UK Renovators: 100 mm That Can Shrink to 18 mm, Measure Reveal Depth

Window reveal depth renovation
Window shutters

Window reveal depth renovation

Reveal depth is not a styling decision. It is set by three constraints working together: where the insulation plane sits, how moisture drains at the sill, and how much clearance the sash or shutter needs to operate. Get any one of these wrong and you either create a cold bridge, block drainage, or end up with a window that cannot open properly. Before you order anything, measure the finished reveal and draw simple head, jamb and sill sections you can hand to your supplier or installer.

TL;DR:

  • Reveal depth varies greatly depending on wall construction, insulation placement, and frame position, making accurate site measurement essential.
  • Positioning the frame outboard, inboard, or centered impacts thermal performance, weather exposure, and insulation continuity, with no fixed regulation.
  • Proper drainage relies on correctly installed cavity trays and weep holes, which must remain unobstructed by mortar or insulation to prevent damp issues.
  • Measuring at the smallest of multiple depth points and photographing all details ensures precision and aids review before ordering bespoke components.
  • Professional surveys are highly recommended for complex or older openings to verify airtight, drainage, and clearance details, reducing costly mistakes.
Shutter World
Get Bespoke Shutters Measured Properly
Shutter World provides professional in-home surveys and made-to-measure shutters for unusual windows and renovation projects across the UK.

Arrange a professional survey

Table of Contents

How to survey and measure reveal depth before ordering

Reveal depth measurement starts with the right tools: a laser measure or steel tape, a spirit level, a set of feeler gauges for gaps, and a phone camera for annotated photos. You are not measuring one dimension. You are recording a set of related numbers that together describe how the opening actually behaves, not how the original drawings said it should.

Work through this sequence on-site:

  1. Record the structural opening brick-to-brick or block-to-block, then separately record the finished face-to-face dimension once plaster, tiling or render is accounted for. The two rarely match, and the gap between them tells you how much the reveal has been built out.
  2. Take three width and three height measurements at top, middle and bottom of the opening, plus both diagonals. A square opening has matching diagonals; anything more than a few millimetres out signals a distorted frame, and you should plan for scribing or packing.
  3. Measure depth separately at head, both jambs and the sill, since older properties frequently have inconsistent reveal depth around a single opening, particularly where render has built up unevenly over decades.
  4. Check sash sweep and handle clearance by measuring how far a casement or sash needs to travel and whether a handle or lock will clear an internal shutter, blind box or curtain pole once fitted.
  5. Note the smallest dimension as your controlling figure. Manufacturers work to the tightest measurement in the set, and most bespoke joinery allows a small manufacturing deduction, typically a few millimetres per side, to guarantee the item still fits if the opening is slightly out of square.

Photograph everything before you close up any reveal: lintel condition, whether a cavity is present, any visible damp-proof course, and how the sill currently sheds water. According to the surveying code of practice set out in BS 8213-4:2016, this record-and-review approach is exactly how professional surveys avoid remakes on non-standard openings.

Choosing reveal depth: frame position, insulation and airtightness

Where you position the frame within the wall thickness changes how the whole junction performs thermally, and this is the decision most DIY renovators get wrong. Push the frame outboard, near the outer face, and you gain a deeper internal reveal that suits deep cills and shutter boxes, but you risk exposing more of the frame to weather and cold. Set it inboard, aligned with the inner leaf, and insulation continuity becomes far easier to achieve, though you lose some external shading benefit. Centre it within the cavity and you compromise on both.

  • Outboard position: deeper internal reveal, useful for shutters and window seats, but the frame sits closer to the weather line and needs careful cavity-tray detailing above it.
  • Inboard position: insulation from the cavity can run almost unbroken past the frame, cutting the risk of a cold bridge at the reveal.
  • Centred position: a compromise that works when the wall build-up genuinely has no better option, such as in a solid wall retrofit.

Building regulations do not fix a single correct answer here. Approved Document L allows U-values to be calculated using the actual size and configuration of the installed window, but installation junctions must still be detailed separately from the window’s own U-value figure. That distinction catches out a lot of renovators: a perfectly good window can still perform poorly if the reveal junction around it is not detailed with continuous insulation and a clear airtight line.

On your section drawing, mark the insulation continuity with a solid hatch line and the airtight line with a separate dashed line, ideally in a different colour. Where they diverge (which happens at almost every reveal to some degree) note how the gap will be closed, whether with a cavity closer, expanding foam tape, or a taped membrane lap. Approved Document L guidance for dwellings also confirms that sills, thresholds and frame extenders should be included when U-values are determined, so don’t treat the sill as a separate, unregulated afterthought.

Moisture, drainage and sill detail at the reveal

Water management at the sill is where most reveal failures actually happen, and it is almost always avoidable with correct sequencing on site. The cavity tray sits above the lintel, stepped so that any water running down the cavity face is directed outward through weep holes rather than tracking inward across the inner leaf.

  • Position the cavity tray correctly relative to the lintel. NHBC technical guidance recommends cavity trays over openings with soldier courses or stone lintels, tied in with appropriately placed weeps.
  • Space weeps to clear water reliably. NHBC’s own detail-level guidance cites weep holes above cavity trays at 450 mm centres in relevant brickwork openings, a spacing worth checking against on any masonry reveal you open up.
  • Never let insulation or mortar droppings block the weep path. A weep hole clogged during construction defeats the entire cavity tray, and the fault often only shows up as damp staining months later.
  • Slope the sill outward and detail any sub-sill so it sheds water clear of the wall face while still allowing insulation and an airtight seal behind the internal trim.

Approved Document C includes diagrams for reveals in areas of severe and very severe exposure to driving rain, worth checking if your property sits in an exposed coastal or upland location.

Pro Tip: Photograph the cavity tray and weep positions before you close the reveal, even if you think the detail looks standard. If damp appears eighteen months later, that photo is the only evidence you’ll have of what was actually built.

Cavity tray and weep positions before closure

Reveal detailing by wall type: masonry, timber frame and SIPs

The construction behind your reveal changes almost every decision you make about depth, drainage and insulation continuity.

  1. Masonry cavity walls. The outer leaf thickness, cavity width and lintel type together set your reveal depth. Cavity tray routing has to clear the lintel end and step down toward the weeps; note the tray position and lintel bearing length on your drawing.
  2. Timber frame. The insulation plane usually sits within the frame itself, with sheathing board providing racking strength and an internal lining creating a service void. Frame fixings for the window need to land in structural timber, not just packing, which can shift the achievable reveal depth by 10 to 20 mm compared with a masonry equivalent.
  3. Structural insulated panels (SIPs). Never compress the panel’s insulating core when fixing through it; use the fixing points the panel manufacturer specifies. Detail the reveal so thermal bridging through fixings is minimised, and flag on the drawing exactly where fixings penetrate the panel skin.

For each construction type, your head, jamb and sill notes should specify the insulation material and thickness, the fixing type and spacing, and where the airtight membrane laps onto the frame. A generic “insulate around frame” note is not enough for a manufacturer or installer working to a fixed reveal.

Worked drawing checklist and renovation workflow

A usable reveal drawing is not complicated, but it needs to show the right things in the right order. Record six opening measurements (three widths, three heights) plus both diagonals, then take depth readings separately at head, both jambs and the sill.

  • Expose the reveal where necessary to see the actual construction rather than guessing from old plans.
  • Measure twice and photograph everything, including any existing cavity tray, lintel and DPC.
  • Draw a scale section showing the airtight line, insulation continuity, cavity closer, lintel detail, cavity tray and weep positions, and sill or sub-sill arrangement with trim allowance.
  • Review the drawing with your manufacturer or a competent installer before you commit to an order, following the survey-and-review sequence set out in BS 8213-4:2016.
  • Only order once the section is agreed.

If you are planning made-to-measure interior shutters as part of the renovation, the finished reveal, not the rough opening, controls everything: panel depth, handle clearance and how far the sash needs to sweep before a shutter panel would foul it. Shutter World’s own step-by-step measuring guide walks through exactly this once your reveal drawing is settled.

Common mistakes and on-site troubleshooting

Most reveal failures trace back to the same handful of causes, and all of them are catchable before you close anything up.

  • Blocked weeps or a missing cavity tray, usually from mortar droppings or insulation pushed too far into the cavity.
  • Insulation falling short at the sill, leaving a cold bridge that shows up later as condensation or mould.
  • Insufficient clearance for sash travel or handles, often only discovered once trim or a shutter panel is fitted.
  • An airtight line that breaks where the frame meets the wall, undermining otherwise good insulation elsewhere in the reveal.

Catch these with a simple check: shine a torch along the weep line, run a hand along the insulation for gaps, and test sash or handle movement before finishing any trim. If you find a problem, photograph it, redraw the section, and get it reviewed before proceeding.

Why finished reveal measurement matters more than the rough opening

Why finished reveal measurement matters more than the rough opening — overview diagram

Assumed clearances have a habit of disappearing once finishes go in. The Passivhaus Trust documents a case where a 100 mm clearance assumed at design stage shrank to just 18 mm once insulation and internal linings were installed, leaving barely enough room for the window to function as intended.

That is why complex or older openings deserve a professional survey before you commit to bespoke joinery or shutters. A competent installer checks the airtight line is genuinely continuous, confirms the weep path is clear, and verifies sill continuity on site rather than trusting the original drawing. Assumptions made on paper rarely survive contact with an actual wall.

— Ken

Getting your reveal surveyed and fitted professionally

Measuring your own reveal is entirely achievable for a straightforward opening, but complex, angled, arched or bay windows are where DIY measurements most often go wrong, and where a professional survey earns its keep. A professional in-home survey records the finished reveal depths at head, jamb and sill, checks sash sweep against handle clearance, and notes the airtight line and any cavity or drainage detail that could affect fit, all backed by annotated photos and sections rather than a single tape-measure figure.

Shutter World

That survey feeds directly into bespoke manufacture from ranges including the Seattle, Hollywood, Boston, Georgia and Montana ranges, followed by expert installation from Shutter World’s own fitting teams and backed by product and workmanship guarantees. If your renovation involves anything beyond a simple square opening, a managed survey removes the guesswork that catches out even careful DIY measurers, particularly on older masonry with inconsistent reveal depths. Get in touch through Shutterworld to arrange a professional in-home survey and see exactly how your finished reveals translate into a bespoke shutter fit.

Sources

FAQ

What is the standard reveal depth for a UK window?

There is no single standard figure. Reveal depth depends on wall construction, insulation thickness and frame position, so it must be measured for each opening rather than assumed from a typical figure.

How do I measure reveal depth accurately?

Measure depth separately at the head, both jambs and the sill, using the smallest reading as your controlling dimension. Check diagonals for square and record sash sweep and handle clearance, following the survey approach set out in BS 8213-4:2016.

Does reveal depth affect insulation performance?

Yes. Frame position within the reveal determines how continuous the insulation and airtight line can be, and Approved Document L requires installation junctions to be detailed separately from the window’s own U-value.

What causes damp problems at a window reveal?

Blocked weeps, a missing or poorly positioned cavity tray, and insulation pushed too far into the cavity are the usual culprits. NHBC guidance recommends weep spacing at 450 mm centres in relevant brickwork openings to keep drainage working.

Should I measure my own reveal for shutters or use a professional survey?

Straightforward square openings are manageable to measure yourself using Shutter World’s measuring guide. Complex, angled or older openings benefit from a professional in-home survey to catch clearance and drainage issues before manufacture.

Tags: