Structural Report: Rear Extension
Structural Letter Report: Two Storey Detached Property with Single Storey Rear Extension
This structural inspection letter report was prepared and commissioned under the instructions of our client, Natalie Pratt. We have been requested by our client to carry out a general site inspection of the whole property, where previously subsidence was identified due to leaking drains, located at Ember Lane, Esher, Surrey. A single, non-disruptive site visit inspection was conducted on the 28th Friday February 2020. The weather condition at the time of the inspection was wet and cold.
This report is intended primarily for information and is to be read in conjunction with any other specialist reports and investigations. In accordance with our standard practice we must point out that this report is based upon our inspection of the premises and any other information made available to us, both written and oral, which we have assumed to be correct. Structural Engineering Services will not accept liability to any third party for any loss, consequential or otherwise, for whole or parts of the conclusion and recommendations provided in this report.
Structural Letter Report: Cracking Along Steel Frame Column
Completion Date: March 2020
Building Type: Residential
Project Type: Structural Report
Location: Cabul Road, London
Structural Engineer: John Murphy
Brief Description of The Existing Property
The property is a two-storey detached house build approximately around the 1920 although the exact age of the building could not be determined. At the rear of the property a single-storey rear extension with a pitched roof was added. The property appears to be formed with a brickwork façade on ground floor and render at firsts floor, presumably with timber flooring and traditional cut roof. The property has suffered from subsidence due to leaking drains around 2013. Various drainage and structural repair works had been proposed in 2013 as per SubsNetuk drainage investigation report and JW Cannon Schedule of Works. However, no underpinning works have been specified.
The local geological map has been highlighted in the figure indicated below. The property is situated above the Langley Silt Member, which a mixture of Clay and silt. The underlying bedrock is London Clay Formation which is a mixture of clay and silt. We carried out a non-intrusive general site inspection of the property to observe and note any structural concerns that may have occurred and provide any necessary recommendations for structural repair work
Upon inspection of the exterior of the property:
- No structural issues have been observed on the front elevation of the property. Similarly, no structural issues have been identified to the rear of the property above the timber decking. No comments can be made on the condition of the masonry below the decking as no access was available at the time of the visit.
With regards to the left flank wall of the property:
- The left flank wall of the property shows signs of previous patchworks but is generally in reasonably fair condition, refer to. Small number of cracking have been observed but are deemed to be either nonstructural or within acceptable limits.
- There are signs of small cracking to some of the brickwork. Although the cracking is small, over time the cracks may increase in size and allow water ingress causing dampness in the property.
- The mortar in the original brickwork next to the door shows signs of deterioration. Various patching up works around the drain pipes from the kitchen are observed.
- Small tapered crack is visible above the doorway lintel bearing. Appearing to reduce in width at the top and approximately 2-5mm wide. No signs of cracking on the internal face of the flank wall,
- Discolouration of brickwork next to drains.
- Poor workmanship in toothed brickwork where the window opening has been reduced.
- Small hairline crack above corner of window opening next to drain pipes. Cracking occurs vertically and cuts through three bricks. Crack width approximately 5mm.
Upon inspection of the interior of the property:
- Three small hairline cracks running parallel to the ridge line are observed.
- Cracking is observed in the Dining room ceiling above the original rear elevation wall. The cracking runs from side to side and along the wall corner on the right flank wall. Cracking is also observed on the external face of the flank wall.
- Small hairline crack is observed in the first floor landing ceiling.
- Small hairline crack is observed in the rear bedroom.
- Cracked roof tile over the extension roof.
Conclusion & Recommendation
The property had been identified to have suffered from an episode of subsidence due to leaking drains to the rear of the property as per ‘Claims Assessment’ by Lloyds TSB letter dated 24th July 2013. The JW Cannon Schedule of Repair Works by Lloyd Bank Home insurance dated 28th April 2014 identifies various repair works but does not propose an underpinning sequence to stabilise the existing foundations and prevent further movement and cracking. This is quite unusual as any remedial works to the property and cracking would be temporary and likely to reappear as the existing foundations continue to suffer from subsidence.
Furthermore, the property is situated above the Langley Silt Member superficial deposits, which are prone to loss of fines and erosion due to flowing water i.e. leaking/broken drains. Additionally, due to the clay content and the proximity of nearby trees the ground can suffer from root induced heave and shrinkage which could also cause subsidence.
The drain repair works specified by SubsNetuk in their Drainage Investigation report dated 05/12/2013 has identified various defect, broken and leaking drains. It is important that these repairs have been conducted as any leaking drains would cause fine particles in the soil to erode and potentially cause the foundations to subside.
It is also worth noting that the existing building is likely to sit on relatively shallow foundations compared to the extension foundations. Due to this it is possible that differential settlement is occurring between the two foundations and manifested as a crack along the line of the extension.
With regards to the right flank wall where vertical cracking is observed between the original building and the rear extension. Repointing of external brickwork will be required to maintain weather tightness. Internally, the cracks can be masked using suitable linings but may reappear due to continuous ground movement.
Apart from the above mentioned points the property seems to be in relatively good condition as only small hairline cracks have been observed which are fairly common for properties of this age. With regards to the left flank wall, repointing of the brickwork and replacement of some broken and cracked brick will be require to maintain weather tightness and prevent dampness.
The original and new brickwork should be toothed in properly and small cut section of bricks should be replaced. The brickwork adjacent to the left flank wall door opening will need to be repointed to maintain weather tightness.
- Further investigations could be undertaken to verify the condition of the foundations below the right flank wall where cracking has occurred between the original building wall and the extension wall
- We suggest trial holes to be dug at the right flank wall to inspect the foundations and the condition of soil. This would indicate whether significant localised settlement is occurring or whether there is a noticeable presence of tree roots below the base of the foundation.
- Disused drains are advised to be disconnected from the sewer system as near as possible to the point of connection. Disused drains can collapse and cause subsidence or be subject to surcharging from connecting flooding drains or sewers.
- An arboricultural report could be undertaken by a tree specialist to determine the influence of the trees to the property. Their surveys may indicate whether subsidence has occurred due to tree roots and may advise, for example, on the potential risks of removing trees near buildings. They may suggest using tree root barriers made of high-density polyethylene to protect your property, if you are unable to remove the trees or provide suitable structural solution.
- If subsidence is found subsequently, an appropriate structural remedy would be necessary. Either cellular structures could be installed beneath foundations to reduce the downward movement or underpinning could also be necessary to stabilise the structure where the soil itself is prone to settlement.