Letter Report: Existing Roof Space Inspection
The structural inspection report has been prepared on the instructions of Jon Gill our client. We have been requested by our client to carry out a site inspection on the existing roof space as there has been some movement. The visual survey was undertaken on the morning of the 28th of February 2019. The inspection took place internally within the roof space and the weather was cold and wet. No intrusive investigations were carried out at this stage.
The following other limitations in respect of this report should also be noted:
– The inspection was carried out from within the rear of the property at ground floor level.
– The inspection was of a purely visual nature.
Services within and to the property have not been inspected. 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.
This report is intended primarily for information and is to be read in conjunction with any other specialist reports and investigations. The report is for the benefit of Jon Gill our client. Structural Engineering Services cannot accept any liability to any third party for the whole or part of its content.
Description of The Existing Building
The property at No 39 is a two-story terraced house located on Parklands Road, London. The houses within the street are formed of masonry wall construction generally with timber floors to all levels and timber roof construction originally covered in slates but the roof is now covered in concrete tiles and these were changed approximately 30 years ago. The adjacent properties are constructed in a similar way.
Defects Recorded Within The Existing Roof Space
The existing roof is constructed with timber rafters which are supported at roughly mid-height off timber purlins which are supported via diagonal bracing members back down onto the central load bearing wall, this is a traditional form of roof construction for properties of this age and type.
There are three sets of diagonal bracing within the existing roof construction, one pair centrally located, one pair against the Party Wall above the access hatch and two smaller diagonal props built off the existing gathered chimney breast. The ends of the purlins are not built into the Party Walls which is unusual and stop short of the wall which is unusual as they are normally built into the walls or supported off cantilever stone elements.
Over a long period of time the roof framing has spread due to the increase in load from the tiles. The original slate tiles would weigh significantly less than the current concrete tiles which has overstressed the existing roof and supporting members. There is noticeable bending and deflection within the purlin members and there has also been excessive movement to the rear diagonal prop off the existing gathered chimney breast.
The movement to the rear brace off the chimney is in the region of 30mm to 35mm indicating that where the front of the roof has spread the rear section of the roof has also moved. (Refer to picture 7). The area of rafters to the front of the property located above the entrance area have moved out and this is clearly visible both internally and externally as viewed from the street level.
As highlighted above the roof has been suffering from roof spread which has been taking place for a sustained period of time. Based upon the level of movement, we would recommend the following remedial works are undertaken within the existing roof space to stabilise the roof framing.
- Installing two areas of further diagonal bracing to limit any further movement and to limit any further deflection taking place within the existing purlins. These members can match the existing bracing. 50mm x 100mm Grade C24 timber elements can be used. The braces should be located between the existing three sets of diagonal braces.
- Install high level 50mm x 100mm timber collars between the existing rafters and fixed into position using a minimum of 4 No 4.50mm diameter wood screws x 75mm long. The collars should be located approximately 600mm below the underside of the existing ridge board.
- Reconnect the existing rafter feet onto the existing timber wall plate. This can be carried out after the front fascia boarding has been removed and the ends of the rafters and plate can be clearly seen. Galvanized framing angles or similar fixings can be used, fully fixed into place.
Following the completion of the remedial works the structural integrity and stability of the existing roof structure will be stabilised thus limiting any further movement.
- We would suggest the following investigations/works are carried out:
1. A full CCTV survey report is carried out on all the local drainage to assess for any damage or leaks. If damage or leaks are found, they will need to be repaired as per the drainage report.
2. The trial hole dug indicates there is no discernable foundation so localized underpinning will be required to
stablilise the chimney. This can be carried out externally with minimal disruption to the existing property.
- Once the underpinning has been completed, we would suggest the following remedial works be carried out on the damage to the property:
1. Localized rebuilding works to the base of the chimney to reinstate the damaged and loose brickwork. It may be
feasible at this stage to try to realign the brickwork where it has moved previously and this may involve some
temporary propping to facilitate this.
Based upon the BRE digest 251 Table 1, Classification of visible damage to walls with particular reference to the ease of repair of plaster and brickwork or masonry and this information is listed out below for your information. Damage categories with descriptions of typical damage. Ease of repair in italics.
0 – Hairline cracks of less than about 0.1 mm which are classed as negligible. No action required.
1 – Fine cracks that can be treated easily using normal decoration. Damage generally restricted to internal wall finishes; cracks rarely visible in external brickwork. Typical crack widths up to 1 mm.
2 – Cracks easily filled. Recurrent cracks can be masked by suitable linings. Cracks not necessarily visible externally; some external repointing may be required to ensure weather-tightness. Doors and windows may stick slightly and require easing and adjusting. Typical crack widths up to 5 mm.
3 – Cracks that require some opening up and can be patched by a mason. Repointing of external brickwork and possibly a small amount of brickwork to be replaced. Doors and windows sticking. Service pipes may fracture. Weather-tightness often impaired. Typical crack widths are 5 to 15 mm, or several of, say, 3 mm.
4 – Extensive damage which requires breaking-out and replacing sections of walls, especially over doors and windows. Windows and door frames distorted, floor sloping noticeably. Walls leaning or bulging noticeably, some loss of bearing in beams. Service pipes disrupted. Typical crack widths are 15 to 25 mm, but also depends on number of cracks.
5 – Structural damage that requires a major repair job, involving partial or complete rebuilding. Beams lose bearing, walls
lean badly and require shoring. Windows broken with distortion. Danger of instability. Typical crack widths are greater than 25 mm but depends on the number of cracks.
In general, categories 0, 1 and 2 with crack widths up to 5 mm can be regarded as ‘aesthetic’ issues that require only redecoration. Categories 3 and 4 can generally be regarded as ‘serviceability’ issues, that is, they affect the weathertightness of the building and the operation of doors and windows. Category 5 presents ‘stability’ issues and is likely to require structural intervention. BRE Digest 251, and in particular the information above, is now used widely in the industry as a way of categorising cracks and determining whether any intervention is necessary.
It should be stressed that these comments are a simplification of the assessment needed to properly classify damage to housing. Several factors, including whether the widths of the cracks are increasing with time, can affect the classification. BRE Digest 251 should be consulted when carrying out any assessment and a building professional should be consulted where damage is significant.
This letter report is considered professional opinion and is not a warranty or guarantee as regards to the works undertaken and no liability shall be attached to us except to the extent that we have failed to exercise reasonable skills, care and diligence in the provision of our structural design services.