Making a Splash | Why timber is a great choice for a swimming pool structure

making a splash

Many people might incorrectly think that timber is a risky choice for swimming pool construction. Quite the opposite: timber is a resilient, healthy and robust material to use. It also looks amazing…

10 metre beam, used with a king post truss pushes the boundaries of what can be achieved with green oak in Burdon, Devon

10 metre beam, used with a king post truss pushes the boundaries of what can be achieved with green oak in Burdon, Devon

Carpenter Oak have designed and built many pool-houses across the UK, and one in Russia. Adam Milton, Founding Co- Owner, talks us through timber swimming pool design and installation, and explains why wood is an excellent choice of pool-house material.

Why wood?

“The beauty of timber framing is being able to span large spaces without the need for supporting walls – the perfect method for building over an existing swimming pool,” Adam points out. “Oak frames and swimming pools work very well together,” he continues. “The humidity of a pool gives ideal conditions for slow and steady seasoning of an oak frame.”

Timber may even have acoustic benefits over other materials with hard metal or plaster surfaces, especially when using common rafters. Having a board ceiling with gaps is even more absorbent of sound.

Which woods?

Green oak is Carpenter Oak’s timber of choice, though they also work with green Douglas fir or green larch – the latter is used for any exterior wood. Green timber is not dry. It has been freshly felled (generally not more than 18 months previously) and will typically have a moisture content of around 70%. The beams and big-section timbers used to frame a pool-house will have a lot of water within. This is a good thing.

Timber Frame

Timber Frame

“Swimming pools are obviously a relatively damp ambience, and that’s very different to a domestic house,” Adam tells us. “In a house the timber is slowly drying out at around an inch each year. As soon as the building’s finished, wrapped up and airtight, the heating system is turned on.”

“We always tell clients to do this very slowly because, for the first year or two, timber is drying out and there will be movement. But in a swimming pool you won’t get that. It’s warm but relatively moist, and a timber structure will perform really well.”

Carpenter Oak leave all their internal timber untreated. However, the boarding, above the rafters, is fire treated and sometimes stained.

Use of CLT and glulam

Swimming pool roof

Swimming pool roof

“We are doing more with engineered timber,” Adam points out. Sometimes this is because of client concerns over movement with green timber, but more often it is simply the contemporary
look of the longer spans that are achievable. “With green oak, there’s a length limit of around 9 or 10 metres. But with laminates it’s longer.

Various laminate woods might be used. From oak at the high end – it is more expensive – moving down to Douglas fir, larch or white spruce.”


Posts, saddle stones and surrounds

Posts need to be kept clear of the water area, and the wood prevented from receiving sustained splashing. They need to be held down with steel shoes or saddle stones. The latter could be made of concrete, granite or other stone.

Carpenter Oak have used many different materials, but often use cast concrete piers from 5degree, a UK manufacturer that can supply a “scabbled” exposed finish that Carpenter like. It’s textured
rather than smooth and looks a lot like stone.

When it comes to surrounds and aisles, Adam recommends stone. It’s a slippery and constantly wet environment – he suggests that you don’t really want decking inside.

Swimming pools and scaffolding

Pool-houses are boarded at pool level and sometimes at eaves height in order to put in the scaffolding necessary for construction. If the project is a complex one, a lot of consideration has to go into the scaffold system beforehand.

“We can actually do a lot from the outside and don’t need to do much internally,” Adam says. “But various follow-on trades need a ‘crash deck’ – a scaffold deck – in order to get access to work.”

Swimming pool using green Oak, steel, bolted connections and tierods.

Charlton Hawthrone, Dorset: this is a cleaner look, using green Oak, steel, bolted connections and tierods.

Assembling the frame

“Everything is preassembled in our workshops in 2D, as it were,” Adam states. “For a cruck frame, for instance, we would build however many cross frames lying flat on ground, then take the post and roll it 90 degrees before building the walls. Every elevation is prebuilt in the workshop. As we build it, we’re thinking about how to put it up.”

The same team that builds the frame fits it onsite – they know it intimately. All the elements are coded up and numbered in sequence, ready for assembly.

Watch out for the tannin

Swimming pool with cut and dressed for the saddle stones.

Lyons West Sussex: stainless steel tie rods restrain the spread of the roof rather than timber tie beams. Local stone has been cut and dressed for the saddle stones.

One thing to watch out for if using green woods is the reaction between tannic acid and mild steel.

“If you have a piece of green oak and leave a mild steel object on it, it will turn blue,” Adam warns. “So we always use 316 stainless steel, which won’t react the same way.”

He suggests you do everything you can to protect the floor and surrounds of your pool-house from being stained by tannins, which will leak out when it rains.

“Protect areas such as the stone surrounds. If rain gets onto the frame before the roof is on, you’ll see tannin come pouring down the posts onto any surface. When the roof has been put up, this should stop.”

Ventilation, ventilation, ventilation

Timber frame swimming pool

Coldstone, Gloucestireshire, demonstrates the warm and welcoming effect that timber can provide.

A crucially important consideration is the air handling. This isn’t really a timber issue, but anyone interested in swimming pool work needs to appreciate it. The air circulation is all generally mechanical.

“A swimming pool is not a domestic bathroom – it’s not just like turning on a hot tap. There’s a lot of hot air in a pool environment,” Adam says. “You need a good M&E services specialist to provide this level of detailing, who has expertise in systems to get rid of the hot air.”

“Pipes which are near flat can be designed into the roof zone. We’ve done some where they’re 200mm, circular and galvanized and they are a visible feature. You have ducts coming out and they are exposed rather than hidden behind ceiling buildups.”

He points to the Coldstone, Gloucestershire pool-house, where vents in the floor produce a gentle current of hot air that goes up the glazed wall and prevents condensation. A proper ventilation strategy will prevent water building up and pooling in any areas where it shouldn’t.

Wood has the wow factor

“The design potential of an oak framed swimming pool building is limitless, and there are a myriad options for adding style and sophistication,” Adam says. “Our clients say the benefits of investing in an oak framed swimming pool building really gives you the wow factor when you walk into the room.”

*  This article was originally printed in the #5 issue of Designing Timber