Timber frame house in Batumi: built on site from kiln-dried timber

A timber frame house in Batumi is built from sawn timber right on the plot. The walls, floors and rafters are put together from studs and joists in a few standard sizes, and the wall gets its stiffness from sheathing boards nailed to the frame at a calculated nail spacing. The Adjara coast tests this method in one place: moisture. Almost everything we do differently from a dry climate is about how the wall gets wet and how it dries out again.

How a timber frame wall is built

We describe the wall layer by layer from the inside out, because here the order of the layers matters more than their thickness:

• an inner lining of plasterboard or tongue-and-groove boards, with battens behind it that leave a service void for cables, so that sockets don't cut through the membrane;
• a vapour barrier membrane with taped overlaps;
• studs of kiln-dried timber, with mineral wool between them and no gaps;
• external sheathing: OSB/3 or a vapour-permeable wind barrier board, depending on the wall calculation;
• a breather membrane that lets vapour out and keeps water out;
• battens for the ventilated air gap, usually 40–50 mm in our houses, and the facade cladding on top.

The floors are built from joists of the same dry timber, only in a larger section. The house goes up one storey at a time: a floor platform, the ground-floor walls on top of it, then the next platform. Each floor also closes off the wall cavity, so fire or a draught can't travel up it from one storey to the next.

A vapour barrier for air conditioning and heating

In a northern climate vapour moves from the warm house outwards, and a dense membrane on the inside stops it. In Batumi that is how it works in winter, but in summer it is the other way round: the air conditioning is on indoors, it is hot and humid outside, and vapour moves from the street into the wall. If the inside has a membrane that lets no vapour through at all, in summer moisture condenses on it on the insulation side and has no way out into the room.

So the engineer checks the wall build-up with a moisture calculation, and we more often use a membrane with variable vapour permeability: in dry air it is almost closed, in humid air it opens slightly. The calculation means little if the membrane is torn. We tape the overlaps, fit sealing collars around pipes and cable entries, and take the membrane onto the window frames. The site foreman photographs every wall before the boarding goes on, and the photos have to show that the tape is pressed down along its whole length.

Timber, drying and preservative

Green timber hardly dries in humid air. Closed up in a wall, it gives its water to the insulation, warps and pulls the plasterboard with it. So for the frame we use only planed, kiln-dried timber, and we check every delivery with a moisture meter when it arrives: timber wetter than 18–20% doesn't go into the wall and goes back to the supplier.

We apply preservative before assembly, on all sides of each piece. Ends cut on site get a second coat, because water gets into wood mainly through the end grain.

Timber, boards, mineral wool and membranes are sold at building markets and shops in Batumi, but planed kiln-dried timber in the right sections is not always in stock in the quantity a whole house needs. So we order it in advance from the timber schedule in the design, leaving time for delivery.

Foundations and plinth

A timber frame house is light, and its foundations are simpler than those of a masonry house. On a level plot we usually pour a cast-in-place slab, which also serves as the base of the floor. On a slope we more often end up with bored piles tied together by ground beams: the house stands level without major excavation, and there is no need for a retaining wall under the whole footprint. How the choice depends on the ground is explained in the comparison of pile and slab foundations, and terracing a plot is described on the page about houses on a slope.

Screw piles suit a small timber frame house set back on the plot, if the calculation confirms them and the shaft is protected from corrosion. For a house on the seafront, where the ground is wet and salty, we don't use them. Either way, the sole plate goes on a damp-proof course on a plinth that we raise at least 30 cm above the ground, and if the house stands on piles, the crawl space is ventilated through vents covered with metal mesh.

Earthquakes and wind: what holds a light house in place

A timber frame is much lighter than a masonry house of the same floor area, and an earthquake puts only a small force on it. Its weak point is elsewhere: the connections. In a tremor, and in a storm wind off the sea, a light house tends to slide on its foundations, and the corners of the walls tend to lift.

So the structural engineer calculates three things: the hold-down anchors at the ends of the walls, the nail spacing in the sheathing, which works as a shear wall, and the connection between the rafters and the walls. A large window takes away the length of wall over which the sheathing works, so a panoramic opening usually gets a steel frame or an extra solid wall next to it. Heavy roof tiles or a concrete screed on a timber floor add mass at the top, and this goes into the calculation too. How a timber frame differs from a steel one by the sea is set out in the comparison of light gauge steel and timber frames.

Fire safety in a timber frame house

Timber burns, and the fire safety of a timber frame house depends on the layers around it. Mineral wool doesn't burn, and on the inside the studs are covered with plasterboard, which holds back fire. Rafters and joists in the loft, where the timber stays exposed, get a fire-retardant treatment. Where the flue of a fireplace or stove passes through a timber floor and the roof, we fit a non-combustible firestop with a clearance to the joists. Cables inside a timber frame wall run in protective conduit, and junction boxes go only where they stay accessible. Setbacks from the neighbours and distances between houses are set by the zone rules, and we check them against the extract for the plot at the sketch stage.

Where a timber frame makes sense and where it doesn't

A timber frame works well on a slope, where every tonne of the house adds to the cost of the foundations, in the mountains, where concrete is hard to bring in, and wherever the shell has to be closed in before the autumn rains. It also suits a guest cabin next to the main house. If you would rather have the walls assembled in a workshop and only installed on the plot, take a look at houses built from factory-made panels.

A timber frame copes less well with long winters with nobody in the house: in a locked, unventilated house the humidity stays high for months, and the sole plate is the first to feel it. A lower ground floor that holds back the soil of a slope needs concrete, and the timber frame can go on top of it. For upkeep and repairs, a timber frame is compared with solid timber walls and with panels on the pages log house vs timber frame house and SIP panels vs timber frame.

How much a timber frame house costs in Batumi

The shell of a timber frame house costs from 750 to 900 ₾ per m² of floor area. This price covers the foundations, a frame of kiln-dried timber, insulation, sheathing, floors, the roof and windows. Building services, finishing and the design are not included. A turnkey timber frame house starts from 1,620 ₾ per m².

An aerated concrete shell costs the same, from 750 ₾ per m², so a timber frame doesn't win on the price of the walls. The savings come on the foundations on a slope and on time: the house is closed in against the rain sooner, and the crew loses less time to bad weather. Two separate lines come up most often: a screw pile with a head plate for a small house costs from 300 ₾ each, and a timber facade of rhombus boards on a subframe adds from 260 ₾ per m² of facade.

Costs go up with a slope that has no vehicle access, panoramic windows and snow in the mountains, and come down with a simple shape and a gable roof on a level plot. We send a preliminary estimate from the plan and photos of the plot within 24 hours on business days, and the final cost is set on the basis of the design, after a visit to the plot and once the estimate is agreed.