We talked about the main garden office construction methods in our Garden Office Structure guide, but here we explore the different aspects of floor construction further.
- A sturdy floor is a must
- A floor needs ventilation
- A good floor is well insulated
- The floor and foundation work together
- Not a home for rodents
When reading specifications, or visiting show buildings, you need to pay particular consideration to the floor construction. There’s nothing worse in a timber garden building than a ‘springy’ floor! How can you avoid this we hear you ask, well you need to ask your supplier how close together the joists are. A good quality floor will have the joists close together – between 400mm and 600mm apart, some cheaper garden offices space the joists further apart to save on materials!
As well as having the joists at narrow spacings, a perimeter framework will strengthen the floor frame, it seams obvious, but we have seen some garden office designs with open ended joists, which without the strength of the perimeter framework are liable to twist – not good for creating a level floor!
Finally a good garden office floor is strengthened with the addition of a structural floor deck – a layer of structural board laid over, and fixed into the joists. This floor deck is also beneficial in forming a firm basis for the final floor covering, whether that’s laminate or carpet.
If you get the opportunity to visit a garden office before you buy, take a small jump up, and see how much ‘bounce’ there is – you should expect the floor to be as firm as those upstairs in your house.[hr]
Its important that air can circulate under the floor of a garden office, particularly with timber floors. If the floor area is not ventilated you are at risk of damp penetrating, rot and fungal attack – all bad things.
Designers are well aware of these problems and design their floors so that air can freely flow. Some of the foundation systems used in garden office design create a suspended floor, which lifts building up from the ground allowing air to freely circulate. When a concrete slab foundation system is used designers either lift the floor frame off the slab with bearers, or create ventilation slots at both ends of the floor.
A floor should be as well insulated as other elements of a garden offices structure, but in some designs the floor insulation is compromised and in a few cases non existent.
Like in other elements of the structure, there is a lot of choice when it comes to the type of insulation used in the floor, the most common today is rigid sheet insulation’s which have high thermal values but are relatively thin.
Many garden office suppliers try to meet and in many cases exceed the insulation values laid out for new houses, this is a good thing. The thermal performance is known as the u-value, and is measured in W/m2k, its worth noting that the lower the value the better the performing when comparing different structures.
Its not only the insulation that has a u-value, all the layers in the floor make-up have a value too, and your supplier will have used software to assess the value of the whole structure.
There are lots of options when it comes to the foundations used in a garden office construction, we explain them in detail in our foundations guide, but the foundation system you choose will have an effect on your floor structure.
Concrete slab foundations are popular, as they form a solid base that spreads the weight of the building evenly. In some designs of garden office this concrete slab also forms the floor structure of the office. In this case there is no timber sub floor, and the wall frames sit directly on the concrete slab – this is quite a useful trick when designing a garden office that’s going to come in at less than 2.5m high, as the concrete slab is usually flush with ground level.
Its important if the concrete slab is being used as the floor of the garden office, that it is insulated – a standard concrete slab foundation isn’t, so check with your supplier. There are two key ways of insulating a concrete slab – one is to place the insulation under the concrete slab i.e. lay the insulation boards before the concrete is poured. The other option is to lay the insulation on top of the concrete slab, it all depends on the designers preference and how they want to achieve their target u-value.
Timber floor on concrete slab
A concrete slab also creates a good basis for a timber floor. A framework made of a perimeter frame filled with joists is laid over the concrete slab, some suppliers use timber bearers running in the opposite direction to the joists to lift the frame slightly to allow good ventilation, other suppliers lay the joists directly on to the concrete slab.
A timber floor on a concrete slab is the best of both worlds, you have the solidity of the concrete slab supporting the joists, with the thermal performance of a well insulated timber floor.
Pad foundations create suspended floors
Spaced pad foundations have become very popular in garden office construction. They are spaced evenly beneath the floor, and a timber floor connects the pads, and spread the weight evenly across the foundation. This setup creates a suspended floor system, which allows good air flow underneath the floor, and keeps the structure of the floor away from the ground. The space between the joists provides valuable space for insulation.
SIP’s are used for floors too
We’ve mentioned the word joists several times in this article, but not all garden office floors have traditional joists, this is because they are made from Structural Insulated Panels (SIP’s), these are sandwiches of structural boards with a form insulation in-between, because of the structural board the load is distributed evenly without the need for joists. Although SIP’s floors don’t have joists every 400mm or so, they do normally have a timber perimeter framework.
SIP’s are highly thermally efficient because there are no timbers breaking the run of insulation.
Custom floor frames
Some suppliers have created a metal framework which has legs which sit on concrete pad foundations. This grid work is designed to take insulated floor panels which are made up of a sandwich of structural board and insulation. This system lifts floor off the ground, so air will freely flow.
Whilst it is important to allow air to flow under the garden office, a void under the building can potentially form a habitat for rodents, who are attracted to the warm dry space. There are two schools of thought on how to deal with this issue, one is that if there is a gap all round the base of the building the rodents can run through the space. Other suppliers drop the wall cladding down beyond the floor level to close the gap around the floor, however with this solution you have to be careful not to block off the air flow!
It is this concern that leads many people to choose a concrete slab foundation, with the feeling that there is no void for the rodents to set up home in.
Rodents are burrowers and they are known to burrow under the floor, some suppliers have been known to bury wire mesh around the perimeter of the garden office, they fit this vertically and deeply with the theory that the rodent won’t burrow through.
Another measure you can take is not to make the area inviting for rodents. Rodents are looking for materials they can make a nest with, and so try to leave materials that will assist them away from the floor. For instance we wouldn’t use a fiber insulation that is exposed in the floor