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Construction Sequence

Please watch our animation of a Block Deck base assembly and the construction of a timber frame garden room studio.

Construction of SIP panel garden room studio


It is important that your building is situated on good solid ground. If your excavations bring up fill material such as bricks and builders debris, additional excavations and stone infill will probably be necessary. Please contact us for advice. The base of the excavation should be level and sides should be battered back by a maximum of 45 degrees.

The depth of excavation depends on the thickness of your base insulation and the height you want the finished floor above ground level. It is generally recommended that the top of the Block Deck framework finishes 150mm above your external ground level, but this can be reduced if a gravel margin is added around external areas of hardstanding to prevent splash back. Typically, an excavation depth of 250mm is suitable. Also consider down pipe locations, drainage and ducting of services at this stage.

Hardcore base

The hardcore material can be natural quarried stone, or recycled aggregate, dependent upon local availability. This can be supplied by a local aggregate supplier, or in 1 Tonne bulk bags from builders merchants.

The hardcore material should ideally be MOT Type 1 material but this depends on your local availability. The stone must be compacted in 100mm layers using a vibrating (Wacker) plate to ensure maximum consolidation. Wacker plates are widely available at hire shops.

Sand blinding

A blinding layer of sand is laid onto the compacted hardcore to ensure there are no sharp stones sticking up which may pierce the layer of damp proof membrane DPM plastic sheeting. A depth of 30mm is sufficient. This material can either be builders sand, sharp sand or recycled sand.

Once laid, the sand needs to be compacted again using the Wacker plate. The top of the sand must be finished level.

Damp proof membrane

The Damp Proof Membrane (DPM) should be a minimum of 1200 Gauge (300mu) and should be laid out flat onto the sand blinding.

Cut the DPM 500mm wider all the way round the footprint of the building to ensure there is sufficient lap up the sides of the Block Deck framework.

Floor insulation

Typically, around 15% of the heat loss within a building is lost through the floor, therefore the level of insulation needs to be determined, dependent on the use of the building. This should be a minimum of 50mm to be effective.

The insulation needs to have sufficient compressive strength to resist the weight of the Block Deck, the structure of the building, internal floor loadings and any point loadings e.g. heavy gym equipment. The floor insulation is a high compressive strength rigid thermoset PIR board and is different to the insulation generally used in wall cavities. A typical product would be Kingspan TF70. Please contact us for recommendations.

Over boarding insulation

Lay 2400mm x 1200mm sheets of timber boarding on top of the insulation to prevent damage by any point loadings or accidental damage during construction.

This should be 18mm thick OSB3 boarding, which is suitable for humid conditions.

Assembly of block deck framework

The Block Deck can be assembled in situ following the assembly instructions. All steelwork and holes are supplied the correct length and position and it is a simple case of bolting together the framework using the bracket and bolt package supplied.

The Block Deck is designed so the concrete blocks can be installed after the Block Deck is assembled. Think about services you may wish to install below the floor deck and drill through any holes in the Block Deck base required for services at this stage. Fit rubber grommets to any drilled holes to prevent damage to the cabling. Predrilled holes can be provided in the Block Deck base if you let us know your requirements.

Fixing of building sole plate timbers

To ensure your sole plate is correctly positioned, clamp the timbers into position and drill through the timber and the Block Deck steelwork, as required. This is generally at 600mm centres. Fixings such as galvanised coach bolts (M16) are then used to fix the timber sole plates to the Block Deck framework.

If the sole plates are to be used for a SIP building, ensure the timbers are inset by the thickness of the outer boarding of the SIP panels (11mm) and allow clearance for the panels internally at the corners for the SIP panels to run through.

Fitting of concrete blocks

It is important that the concrete blocks used for infilling the Block Deck are Standard 100mm thickness dense concrete blocks, rather than medium density, aerated or “Thermalite” type lightweight blocks. The blocks should be around 18kg each.

Lay the blocks between the Block Deck beams, placing one end of the block into the closed channel first, before lowering the other side into position along the open sided channel. Leave gaps of around 10mm between blocks.

Pea gravel infill

The concrete blocks are bedded into position by infilling the voids between the Block Deck framework and the concrete blocks. The Pea gravel should be between 6mm to 10mm, dependent upon your local availability.

Pea gravel is available from your local builders merchants in 1 Tonne “jumbo” bags. The bags can be lifted above the Block Deck using an excavator and the bottom cut to let pea gravel run free. Alternatively, purchase 25kg bags. Fill to the top of the Block Deck framework, leaving approximately 10mm gap at the top.

Addition of side insulation

The sides of the Block Deck framework have a perimeter lip, under which should be installed the perimeter insulation. This insulation can be the same as the base insulation if you have some left over and should be 50mm thick. The side insulation does not need to be the same high density as the base – normal wall insulation is perfect which is generally cheaper and more readily available.

The insulation is cut into strips so that the side insulation sits directly onto the base insulation. Also infill the open ends of the side channels where required.

Lapping of DPM over block deck

The Damp Proof Membrane (DPM) laps up the outside face of the side insulation and up onto the top face of the timber sole plates. The DPM is shown in blue on this drawing. Trim the DPM neatly along the inside face of the sole plate timbers and fix into position.

Trim to perimeter DPM

To protect the DPM at the sides, especially during the construction of the building, fit a 300mm wide damp proof course DPC to the side of the base. This can be fixed to the outside of the sole plate timbers. The bottom of the damp proof course can either by glued onto the DPM or secured by backfilling the ground up to it. DPC is generally available from builders merchants or Screwfix.

Laying of flooring

Lay your tongue and groove flooring onto the Block Deck grid. The flooring is generally supplied in 1200mm x 600mm panels and should be 18/22mm thick, running across the Block Deck framework. The flooring is fixed to the Block Deck steel beams using timber Tek screws. These screws have a drill point at the end to drill through the timber and the steel and then fix into position.

Ensure the flooring is water resistant, for example 22mm Egger Protect tongue & groove chipboard. Joints are glued following manufacturers recommendations and to stop squeaking.

Installation of SIP panel wall sections

This shows the simplicity of constructing a garden room in SIP panels. The SIP panels are recessed to all sides and simply slot onto the sole plate timber below and each panel slides into the next using simple joints. Each joint is either made up using a SIP’s insulated spline section which slots into the recess, or using standard timber joists. The timber joists added to the top of the SIP’s panels secure the wall panels and provide fixings for the roof panels. The SIP wall panels are 100mm thickness.

Consider covering the chipboard flooring with a waterproof non-slip material if assembly is taking place in wet weather.

Install SIP panel roof sections

The roof panels are thicker than the wall, dependent on the span of the building required, but generally the roof is 150mm thickness. They are installed the same way as the wall panels, with timber joists set into the recesses of each panel to secure them into position.

The edges of the SIP panel roof are finished using 150mm thickness timber joists around the perimeter. This provides a fixing for the gutter, the roof membrane sides and also provides an overhang to all sides of the roof to help protect timber side cladding.

EPDM Roof membrane

The roof has a slight fall front to back, which is created by mounting a 100mm wide firring strip to the top of the SIP panel walls. The fall is minimal so that the building has the maximum height inside the building whilst allowing rain water run-off. Rather than fitting a traditional box profile roof cladding which generally requires a greater roof pitch, it is common to fit an EPDM roof membrane. This is similar in texture to a pond liner fabric, but is especially made for this type of roof and is generally available with a 20 year guarantee.

The membrane is supplied in one piece, like a roll of carpet and is rolled onto the roof and glued into position. The sides are fastened to the sides of the SIP panel roof sections.

Clad walls & fit doors & windows

It is important to fit the roof membrane fairly quickly once the SIP panel construction is complete to provide protection from the weather. The SIP panels are constructed using OSB3 timbers, which are resistant to moisture, but it is best to make the structure water tight as quickly as possible. Once the roof is on, the walls can be clad and the windows and doors can be fitted.

It is standard practice to wrap the SIP panel structure in a breathable building wrap material before fitting timber battens to the outside of the SIP panels and then fitting the wall cladding. This air gap between the SIP panel structure provides air flow to prevent condensation and mildew. The wall cladding can be either vertical or horizontal.

External finishes

Complete external works to the building ensuring a sufficient vertical gap between hard surfacing (concrete slabs) and your finished floor levels. The finished height will be established from the depth of your initial excavation and can be varied dependent on insulation thickness, stone depth etc.

Consider using 150mm gravel border around areas of hardstanding to help prevent splash back onto timber wall cladding.

Install guttering and downpipes and connect to drainage.

Internal finishes

Fit your choice of internal finishes, such as plasterboard or cladding to the walls. Install laminate flooring to the chipboard flooring deck and fit architraves, skirting boards and complete electrical installation.

Please contact Foundation Buildings on:

07538 775060

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