The framework of the floor should be as sturdy as the rest of the building. There are different types of floor construction which all have different merits; the most important thing with all construction types is that the joists are of appropriate size and spacing.
The most common method of floor construction is to use treated timber joists, which if built on a concrete slab should be laid over a damp proof course (DPC) so that moisture from the concrete doesn’t get sucked up into the timber. The joists should have spacing’s of approximately 400mm, joists may be closer together near the edges of the floor to support the walls and catch the ends of the floorboards. To strengthen the floor, timber should be fixed between the joists, these are known as noggins.
Air should be able to move freely under the floor so small vents should be drilled in the noggins and header and footer joists. Insulation used in the floor will need to be supported so as to maintain this airspace. The insulation can be supported with an insulation netting or on treated battens fixed to the side of the joists, the problem with these methods is that the insulation is open to insect or rodent attack, a better method is to use a layer of plywood under the insulation supported by treated battens.
Rigid insulation offers the best performance by thickness – a much thicker layer of fibreglass insulation would be needed to match the performance of a rigid insulation, but both types of insulation are suitable if a plywood deck is used to support it (as described above).
A vapour membrane is laid on top of the insulation and any joints should be well lapped and taped with a moisture resistant tape. A vapour membrane is important in timber frame construction as it helps control condensation formation.
Finally the floorboards are laid over the vapour membrane, these should be fixed using a secret nailing technique where by the nail is driven into the tongue of the floorboard at an angle of 45°, this forms a secure fixing which is invisible to the eye.
Rather than use treated joists some firms use I-joists which as the name suggests are in the shape of a capital I, the top and bottom of the joist are made from wood and a web of structural board joins them. This engineered product is very strong and particularly useful on big buildings where the joist needs to be very long. Like treated timber joists insulation can be fitted in-between the joists.
The final type of garden room floor construction is a SIP’s floor. SIP’s stands for Structural Insulated Panel’s, they are a sandwich of structural boards with rigid insulation glued in-between. These panels lock together and remove the need for floor joists although they should not be put straight onto a concrete base. SIP’s are very thermally efficient.