The Original Orlit Prototype House

Photograph of original orlit house at colnbrook
  • Manufacturer: Orlit
  • Variant: 1945 Version
  • Construction Type: Steel Framed
  • Consultant:
  • Period Built: 1945-48
  • Number Built: 1600
  • AKA: Atholl Post-War
  • Example Build Location: Colnbrook

PROTOTYPE Orlit House at Colnbrook

Photograph of original orlit house at colnbrook
Orlit House Colnbrook

The Orlit system of construction relied on site prefabrication, but never achieved complete dry assembly. The horizontal frame members are jointed by pouring concrete into a socket, formed between the mating points and the wall. Panels were jointed and pointed in gauged lime mortar. One positive aspect to this form of construction was the wide availability of the basic materials required. Orlit, also claimed that fewer skilled workers were required for this form of construction and erection, which was a welcome statement during the post-war skills and manpower shortages at the time.

The foundations consisted of separate  mass concrete  footings  which  carry columns of the reinforced concrete  frame. The footings were exactly spaced and levelled by using a specially designed jig.

The structural frame consists of precast reinforced concrete beams and columns, connected on the site to produce a continuous load-bearing structure. 
The columns are 6 3/4″ x 6 3/4″ in section, spaced at about 12 ft. centres; 15 columns are required for a pair of houses. The columns are made in three sections which are bolted together by with end capping plates.  
The base sections are short lengths of 2 ft. 6 in. which were suspended over the footing excavations, set out and levelled by means of a framed jig, after which the concrete bases are cast around them.
The middle sections extend to first floor level and have short lengths of the first floor connecting beams cast on to them. The top sections are similar to the middle sections, having  short lengths of the roof beams cast on.
The first floor and roof beams are 6 3/4″ x 9″ in section and have rebated slotted ends with projecting lengths of bar reinforcement at the top and bottom. The beams are supported on the rebated ends of the short beam projections on the columns, and the connection is made by filling in the slot with in-situ concrete. The upper reinforcing bars have a lap of about 10 in. The frame is separated from the cladding by felt strips and can move independently of it.

Orlit House Under Construction

The external walls are of cavity construction, consisting of an external leaf of dense concrete slabs and an internal leaf of foamed slag concrete units of similar size. The units are of a maximum length of 4 ft. x 1ft. 4″ high, and are 2″ thick on the edge and 1″ thick in the tray. The two skins are built up simultaneously and are held together by galvanized steel ties.
The mortar for bedding and jointing is a mix of 1:1:6 cement:  lime: sand for the external skin and  1:5:18 cement: lime: sand for the internal skin. Special concrete bonding units laid in continuous courses at window cill and head levels are employed to stabilize the two skins. These bonding units are channel shaped on the top and are perforated at intervals to provide drainage.
The internal finish is a single finishing coat of plaster. Door and window surrounds are of precast concrete and are made as complete frames with the metal sub-frames cast in; they are built in as the walling (sic) proceeds.

The party wall consists of two leaves of foamed slag concrete blocks with mortar joints similar to those in the external walls. Bonding units laid in courses and metal ties are used to increase the stability.

Partition walls are composed of lightweight foamed slag tongue & grooved concrete blocks 2 inches thick laid in mortar with a finishing coat of plaster on both sides.

The ground floor consists of a concrete slab 4 1/2″ thick, laid directly on the ground, and covered by a 1/2″ layer of a proprietary thermal insulating material. A 1/2″ thick cement mortar screed is used to provide a base for a 1/4″ thickness of a proprietary jointless flooring. The promoters state that the insulating material is composed of 3 parts by volume of granulated cork to 2 parts foamed slag passing a 36 mesh sieve, bound together by 1 1/2 parts of bitumen emulsion. The proprietary finish consists of a mixture of reclaimed rubber latex and cement, with a proportion of hardwood chippings as an additional filler. It is laid cold and finished with a trowel.

The first floor is constructed of precast concrete units 6″ deep overall of inverted trough shape spanning between the outer and central longitudinal floor beams. The floor surface consists of 3/4″. Tongued and grooved flooring nailed to 2″.x 2″. Timber battens at 16″ centres. The latter are grouted into spaces between the concrete floor units. The ceiling is of 3/4″. Plasterboard fixed to dove-tailed wood battens fitted between the floor units.
The construction of the structural roof is similar to that of the first floor, but it is covered with a 2 in. screed of foamed slag concrete laid to falls and finished with two layers of two-ply bitumen felt. The first floor ceiling is similar to that of the ground floor.

Calculations show that the structural framework has been designed in accordance with the Code of Practice for reinforced concrete. A modified frame of similar layout, but with the column and beam sections considerably reduced, was erected and tested at the Building Research Station. The test showed that the strength of the frame was adequate, that deflection under the proof loading was small, and recovery after removal of the load was satisfactory.

The roof covering consists of two layers of two-ply bitumen felt. When laid by skilled men and using the bitumen compound supplied by the makers, this finish provides a satisfactory roof covering. The outer leaf of the external walls consists of 11/2″ thick vibrated concrete blocks with rebated vertical and horizontal mortar joints. The concrete slabs are expected to resist moisture penetration, but the vertical joints may crack and admit moisture. Normally, any moisture penetrating through the joints would find its way to the concrete sole-plate at the bottom of the wall and through the weep holes provided. However, the cavity is interrupted at various levels by concrete bands, and it is difficult to ensure that all water finding its way through the outside leaf will be thrown clear of the foamed slag concrete inner leaf. There is an appreciable risk of moisture penetration with any construction of this kind. The insulating compound applied to the ground floor has a bitumen emulsion as a binding emulsion, which is considered to be inadequate as a water vapour resistant material. Furthermore, the employment of cork and foam slag fillers will likely result in a porous material.

The thermal insulation of the roof and ground floor is satisfactory, but that of the external walls falls considerably short of the standard suggested in P.W.B.S N0.I and is slightly better than that recommended in the Housing Manual, 1944.

All sanitary fitments are grouped centrally near to plumbing ducts and provided with access panels for maintenance and inspection. Electrical conduits in pre-cut lengths are laid in the walls and floor cavities, or behind hollow skirting.

Site Man Hours & Cost
Frame Assembly – 30 man-hours.
Complete Structure – 230 Man-hours
Completed House – 800 – 900 Man-hours
Cost at time of erection £880 for 880 sq ft