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Glossary

Asphalt Shingles

Made from asphalt and available in a wide range of colours asphalt shingles have the effect of a tiled roof. Asphalt shingles come in strips of three tiles which are butted together horizontally and are lapped vertically. Asphalt shingles are fixed over a under felt and on top of a roof deck such as plywood or OSB.  Each strip of shingles has a bitumen strip which needs to be heated to glue it to the shingle on top – this stops the shingles lifting in the wind.

Batten & Board Cladding

A type of external cladding where wide boards are fixed vertically and narrow battens are fixed over the joints in the boards.

Blanket & Batt Insulation

Blanket and batt insulation is normally packed between rafters, studs or joists. It comes in several types such as fibreglass, rock wool insulation or natural materials such as sheep’s wool or hemp. The disadvantages of this type of insulation are that it creates cold spots, areas where there is no insulation such as where the joist, studs and rafters are.

Breather Membrane

Breather membrane is fitted over the outside of timber sheathing and is designed to stop moisture entering the building framework. Bitumen papers used to be used but modern fabrics are now replacing them. Joints of breather membrane should be well lapped and sealed.

Building Regulations Part L

Whilst garden rooms don’t have to comply with Building Regulations unless they are being used for sleeping accommodation they are better performing if they are built to Building Regulation standards. Part L of the Building Regulations is particularly relevant as it addresses the insulation values of building elements. The Building Regulations state target u-values for walls, floors and roofs as well as doors and windows.

Cedar Shingles

Cedar shingles can be used as an external cladding on walls but are most commonly used as a roof covering. Made from Western Red Cedar they come in random widths, each shingle is nailed with two stainless steel nails in the middle of the shingle, into treated tile battens. The overlap of each course depends on the pitch of the roof but at any point on the roof there are at least three layers of shingle. A cedar shingle roof is very durable and will last for more than 25 years. Cedar roofs start out a reddish brown colour but weathers to a silver grey colour with time.

Cold Roof Construction

Cold roof construction is when the roof insulation is fitted between the roof rafters. There must be an air space between the top of insulation and the roofing felt, and this air space must be ventilated at the eaves with soffit vents. You often need a greater thickness of insulation in a cold roof to achieve the same u-value as you would get with a warm roof. A cold roof has ‘cold spots’ over the actual rafters where there is no insulation.

Cold Spots

Cold spots are areas in a building where there is no insulation such as corners of a building where timber frames meet. Cold spots are common in timber frames where the insulation is only found between the joists, studs or rafters. The solution to cold spots is to have insulation wrapped around the outside of the timber frame or use a structural insulated panel system (SIP’s) where there are no timbers to break the thermal envelope.

DPC

A Damp Proof Course is normally used in brick wall construction but it is also used in timber frame construction. Normally a reinforced plastic strip is used, this strip is placed under the joists if they sit on a concrete slab, the DPC stops moisture from the concrete permeating into the timber joists.

DPM

Damp Proof Membrane is high density plastic sheeting that is laid under a concrete slab and stops moisture from the ground being sucked up into the concrete.

EPDM Roof Covering

EPDM is a form of rubber roof covering normally used on flat roofs; it has replaced the use of bitumen roof coverings in such instances. EPDM is extremely durable and has a life expectancy in excess of 50 years. For best results EPDM should be laid in one piece.

External Cladding

On garden rooms the external cladding is commonly made from wood, the wood used is shaped into different profiles such as tongue & groove, shiplap, tapered weatherboard and board & batten. Timber claddings are made from many different woods such as cedar, oak and larch. External cladding can also be made from UPVC or metal which is often lower maintenance than timber cladding.

Featheredge Boards

A type of external cladding, also known as Tapered Weatherboard. Boards are lapped horizontally one on top of the other – at any point there are two layers of board.

Floorboards

Floorboards are fixed over the joists and many different types are used in garden room construction. Composite sheets such as chipboard are the most basic and require a further decorative cover such as carpet, softwood tongue & groove floorboards are sometimes used and can easily be stained and varnished. Hardwood floorboards such as oak or walnut offer a luxury finish but wood effect laminate is also popular.

The flooring chosen will dictate how it is fixed, some laminates will float over a sub floor such as chipboard, whilst floorboards will be nailed directly into the joists, and a secret nailing technique is used so no fixings are visible to the eye.

Gable End

The gable end is the pointed section of wall between two planes of the roof.

Hipped Roof

A hipped roof is a roof with a pitched slope on all four sides of the building. The pitch of the slope is the same on each side. The point where the slopes intersect is called the hip.

Joists

Joists form the framework of the floor; they are normally made from wood but can also be made of materials like steel. Joists are fixed to header and footer plates which run in the opposite direction to the joists. To strengthen the floor noggins are fixed between the joists. For long expanses of joist, engineered timber joists are used, these are called I-joists and as the name suggests are shaped like a capital letter I.

Living Roof

Also known as a Green Roof or Sedum Roof a living roof is a roof covering made from plant life. The roof is protected by various layers of waterproof membrane, on top of this there is a soil substrate and the layer of vegetation. Many types of plants can be used in a living roof such as grass, but the most common and low maintenance in garden room construction are sedums. Living roofs are good for the environment as they absorb CO2 from the atmosphere and replace an eco system for birds and insects.

Metal SIP’s

Commonly SIP’s (Structural Insulated Panel’s) are made from structural wood sheets and ridged insulation but they can also be made of metal. Steel or aluminium is laminated (glued) to a ridged insulation to form a structurally insulated panel. The metal panels are often finished with a powder coating which is very durable and low maintenance.

Noggins

Noggins are small lengths of timber fixed between joists or studs to strengthen the framework.

OSB

OSB stands for orientated strand board. OSB is an engineered wooden product made by layering and gluing strands of wood in different directions, its layering the wood in different directions that makes the board strong and suitable for structural use. OSB is often used as the structural panel in SIP’s construction.

Pilkington K Glass

Pilkington K is a type of energy efficient glass produced by Pilkington Glass. Pilkington K is the coating on the inner pane of glass in a double glazed unit, this special coating reflects heat back into the room. In the summer it has the opposite effect and keeps the room cooler. Pilkington K glass helps doors and windows achieve high energy ratings to comply with Building Regulations Part L.

Rafters

The structural timbers on a roof are known as rafters. On a pitched roof the rafters run from the top plate of the wall to the ridge board. For wide spans the rafters may have a horizontal brace to strengthen them. Rafters are normally spaced between 400 and 600mm apart, the closer the spacing the stronger the roof structure.

Ridge Board

The ridge board runs between the gable ends of a building and the top of the rafters are connected to it.

Rigid Insulation

The main advantage of rigid insulation is that a high u-value can be achieved with a relatively thin layer of insulation. Rigid insulation types include extruded polystyrene, expanded polystyrene, polyurethane and polyisocyanurate.

The environmental benefits and disadvantages of rigid insulation have to be weighed up – gasses used in the manufacturing process of rigid insulation can be ozone depleting but the thermal performance of rigid insulation out does other types of insulation, so less energy will be needed to heat and cool the building.

Roof Deck

It depends on the roof covering chosen as to whether a roof has a deck. EPDM and Asphalt shingles are laid over a roof deck. Exterior grade boards such as plywood or OSB are fixed over the roof rafters, on top of this the roof covering is fitted.

Secret Nailing Technique

A secret nailing technique is used on tongue & groove boards whether they are floorboards or cladding boards. The technique involves driving a nail through the tongue of the board at an angle of approximately 45 degrees; the head of the nail is driven flush with a nail press. The groove of the next piece of wood is carefully tapped in place over the tongue and hides the fixing.

Sheathing

Sheathing is fixed onto the outside of timber frames to stop it from twisting, which is known as ‘racking’. Sheathing needs to be made from exterior grade timber, exterior plywood and OSB are commonly used.

Shiplap

Shiplap is a type of external cladding; it gets its name as its profile is used on wooden boats. Shiplap has a scooped top which slots into a groove at the bottom of the piece of wood above.

 Studs

Studs are the vertical members of a studwork wall; they are joined to header and footer plates. Studs are normally spaced between 400 and 600mm apart but are often closer at the corners of a wall or around door and window openings. Studs can be strengthened by the addition of noggins – horizontal timbers fixed between the studs.

Tapered Weatherboard

A type of external cladding where tapered boards are lapped horizontally one on top of another. Tapered weatherboard is commonly found on rustic buildings such as barns. Tapered weatherboarding is also known as featheredge boarding.

Tile Battens

Tile bitterns are normally made out of treated 50 x 25mm (2” x 1”) softwood timber. Tile battens are fixed over roofing felt into the rafters. The spacing of the battens depends on the pitch of the roof and the roof covering used.

Tongue & Groove (T&G) Cladding

Tongue & Groove is a type of cladding commonly used on contemporary garden rooms; it can be fixed vertically or horizontally. As its name suggests the cladding is fixed by inserting the tongue of one piece of wood into the groove of another. T&G can be fixed through the face of the board or a secret nailing technique can be used which hides all fixings.

Under Felt

Under felt is a layer of roofing felt laid under the finished roof covering. Under felt used to be made of bitumen based materials but modern felts are a light weight fabric. Under felt offers extra protection in case moisture creeps through the top roof covering.

UPVC

UPVC or as Europe say it PVCU, is a durable, light and easy to use plastic. UPVC is being used more and more in garden room construction, and is available in a range of colours and wood effects. Garden rooms with UPVC frames are quick to install and require little maintenance. Doors and windows are often made of UPVC  

U-values

U-values are the measurement of how much heat passes through a material. Target U-values for elements such as walls, roofs and windows are stated in the Building Regulations, for example the target U-value for a wall is 0.27.   The lower the U-value the better performing the building element i.e. the less heat is lost through it.

Vapour Membrane

Vapour membrane is used on the warm side of the wall i.e. the inside. Vapour membrane is plastic sheeting which helps control condensation; joints should be well lapped and sealed with tape. Openings (such as for electrical sockets) in the vapour membrane should be minimised and sealed with tape.

Warm Roof Construction

A warm roof is where the insulation is fixed over the top of the rafters, this means there are no ‘cold spots’ as there are no areas of the roof that don’t have insulation covering them. Rigid insulation is often used in warm roof construction and thinner layers of insulation achieve the same u-value  as a cold roof construction. Because the insulation is on top of the rafters there is no need to have a ventilation space in the roof and soffit vents are not required.

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