Explained
How a house wall and roof works.
A wall is a sandwich
The walls of a timber-frame house look like one solid thing, but they're a carefully ordered stack of layers. Each layer has a single main job: carrying weight, keeping out water, holding in heat, or giving a finished surface. Slice the model and you can follow those layers from the inside of the room to the weather outside: drywall, studs with insulation between them, sheathing, a membrane and finally the cladding.
The structure is the timber frame. Vertical studs stand between a bottom plate and a doubled top plate, and they carry the loads from the roof and any upper floors down to the foundation. Fixing sheathing panels of OSB or plywood across the outside turns the frame into a stiff box that resists being pushed sideways by the wind. Between the studs, insulation batts trap still air, which is a poor conductor of heat, so less heat escapes in winter and less gets in during summer.
Keeping the water out
Outside the sheathing, a breathable membrane stops liquid water while letting water vapour escape, so the timber stays dry. The cladding goes over the top. Each board laps over the one below, so rain runs down the face of the wall and drips off the bottom instead of being driven in. The roof works on the same idea: tiles hang on horizontal battens and overlap downhill like fish scales, with another membrane underneath to catch anything blown past them. Water is always encouraged to flow down and out, never in.
From roof to ground
The roof structure is a series of sloping rafters running from the top of the wall to the ridge. Each one carries a strip of tiles, plus snow and wind, and passes that load into the wall at a notch called a birdsmouth. From there the load travels down the studs to the bottom plate, through the concrete slab edge and into the foundation. The footing at the bottom is wider than the wall so that the weight is spread over a larger area of soil. In cold climates it's set below the frost line, the depth to which the ground freezes in winter, so that ice forming in the soil can't push it upwards. Follow that chain of parts and you've traced the load path, the most important idea in building design.
Parts glossary
- Foundation
- A concrete footing and foundation wall spread the weight of the house into the ground so it doesn't sink or settle unevenly. The footing is wider than the wall above it to lower the pressure on the soil. Fun fact: In cold climates footings are placed below the local frost line so freezing ground can't heave them upwards.
- Floor slab
- Concrete poured over compacted fill and a damp-proof membrane forms the ground floor. Steel mesh inside it helps control cracking. Fun fact: Concrete hardens through a chemical reaction with water called hydration, not simply by drying out.
- Wall studs
- Vertical timber studs, tied together by a bottom plate and top plates, carry the roof and floor loads down to the foundation. They're spaced evenly so sheet materials can be fixed to them.
- Insulation
- Mineral wool or fibreglass batts fill the cavities between the studs. They trap still air, which slows the flow of heat through the wall in winter and summer.
- Sheathing
- Panels of OSB or plywood fixed to the outside of the studs stiffen the frame against wind. A breathable membrane over them keeps rain out while letting water vapour escape.
- Cladding
- Overlapping boards on the outside shed rain and protect the layers behind from sun and wind. Each board laps over the one below so water runs down and off.
- Rafters
- Sloping timbers run from the top of the wall up to the ridge and carry the weight of the roof covering, plus snow and wind. Where they sit on the wall they're notched with a cut called a birdsmouth.
- Roof tiles and battens
- Tiles hang on horizontal battens and overlap like fish scales, so rain runs from one to the next and into the gutter. Fun fact: Fired clay roof tiles have been used for thousands of years, including in ancient Greece and China.
- Drywall
- Gypsum plasterboard fixed to the inside of the studs gives a smooth, paintable surface and hides the wiring and insulation. Fun fact: Gypsum contains chemically bound water, which is released as steam in a fire and slows the spread of heat.
