Description
The 170 mm white oversill is the step-up depth in our window sill extensions range — chosen on standard 100–140 mm EWI builds when the elevation faces prevailing weather and the minimum drip projection of a shallower trim is not enough margin. Powder-coated brilliant white aluminium at 2.5 m length, factory PVC film applied, the profile delivers roughly 40–60 mm of overhang past the finished render face — the headroom that exposed south-west and west-facing walls need to keep wind-driven rain off the rendered facade. Trade self-collection from our Southampton warehouse.
What the 170 mm White Oversill Does on Exposed UK Elevations
The 170 mm White Oversill carries the original window sill outward over EWI insulation builds of 100–140 mm, delivering a 40–60 mm drip projection past the finished render face. That is meaningfully beyond the 30 mm minimum that BBA-certified EWI systems expect, and the additional overhang is the point of the profile — it is the depth chosen specifically when site exposure assessment shows that the minimum projection of a shallower oversill would leave the facade vulnerable to wind-driven rain.
UK prevailing weather pushes rain at horizontal angles exceeding 30 degrees during storms, particularly on south-west and west-facing elevations. At those angles, a 28 mm drip from a 140 mm oversill is geometrically marginal; rainwater is increasingly likely to track back onto the render face rather than fall clear. The 170 mm depth lifts the overhang into a band where the trim sheds water reliably even when wind direction is working against the detail. Brilliant white powder coat keeps the trim line aligned with the standard uPVC frame colour across most UK housing stock.
Why Installers Step Up From 140 mm to the 170 mm White Oversill
- Exposure margin where prevailing weather hits hardest: the 40–60 mm projection band lifts the drip edge clear of the render face on west-facing, south-west-facing, and unshielded gable elevations where horizontal rain angles routinely exceed what a minimum-projection trim can handle.
- Same install detailing as the rest of the range: fixing centres, end-cap fitting, and sealant work are identical to the 140 mm trim — the only on-site difference is the cut width, so stepping up depth on exposed elevations adds no programme complexity.
- Matches standard uPVC window frames: brilliant white powder coat aligns with the dominant frame colour across UK housing stock, holding a single sill colour line across mixed-depth elevations where some walls take 140 mm trim and others take 170 mm.
- UV-stable factory finish: the powder coat resists yellowing across the 25–30 year service life of a certified EWI system, so the white trim line stays parity with the window frames over the long term.
- PVC film through the rendering programme: the factory film stays on through basecoat, mesh, and topcoat application — meaningful on exposed elevations where wind-blown render splatter from adjacent scaffold lifts hits the trim face hardest.
- Bridges the standard-to-deep band: sitting between the 140 mm workhorse for sheltered standard builds and the 220 mm for deep retrofit, the 170 mm depth is the practical step-up when the project needs more margin without moving into deep-projection territory.
Technical Specifications — 170 mm White Oversill Data
| Property | Value |
|---|---|
| Profile type | Window oversill extension — upper-mid projection |
| Material | Powder-coated aluminium extrusion, exterior grade |
| Colour | Brilliant white |
| Length | 2.5 m (single unit) |
| Projection (depth) | 170 mm |
| Insulation build accommodated | 100–140 mm (with basecoat and topcoat) |
| Drip-edge projection past render | 40–60 mm typical at correct build depth |
| Recommended fixing centres | 300 mm along rear upstand |
| Surface protection | Factory-applied PVC film, removed at handover |
| Compatible end caps | White verge sill end-cap pair (sold separately) |
| Delivery route | Self-collection only — Southampton warehouse |
| Storage | Flat, dry, protective film retained until install |
The 2.5 m length fits comfortably in a standard panel van, and the warehouse team can assist with loading.
How the 170 mm White Oversill Sits in the EWI Build Sequence
The oversill goes on after insulation boards have been bonded and mechanically fixed but before the basecoat and mesh reinforcement layer is applied — this sequence lets the basecoat be worked tight against the trim edge afterwards, locking the profile cleanly into the finished build. Bed the rear upstand into a continuous low-modulus silicone or hybrid sealant bead against the original masonry sill, then fix mechanically with countersunk screws at 300 mm centres along the upstand.
Set the trim with a 5–10 degree outward fall so rainwater drains clear of the wall rather than tracking back behind the upstand toward the substrate. On exposed elevations — which is where the 170 mm depth most often earns its specification — verify wind-uplift fixing capacity against the project exposure category, since prevailing weather loads work harder against the trim than on sheltered walls. For the full installation sequence covering measuring, cutting, sealing, and end-cap integration as a complete system, our window sill extensions installation guide walks through the worked process across the whole trim family.
Installation Notes — Cutting, Sealing, Multi-Window Runs
For the best result, measure every opening individually. On pre-war and inter-war solid-wall properties — the housing stock where 170 mm is most commonly specified for exposure reasons — original reveals vary by 15–20 mm between openings on the same elevation, and a single measurement carried across multiple windows produces visible inconsistency. Cut each profile to width with a fine-toothed metal-cutting blade, leaving 2–3 mm clearance at each end for the end cap, then deburr the cut edge with fine-grit abrasive so the end-cap seal sits cleanly against the cut aluminium.
Use a low-modulus silicone or hybrid sealant along the rear upstand and again at the end-cap junction; high-modulus products can crack as the aluminium cycles with temperature, opening a path for water onto the insulation layer behind. Leave the PVC protective film intact through basecoat, mesh, and topcoat applications, and peel it only after scaffolding is being struck. On exposed elevations the film earns its value most — the same prevailing weather that drove the depth-up specification will also push render splatter against the trim face during the build.
How the 170 mm Depth Compares to Neighbouring White Oversills
The white oversill family covers the full UK insulation depth band across four projections in a single colour. Picking the right depth is a two-part decision: insulation build first, then site exposure as the modifier.
| White Oversill | Insulation Band | Decision Driver |
|---|---|---|
| 140 mm depth | 60–100 mm | Sheltered standard builds — minimum projection adequate |
| 170 mm depth (this product) | 100–140 mm | Exposed elevations on standard builds — extra weather margin |
| 220 mm depth | 140–200 mm | Deep retrofit specification — insulation drives depth |
On a project with mixed elevations, specifying 140 mm for sheltered walls and 170 mm for exposed walls is a practical balance — install detailing is identical across all depths, so the only on-site difference is the profile width at the cutting stage. Where the project specifies dark trim rather than white to match anthracite frames or contemporary contrast detailing, the matched 170 mm anthracite oversill covers the same insulation band in RAL 7016 powder coat.
Pro Tips From UK Renderers Fitting Mixed-Depth Oversill Runs
- Survey exposure before ordering depth: walk the elevations and identify which face prevailing weather and which are sheltered by neighbouring structures or topography — that survey, not the insulation thickness alone, decides whether each elevation takes 140 mm or 170 mm.
- Mark profile depth by elevation on the order sheet: on a typical detached house with mixed exposure, the order might split as 140 mm for north and east elevations and 170 mm for south-west and west — labelling profiles by destination on collection day avoids cuts going to the wrong elevation.
- Count end-cap pairs by openings, not by depth: the white verge sill end-cap pair closes one oversill run regardless of profile depth, so count caps against window openings and add the same pair count regardless of which depths the project specifies.
- Hold a 5–10 degree pencil mark on the masonry sill: a consistent fall mark across every opening keeps the trim line visually parallel along the elevation, which matters more on a mixed-depth run where any inconsistency reads as installer error rather than design choice.
- Plan film removal as the last task: on exposed elevations, the same weather that drove the depth-up specification will throw render splatter at the trim face throughout the programme — the film stays on through every adjacent trade interface and comes off only at scaffold strike.
Is the 170 mm White Oversill Right for Your Project?
- Exposed elevations on 100–140 mm EWI builds: the right specification for south-west, west-facing, and unshielded gable walls where the extra 40–60 mm of drip projection keeps wind-driven rain reliably clear of the render face.
- Mixed-elevation properties: a sound choice for the exposed walls on a property where sheltered elevations take 140 mm trim — install detailing is identical, so mixed-depth specification adds no programme complexity.
- Sheltered standard builds with 60–100 mm insulation: the 140 mm white oversill covers minimum projection requirements at a narrower profile, avoiding the architectural step that an oversized trim would add unnecessarily.
- Deep retrofit with 140 mm+ insulation: the 220 mm white oversill is the correct projection where the insulation depth itself drives the trim choice, rather than exposure margin.
- Anthracite or contrast-trim facades: the 170 mm anthracite oversill covers the same projection in RAL 7016, matching dark frames and contemporary facade specifications.
FAQ — 170 mm White Oversill Selection, Compatibility, Ordering
When should I choose 170 mm over 140 mm for 100 mm insulation?
Both depths accommodate 100 mm insulation, but they differ in drip-edge projection. The 140 mm oversill provides roughly 28 mm of overhang past the render face — adequate for sheltered elevations such as courtyard walls, urban-terrace rears, or facades shielded by neighbouring buildings. The 170 mm oversill provides roughly 58 mm, which is the more reliable specification on west-facing and south-west-facing walls where horizontal rain angles in storms push water back toward the wall surface. If the elevation faces prevailing weather or is unshielded by adjacent structures, step up to 170 mm.
How many profiles do I need for a typical window opening?
One 2.5 m profile covers most domestic window widths in a single length, which is the cleanest detail because any intermediate joint reads as a visible line along the trim face. Measure each opening width and add 30–50 mm overhang each side past the reveal. For openings wider than 2.5 m clear, two profiles are joined with a manufacturer-approved jointing strip — plan the joint position to fall away from the visual centre of the opening so it sits less prominently.
Can I use different oversill depths on the same property?
Mixing depths is a practical approach on properties with both sheltered and exposed elevations. Fixing centres, sealant details, and end-cap fittings are identical across all four depths in the range, so the only on-site difference is the profile width at the cutting stage. Specifying 140 mm for sheltered walls and 170 mm for exposed walls optimises both cost and weather performance without adding installation complexity, and the white powder coat holds colour parity across the whole elevation.
Are end caps included with the oversill?
End caps are sold separately as a matched pair so each oversill run can be matched precisely to its measured width. Order the white verge sill end-cap pair alongside the oversill — one pair closes both ends of a single run, regardless of whether the run is 140 mm, 170 mm, or 220 mm depth.
Can the 170 mm oversill be cut to length on site?
Each 2.5 m profile is designed to be cut to the measured window width on site using a fine-toothed metal-cutting blade. Leave 2–3 mm clearance at each end for the end cap to seat without forcing, then deburr the cut edge with fine-grit abrasive paper so the end-cap seal beds cleanly against the cut aluminium rather than catching on a rough burr.
What sealant should be used at the rear upstand?
A low-modulus silicone or hybrid sealant rated for facade movement is the right choice. High-modulus products can crack as the aluminium expands and contracts with temperature cycling, and any break in the sealant line behind the upstand lets water track onto the insulation layer behind. Run a continuous bead along the full upstand length and tool it cleanly before the basecoat is applied.
Why is this product self-collection only?
2.5 m powder-coated aluminium profiles dent too frequently in courier transit for white trim to remain economic, since any handling marks show clearly against the brilliant white finish. Self-collection from our Southampton warehouse keeps the profile in factory condition through to install — the 2.5 m length fits comfortably in a standard panel van, and the warehouse team can assist with loading.