Sounds easy enough? Its already got one, shouldn’t need to do much about it? Maybe not but attention to detail in this area may well save a fortune in the long run and any errors made now will be magnified if you have to rework later on. Don’t forget the rules and regulations are always in flux so before you even cost anything up you’ll need to know exactly what the job will entail before you begin.

There are 3 main things to consider: - Internal ceiling height from floor (2.4m taking into consideration finished floor level), roof construction and insulation to current regulations.

There are 3 main types of garage roof: - Pitched, flat and built in / under where the garage forms part of the main house.

Ceiling height

What you need to consider here is the finished floor level. Most garages do not have any type of insulation in the floor so in order to achieve the ‘u’ value specified for habitable floors then it usually has to be raised by as much as 100mm (150 mm in some cases). This obviously lowers the clearance height of the ceiling so in order to bring that back within building regulations the ceiling joists may need raising or in fact the entire construction in the case of a flat roof. (It’s usually cheaper to replace the roof than dig out 8-10” of reinforced concrete and hard-core and re concrete the floor incorporating insulation).

A pitched roof with ‘cut timbers’ (not pre-manufactured trusses) may get away with having the ceiling level joists lifted thereby creating a ‘vaulted’ area (the edges of the ceiling follow the pitch of the roof until it meets the horizontal joist). Usually this isn’t a problem with a standard construction as it should be 2.4m from the DPC anyway but it needs checking.

In the case of a flat roof it is highly likely the ceiling height is about 2.4m or thereabouts from the existing floor level and may entail a lot more work i.e. removal and reconstruction – a lot more expensive. It’s worth asking the question of the building inspector however as some garage conversions done in the past have simply kept the floor level near where it is (so you’d step down into the new room sometimes utilizing ‘space blanket’ type insulation as it’s a lot thinner than polyurethane insulation sheet for the same insulation value).


Roof construction

The first thing to consider would be the roof structure itself. Should the construction be ‘non standard’ e.g. corrugated steel on flimsy rafters then a re-roof would be the way to go.

A pitched roof may need new trusses, felt and tiles incorporating fascia and soffit etc.

A flat roof may need to have the ceiling height raised in which case it’s a complete re-roof again. This usually involves 2 or more courses of bricks built on top of the existing structure and 7” joists on top capped off which fascia on the edges. Insulation would be incorporated into the build before re-felting or fiberglass. (see insulation below)

If the roof is standard and ceiling height falls within limits then a good check over for water tightness and loose tiles will probably suffice.


There are 2 types of roof insulation: - warm roof and cold roof. The difference is the location of the insulation within the roof space.

A ‘cold’ roof has the insulation at ceiling level. This makes the roof space itself the same temperature as the outside hence ‘cold’ roof. The roof will always need ventilation to combat moisture and this could be either ‘breathable’ felt (pitched roof) or by the use of eaves vents and / or ridge vents possibly with the inclusion of ‘tile’ vents (special roof tiles doubling up as vents), or fascia vents (flat roof). Consideration needs to be given to insulating any water pipes within this space and the installation of a loft hatch in a pitched roof situation would be a really good idea. In the case of a pitched roof insulation is achieved in the same way as the main house, loft insulation on a roll. Simple enough.

In a flat roof situation a cold roof can be achieved by insulating underneath the joists with cellotex or similar. This would usually only be an option if the ceiling height permits the lowering of the ceiling. Fascia vents need to be installed to ventilate any cold space above the insulation but underneath the covering.

Sometimes in a flat roof situation ‘u’ values can be achieved without the need for a re-roof and without lowering the ceiling by installing cellotex in between the rafters and possibly another layer (25mm) underneath leaving a gap underneath the finish layer of around 25-50mm which will need ventilating. Technically still a ‘cold’ roof. It all depends on the situation.

A ‘warm’ roof has the insulation at rafter level (in a pitched roof situation) or sits on top of the joists (in a flat roof situation) and is a bit more complex. The reason for having a warm roof would be to utilize the roof space purely for aesthetic reasons dependent on its construction maybe with the installation of one or more ‘skylights’ or velux type roof windows. Another option (pitch roof) would be to convert the space into another room with the use of flooring joists / attic trusses dependent on planning permission and the size of the existing space.

Insulating a pitched roof can be done from the inside once the roof itself has been checked over. Usually consisting of at least 80mm of polyurethane type insulation (kingspan/cellotex etc) between the rafters followed by 25mm of same over/inside the rafters as a ‘thermal break’. This combats any heat loss through the roof timber. Then finally plasterboard. There are currently many different types of insulation ranging from organic compacted sheep’s wool to silver ‘space blanket’ type insulation on a roll, the type you use will depend on budget and the requirements of the job but the one thing you will have to ensure is that it meets current building regulations with regard to ‘u’ value. This figure will be issued by building control upon receipt of your building notice and is an absolute minimum requirement. Its always better to ‘go one better’, especially with rising fuel costs - insulation makes a massive difference to the cost of heating a space. Don’t forget, the lower the ‘u’ value, the better the insulation.

A warm roof in a flat roof situation (normal when re-roofing is required) consists of loft insulation between and to the top edge of the firings (long wedges that sit on top of the joists to form the water run off or very slight pitch) then cellotex of around 80mm thickness followed by 18mm plywood or stirling (cheaper) board followed by the final water proof covering (felt / fiberglass). Water pipes / cables may be routed through the fiberglass and require no further insulation. It is possible to leave out the fiberglass and increase the cellotex on top if the joists need to be visible just as long as the target ‘u’ value is achieved.

There are different maximum ‘u’ values to be achieved in all circumstances and you will need to be sure of the figure before you plan the job and order and materials e.g. an existing flat roof structure converted by means of a cold roof will have a higher value than that of a new warm roof. This is purely to reflect the difficulty of achieving these targets in a given situation. Remember, these figures usually change year upon year.

Built in / under garage

Probably the easiest conversion to carry out, most of the work should already be done for you and its likely to be a relatively simple job.

The internal ceiling height will be the same as the rest of the house and even if that’s not exactly 2.4m or more current regulations allow you to continue provided you don’t make the situation any worse than it already is. It should already be insulated too, if not it should be a simple matter of installing some loft insulation or similar in between the joists before you board over as there will be a habitable room above. It’s not really a roof as such because of this fact.

Before you go ahead and board it though you’ll need to address the electrics and water services required from the conversion. Taking a branch from a radiator in the room above would be the easiest method of installing a radiator in the conversion so routing needs to be addressed. There will probably be a ceiling light but this may require moving and others may need to be installed as well as moving the switch(es) so this is the next thing to consider. Then any electrical sockets will need to be first fixed as the ring main wiring will probably run within the ceiling space and is easy to tap into. Sometimes the electrical consumer unit or even gas meter is in the garage and if these require relocation this is another job. The building inspector / regulations will advise on the level of insulation depending on the use and the type of plasterboard to be used. If the conversion is a workshop it may well require 2 skins of fireproof plasterboard or 1 fireproof and 1 soundproof with a skim finish.

If the room above is a bathroom it would be an ideal situation for an ensuite conversion as domestic water and waste services are very close by and easy to tap into. The same goes for a utility room.

Access door

Something to consider at this time would be the access into the conversion from the main house. If the garage is an extension type construction then the main house wall will be at least 9” thick and probably a cavity wall which means if there isn’t currently direct access and you’ll probably be cutting a hole out from the main house. This opening will need a lintel and 2 courses of bricks removing above the opening to install it. If this will encroach into the roof space, now is the time to do it.

Cavity trays

Something becoming common with local authorities is the regulation to include cavity trays or similar to an installation that shares an external wall above, with an internal wall below roofline. If your garage is a single storey built on extension with a flat roof you may have to raise the height of the roof. This is the time to install your cavity trays.

A cavity tray is basically a course of waterproof material formed in such a way as to direct water back outside that has soaked into the bricks. This is to stop damp manifesting itself underneath the roof structure on the now internal wall having soaked through round the roof structure. The lowest point of the tray must be above the highest point of any flashing so drained water comes out onto the roof and not into the building so make sure you get the calculations right before you cut out the brickwork.

Cavity trays themselves are usually made of plastic designed to interlock with each other as you install them and used with weep vents installed in the perpendicular (perp joint - vertical) joints of the brickwork.

garage conversion roof diagram

The illustration shows roof joists parallel to the fall (slope) of the roof, they can run perpendicular if needed though may need to be doubled up or increased in size depending on the span. Built in joist hangers can be fitted to the wall or a ‘wall plate’ bolted to the house wall and sat on another 4” x 2” wall plate tied down to the external walls with buildings straps. If the roof is not to be upgraded externally it may need a vapour barrier between plasterboard and roof space due to the limited ventilation and / or foil backed (duplex) plasterboard may be specified.

Building control should be able to furnish you with an exact specification if needed once the building inspector has visited the site, the council carry out their own garage conversions so you could even ask them for a copy of their specifications in your area to go with the target ‘u’ values and standard issue regulations. Always remember the building inspector is your friend, their job is to ensure the build meets current regulations but keep them in the loop and they’ll be happy to pop out to site and answer an queries you might have, and may well have some suggestions that will save you time and money. Just don’t argue with them, they’re in that position for a good reason and they’re the people who will pass the job off at the end of the build and they have the power to make you rip it all down and return it to its original state if they’re not happy…

You can safely query any part of any specification and ask them to check anything with regard to a given situation but under no circumstances should you upset them, an unhappy building inspector can make life very difficult for you and the builder….