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Shed Roof Construction Should Be Simple – But Is Not Always Easy!

Reading this guide to shed roof construction should help you along the way.
The main components of a shed roof are:

1. Load bearing structure (rafters and roof trusses)
2. Insulation
3. Roof decking
4. Shed roof covering

1. Load bearing structure

The rafters and roof trusses support the roof covering, roof decking and insulation. The roof framing is determined by the roof shape. The size of the structural components is governed by the weight of the roof covering and the roof span. The heavier the roof covering and the longer the span the larger the members. The roof design loading is a combination of the dead load of the roof covering, wind load, snow load and access load. The size of these loads vary but an allowance of 75kg/m² is usually made for access and snow load.

The layout of the shed roof construction depends on the outer shape of the roof. I hope these diagrams of roof construction help you to understand the structure that lies beneath.

Pent roof

This simple roof deck construction is a beam spanning from front to back, my kids build houses that look this.

Gable Roof

The simplest of roof truss designs is a triangle, called the ‘closed couple’ roof. This roof truss type is great for shed builders, it is simple to build, sheds water well and with minor reinforcing roof truss storage is easily achieved.

shed roof construction2

If roof storage is not so important the horizontal tie can be lifted for extra headroom to form what is known in the trade as a ‘raised collar’ roof.Care must be taken when making a raised collar roof, the timber sizes must be increased as ‘bending stresses’ can become critical in the sloping rafters.

Hip roof

The hip roof is a development of the gable roof, shed roof construction is more complex due to the compound mitre joints at the junction of the hip and jack rafters. The roof in this picture would be for a large garage and is beyond the skill of most d-i-yers. There are simpler forms of this roof for smaller garden sheds.

shed roof construction

Gambrel roof trusses

Traditional Gambrel roof trusses consisted of large frames at 4.5 to 6 metre centres, as shown in the sketch below.

shed roof construction4

When used for constructing sheds the gambrel roof is built in a very different way. A Gambrel shed is a lot smaller than the large barn structures, the roof is formed with a series of connected roof joists. The mitred ends are joined with a nailed plywood gusset plate. The joists are spaced at 400mm or 600mm centres along the length of the shed.

Salt box shed

The traditional salt box roof is a combination of the gable and pent shed roof construction with a steeply sloping roof.

In the UK roof timbers do not need to be preservative treated unless the ceiling encloses the rafters, then there becomes a risk of condensation and damp becoming trapped and initiating rotting. Ventilation between the roof covering and the top of the insulation is introduced and vents are needed at the eaves and ridge.

There are localised areas in the UK where timbers must be treated because of insect pests (House Longhorn beetle). Most of these areas are in London and the Home Counties, check with your Local Authority if you think you may be affected.

2. Insulation

Storage sheds and tool sheds are often left un-insulated. Garden offices and outdoor garden rooms can be insulated for year round use.

3. Roof decking

Typically this will be plywood or OSB (Oriented Strand Board) – not chipboard. The thickness of material used will depend upon the spacing of the rafters (typically 400 or 600mm as this suits the ply roofing sheets which are 1200X2400mm).

4. Shed roof covering

What will it be, Clay tiles, cedar shingles, felt shingles or roofing felt?

see here for discussion of Shed Roofing options

Click here for more on Shed Roofs

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