Designing your panels

When designing the timber frame structure, it is really useful to consider which dimensions to use for the structural grid or layout in order to minimise cutting processes and waste. Since most board products such as sheathing, lining and flooring are supplied in sheets 2400mm long and 1200mm or 600mm wide, the most obvious structural grid dimensions are 400mm or 600mm.

Next, consider the type of structure. The most common method of designing and constructing a domestic timber frame building which you should encounter is the platform frame system, in which each storey is erected so that the previous floor becomes the platform for the next. The ground floor is usually of solid concrete, upon which the ground floor storey is constructed.

An open panel platform system describes a sheathed timber frame of studs, plates and lintels; covered with a breather membrane and complete with openings for windows and doors. Internal linings, insulation, vapour control layer (VCL), services and claddings are all added on site. A closed panel system is a development of the open panel, in that the wall panels are delivered complete with insulation, VCL and internal linings. The windows and doors can be prefitted and service ducts, cables and pipes can also be included.

Floor and roof panels known as cassettes can also be prefabricated in the factory, in both open and closed panel construction.

The advantages and disadvantages of each option can be summarised as follows:

Advantages Disadvantages

Less expensive to produce

Faster to fabricate in the factory

OPEN PANELS

Easier to adjust on site

Less expensive to repair if they get damaged in transport or on site

Easier to dry our if they get wet during construction works

Requires less work on site

Quality control of fittings, openings, airtightness and insulation is easier to manage in a factory than on site

CLOSED PANELS

Less likely to be tampered with by follow-on trades

Doesn’t save as much time on site

Less quality control over the fitting of insulation and membranes on site

Requires more work on site

Can suffer abuse from other trades, eg. notching of timber members

More scope for errors around openings

Requires a detailed design by the manufacturer

Less scope for adjustment on site

More complex to repair if they get damaged in transport or on site

More expensive to produce

More difficult to dry out if they get wet internally during construction works