Structural and civil engineering for St. Albans family home

Project Overview

Architect: Uskuri Theobold Architects
Project type: Bespoke dwelling
Size of project: 3,600 sq/foot
Build cost: Circa £850k + interiors
Location: Midway, St Albans – Hertfordshire
Role: Structural Engineering, Civil Engineering design

Overview

We were appointed to provide structural and civil engineering design services for this bespoke 2.5-storey detached family home. The recommendation came from the architect, and we were engaged directly by the client following planning approval.

Key structural challenges included designing an 8m+ high void from the ground floor to the underside of the roof through the stair void, a double-height vaulted master bedroom with extensive glazing and a dressing room mezzanine, and an external balcony over the habitable space below. These features required smart structural solutions to ensure stability without compromising the architectural intent.

Trench fill foundations for clay soils

Given that the site was located in St Albans, we anticipated clay soils, which was confirmed during site investigations. The absence of significant trees meant that ground movement concerns were minimal, allowing for the use of traditional trench fill foundations.

For the ground floor, a beam and block floor system was implemented. This offered a robust and practical solution, reducing the risk of future settlement issues. The upper floors were constructed using engineered joists, with the second floor initially following the same approach before incorporating the bottom chord of an attic truss for additional structural efficiency.

Cut roof to accommodate vaulted bedrooms

The roof was originally designed as a cut roof to accommodate vaulted bedrooms at both the front and rear. A cut roof consists of individual lengths of timber that are cut and assembled on site, using rafters, ridge boards, joists, and purlins. This traditional method allows for flexibility in design but is limited by the maximum available timber length, therefore required additional engineering of internal supports. When a room has no ceiling, such as in a vaulted space, a truss system is not a viable option.

As the design evolved, the client decided to remove the front vaulted rooms. This presented an opportunity to explore trussed roof construction, which was proposed by the contractor.

At the time of this change, the structure had already reached the first-floor level. This meant that we had to assess the new load paths from the trusses to ensure the design remained sound. Load paths refer to the route all forces take through the building to the ground. After careful consideration, trusses were determined to be the best solution, offering a balance of strength, cost-efficiency, and ease of construction.

Civil engineering

Through asset searches from the local authority, we were able to design a straightforward and effective drainage solution. Both surface water and foul water were directed to the front of the site, where they connected to a demarcation chamber. From there, a length of adoptable pipe connected to the public sewers within the road. This approach minimised disruption and ensured compliance with local drainage requirements.

Key structural features

An 8m+ high void running from the ground floor to the underside of the roof through the stair void, creating a striking architectural feature.
A double-height vaulted master bedroom featuring a glazed gable frame and a dressing room mezzanine. The extensive glazing presented a challenge, as it left little room for structural elements to support loadings and resist lateral forces. Our detailed gable frame design was successfully constructed with minimal queries from the contractor while maintaining and no compromise on the intended aesthetics and spatial efficiency.
An external balcony was installed over habitable space below. We worked closely with the architect to ensure all cold bridge junctions were avoided, preserving the thermal efficiency of the design.

This project exemplifies how collaborative engineering solutions can adapt to design evolution while ensuring structural integrity, practicality, and visual appeal.

Midway, St. Albans