Borescopes: a few centimetres of cavity at one point, and how to say so

A borescope is the instrument that turns “I could not see into the cavity” into “I looked into the cavity”, and that is where the trouble starts. A 10 mm hole in a mortar joint gives you a view of a few centimetres of cavity at one point. It is useful evidence. It is not an inspection of the wall, and a report that treats it as one is making a claim the instrument cannot support.

What it is actually for is seeing what a Level 2 or Level 3 inspection otherwise has to infer: cavity width; mortar droppings bridging the damp-proof course; the condition of a wall tie at the point you see one; whether cavity insulation is present, and its state where the probe is; the void under a suspended timber floor, through a lifted board or a removed air brick; behind a bath panel or boxed-in pipework; a flat-roof or dormer void; a flue.

What it shows

A borescope is a camera on the end of a probe with its own light. A semi-rigid probe of about 5.5 mm diameter and 1 m length carries a small image sensor and LEDs to a place you cannot put your eye and sends the picture to a screen or phone. Most units save stills and video to a card.

The first is the direction of view. A forward-looking (0°) tip pushed through a cavity wall looks at the inner leaf, 50–100 mm away, and shows you a patch of blockwork. A 90° side-view or an articulating tip looks up and down the cavity, which is where the ties, the droppings and the insulation are. You look along the cavity, not across it.

The second is focus. The useful range on these probes is roughly 15–100 mm. A cavity is 50–100 mm wide, so that is adequate. Anything much beyond 100 mm is a blur, and anything the LEDs cannot reach is black.

The third is light. The LEDs light what is close. Mortar dust reflects, so a cavity full of droppings looks bright and busy and a clean one looks dark and empty. A dark image proves nothing: an empty cavity, a tip pointing at nothing and a dusty lens all look the same.

Where it misleads

One point stands in for the wall. The view through one hole covers a few centimetres. Tie corrosion, insulation slump and bridging are not uniform across an elevation. A clean view at one point, or a bad one, says what was seen there and nothing beyond it.

It cannot prove the absence of wall-tie corrosion. This is the claim that ends up in dispute. You may see one tie, perhaps two, and you see its face in the cavity, not the embedded ends where the corrosion that matters sits. A wall-tie condition survey is a different exercise: a metal detector to locate ties and establish their spacing, then a sample of ties exposed and examined, as set out in BRE Digests 329 and 401. A borescope is a useful preliminary. It is not that survey.

It cannot grade insulation. You can usually tell whether blown fibre, bead or foam is present at the hole. You cannot reliably tell from a 5.5 mm probe whether it is wet, whether it has slumped two courses up, or whether a void you can see is local or general. Insulation present is not insulation sound.

There is no scale. A gap that looks wide may be 10 mm from the lens; a tie that looks heavily corroded may be a dusty one. Without a known object in shot you cannot quote a dimension or a grade from the picture.

Using it so the result holds

Scope and consent first. The RICS Home Survey Standard describes Level 2 as a visual, non-invasive inspection; Level 3 is more thorough but still does not include opening up as standard. Drilling a hole is beyond both. It is an agreed additional instruction and it needs the written consent of the property owner. The buyer who commissioned the survey cannot give that consent. No consent, no hole: say so in the report and recommend the investigation instead.

Check before drilling. Run a cable and pipe detector over the area. Choose a mortar bed or perpend joint, never a brick face. A 10 mm hole in a joint is made good with matching mortar and disappears; a hole in a brick does not.

Match the probe to the hole. A probe of 5.5 mm or less will pass a 6 mm hole. An 8 mm probe needs the 10 mm hole. Buy and drill accordingly.

Record the position and the view. Elevation, height above the DPC, distance from a corner or reveal, and an external photograph with the hole in frame. Save the stills and video. Note the view direction (0° or 90°, up or down), what was in frame, roughly how far away, and whether the view was clear or obscured. An image with no position is not evidence.

Use a control where you can. A second hole on a different elevation, or higher up the same wall, gives the first one context.

Make good, and photograph the making good. Matching mortar, pointed, photographed, noted in the report.

What to buy

Model	What it is	Indicative UK price (inc VAT, Oct 2026)	Verdict
Wöhler VE 400 HD	Video endoscope, 5.5 mm × 1 m semi-rigid probe, 0°/90° view, records to SD card	About £520–£595	The default. Thin enough for a joint hole, side view, records.
Wöhler VE 400 kit	VE 400 with 3.9 mm and 5.5 mm side-view probes	About £1,130	Only if you regularly need the 3.9 mm probe.
RIDGID micro CA-350	3.5 in screen, rugged, lifetime warranty; standard imager is 17 mm	About £540–£615 (CA-350x with Wi-Fi about £650)	Good body, wrong head. Useless for a joint hole without the 6 mm imager accessory.
Teslong NTS500	Dual-lens probe, 5 in screen	About £200–£270	Fine for occasional use. Check the probe is 6 mm or less before buying.
Teslong USB-C phone borescope	Probe that plugs into a phone and uses an app	About £90–£100	Works. App and connection faff; less durable.

Prices move; check the live price before ordering.

For most residential work, buy the Wöhler VE 400 HD. Its 5.5 mm probe passes a 6 mm joint hole, the 90° view looks along the cavity, and it saves stills and video to a card with no phone in the loop. At about £520–£595 it is the cheapest unit that does all three. 720p or 1080p is adequate; there is nothing in a cavity that needs more.

The extra money in the Wöhler kit buys a 3.9 mm probe for smaller holes and tighter voids; most surveyors will not use it often enough to justify about £1,130. The RIDGID is a tougher body with a longer warranty and a better screen, but the 17 mm standard imager will not pass a joint hole, so the real price is the unit plus the 6 mm imager head. If you already own one, buy the head. If not, buy the Wöhler.

Not worth paying for: Wi-Fi, a long probe or a large screen. A 1 m probe is enough and a longer one is a nuisance to feed. Not worth skimping on: probe diameter. An 8 mm probe commits you to a 10 mm hole every time.

Recording it in the report

Keep five things separate: the instrument and probe, the position and conditions, what the image showed, what that is consistent with, and what should happen next.

Weak wording: “A borescope inspection of the cavity found the wall ties to be in satisfactory condition.” That is a conclusion about the wall drawn from a view of a few centimetres of it.

Wording that holds: “With the owner’s written consent, a 10 mm hole was drilled in a bed joint on the rear elevation, about 600 mm above the DPC and 1.2 m from the north-east corner, and a 5.5 mm side-view video endoscope (Wöhler VE 400 HD) was inserted. The cavity at this point was about 60 mm wide and clear of debris. One wall tie was visible about 300 mm above the hole, with surface rust but no visible loss of section. Stills are retained. The view covers a few centimetres of cavity at one point and does not establish the condition of ties elsewhere. A cavity wall tie survey to BRE Digest 329 is recommended before exchange.”

The second version is longer. It is also the one that does not need defending.

References

• RICS Home Survey Standard, RICS, 2019 (effective March 2021)
• BRE Digest 329, Installing wall ties in existing construction, BRE, 2000
• BRE Digest 401, Replacing wall ties, BRE, 1995