the system against a fascia, flat-roof edge or parapet, which is the opposite job to an oversill and not interchangeable with one.
• At jambs and arrises, the beads do the work: specify render stop beads at the reveal edges and render corner beads at external angles so the whole opening reads as one detail.
• Order caps with profiles, not after: one pair closes one continuous run, in the matching colour, and a run left open at the ends is an unfinished termination.
• Where the build is borderline between two depths, the £4.00 step between adjacent depths makes the deeper profile a straightforward call once the desk has confirmed the geometry.

FAQ — Quantities, Colour Matching, Comparison and Care

How many profiles and caps does one window need?

One 2.5 m profile covers most domestic openings in a single length, which keeps the trim face unbroken, and one end cap pair closes that run at both ends. Count caps by runs rather than by windows: a bay with three separate oversill sections needs three pairs, while a single wide opening made from one profile needs one.

Can white and anthracite components be mixed?

Caps are colour-specific and pair to their own trim, so a white cap on an anthracite run reads as a mismatch at the most visible point of the detail. Depths and prices are identical across both colours, so choosing colour by the window frames and fascia line costs nothing either way.

What separates a verge trim from an oversill at the same 140 mm?

The direction of the return. An oversill turns down at the front to shed water clear of the wall below an opening, while the verge trim turns up at the back to tuck behind a fascia or coping at the top of the wall. They share colour, length and the same end caps, and the 140 mm verge trim at £29.50 sits above the 140 mm oversill at £23.90.

How are the profiles looked after once the render is finished?

Periodic rinsing with clean water to clear airborne deposits is the whole of it, avoiding solvents and abrasive pads that dull a coated surface. Keeping cementitious runoff off the face during the build is the step that pays off most, since staining on a pale profile usually comes from render residue rather than from the finish itself.

Does a deep profile affect planning on a sensitive facade?

Projection changes the proportions of a window detail, so on a listed building or in a conservation area it is worth showing the profile on elevational drawings and confirming with the local planning authority early. That conversation is straightforward at design stage and awkward once scaffold is up.