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Before you do anything . . .

. . . You have to ask yourself some questions and answer them honestly and accurately.

  • What do I want to do in my studio – record music, mix music, practice with a band/ individually or all of these things?
  • How loud will you be doing this – don’t guess, buy a good quality sound level meter and measure your activity in decibels.
  • How loud is the environment around the studio in all likely conditions? How much noise do you need to keep out? Again, this needs to be measured.
  • How much money am I able to spend on this? Be realistic and bear in mind that, even if you do most of the work yourself, your project is likely to cost somewhere in the region of £1000 per square metre (!)

I must add that I am not an expert in the field of studio/acoustic design, merely a student. I’m hoping that sharing my experiences will be helpful to anyone wanting to build a project studio by collecting relevant, accurate information together in one place and also helping to avoid the pitfalls that I walked straight into! I should also add that I am not paid to write this blog, receive no advertising income from it or have any commercial incentives. I would like it to be understood that any products, service providers or online entities that I mention should not be considered as endorsements, this is purely a record of my studio build and I assert my copyright.

© John Steel 2020

Getting Started

There is no ‘one size fits all’ method of designing a sound insulated space. To quote the great Steveland Morris (Stevie Wonder to you and me), ” . . . you gots to work with what you gots to work with.” This could be a specifically designed and constructed new build (the ideal solution) or an existing room within or outside your building. In my case it’s a double garage next to my house. There is a lot of advice online about this topic and be warned, most of it is utter nonsense. There are good sources of information which I’ll mention in due course but the first step I recommend is to read two books.

The first one is ‘Home Recording Studio: Build It Like The Pros’ by Rod Gervais, an experienced designer with an international reputation. This is a must read, not only because it details nearly all aspects of studio building but it also explains the physics of sound insulation without being too technical. If you want to build your own studio, you will have to learn to calculate the physical requirements of the space – how much mass to put in the structure, how much air for adequate ventilation and how much electrical power it will need and so on. This book will introduce you to all of this.

The second is ‘Master Handbook of Acoustics, Sixth Edition’ by F. Alton Everest, one of the pioneers of modern acoustics and updated by Ken C. Pohlmann, a professor emeritus and former director of Music Engineering at the University of Miami. This is a more technical book that is focused on acoustics but is still readable and is also essential. Now I’m not suggesting for a moment that this is all the reading necessary – far from it. It’s impossible to know too much about acoustics but these two books are a great place to start, especially if you are new to the subject.

© John Steel 2020

My Design

The first thing I should say is that I’m currently up to the 60 or 70th iteration of this design (and it’s still changing). Of the three years I have been planning, most of my time and effort was put into learning to use a 3D design tool called ‘SketchUp’. I would urge you to do this too, even though it involves a fairly steep learning curve, because it lets you ‘build’ your studio digitally and iron out potential problems in advance. It also has tools for estimating quantities of materials and there is still a free download version available (as of March 2020, look for ‘Sketchup Make’ here:

https://help.sketchup.com/en/downloading-older-versions

I have been told that the free online version is not worth bothering with). All the illustrations on this blog were drawn in SketchUp.

The only really effective way to achieve good sound insulation on a budget is to build an airtight room inside another airtight room. The air trapped between the two layers of mass (or ‘leaves’ as studio builders refer to them) creates a cushion (or spring) which progressively inhibits the transmission of sound waves. Because of this you will most definitely need to install an HVAC system (heating, ventilation, air-conditioning). Some would-be project studio builders might consider this as an unnecessary expense; regarding it as a refinement or luxury.

It is (and I can’t stress this enough) absolutely essential!

Firstly, without it, you won’t be able to breath (or sing or play properly) and secondly, your studio will rapidly become damp, mouldy and infested by insects and vermin. Trust me, I have worked in a ‘studio’ with inadequate ventilation and the four day session left me and the other musicians in the band with chest infections that took weeks to shake off. So be sure to leave enough money in your budget for HVAC – a project like this simply can’t be done properly without it. I have found it’s also by far the most difficult part of studio design to understand, for a number of reasons. As we already know, we’re making an airtight room inside an airtight room. But because ventilation is needed, we have to knock at least two holes through the outer leaf and another two through the inner leaf for the intake of fresh air and exhaust of stale air. Luckily there is a lot that can be done to mitigate the loss of isolation this causes and yes (you’ve guessed it) it involves more thought, planning and sums! I’ll go into more detail about this later, but in short you need to build silencer or baffle boxes to control both airflow and isolation loss in the ventilation system.

Garage Complete~4.jpg

The space I want to insulate is a double garage next to my house. It’s in the South East of England which has a temperate climate, is situated in a residential area, on a hill, and among woodland. The garage is at the edge of a leveled area cut into the side of the hill and the South-Eastern aspect of the building is built into the earth to a height of 131cm (4’4”). It sits on a 4” (10cm) concrete base and the internal dimensions are 4.85m x 4.98m (15’11” x 16’04”). There is a half-hipped roof which is 4.46m (14’7”) at the highest internal point. The walls consist of a single tier of brick and are buttressed on all sides with breeze block and brick columns (or piers), roughly at the centre point of each wall. There is a structural beam which runs from from the North-East wall to the South-West wall at a height of 2.24 M (7’4”). The North-West wall has an outward opening door and window and the South-West aspect has two large openings for the garage doors.

Garage Walls + Joists6.jpg

I have measured the ambient sound level in the area surrounding the garage at 35dB on a windless day and up to 68dB when the wind shakes nearby trees. Passing traffic sometimes elevates the level to 70dB and the occasional light aircraft passing nearby will give a reading of 55dB.
Typically, the loudest instruments I am going to record and play are drums and electric bass guitar. I have measured the drum kit peaking at up to 116 dB. Local noise regulations are non-specific when it comes to defining an actual level at which sound becomes a nuisance, but as we have neighbours at both sides (6m and 12 metres) I am aiming for a sound transmission reduction of 55dB or greater and while this is ambitious, transmission loss calculations suggest that it’s possible (although it’s approaching the limit of what is achievable for a home build studio). I’m going to undertake as much of the work as I practically can (although I will not attempt any electrical work, fitting the air conditioner or acoustic windows or doors – I would rather pay experienced professionals to do those parts of the job).
I plan to build a single room within a room consisting of two leaves (where possible). The outer leaf will consist of the existing single tier, solid brick wall which will be sealed, up to a height of 221cm, then a combination of 18mm OSB and 12.5mm cement fibre board to meet a ceiling of similar construction, supported by the existing ceiling chords.

Upper Outer Leaf 2.jpg

It’s important to remember that sound insulation is dependent on something called ‘mass law’. Broadly speaking, the more mass you add, the more you raise the level of sound insulation that can be achieved ( it also depends on how the mass is arranged, but we’ll get to that later). Be sure to ask a qualified structural engineer if your existing or planned building will support the extra mass that you will be adding. Again, this is not something to be guessed at! I commissioned a report from a structural engineer who advised that with the correct reinforcement to the chords and lowest beam, this would be both possible and safe. He suggested forming a ‘flitch’ beam (i.e. adding a 10mm steel plate with two staggered rows of bolt holes, sandwiched between another identical beam and held together with M12 bolts).

Flitch_Beam_4.jpg

The existing ceiling chords also need be bolted to the joists with M12 bolts.

Chord_Reinforcement_2.jpg

The South-West aspect of the building has two large “up and over” garage doors which will need immobilising, sealing and damping. Then the gaps will be framed with timber, insulated with rock wool and an inner wall of one layer of 12.5mm cement board and one layer of 18mm OSB with green ‘glue’ in between.

garage door gap.jpg

The inner leaf structure will be a conventional stud frame supporting timber framed modules which will be capped with one layer of 12.5mm cement board and one layer of 18mm OSB with green ‘glue’ in between. The remaining space within the frame of each module will be filled with rockwool. This will be anchor bolted to the concrete floor with an air gap of at least 20 centimetres between the leaves, which will be loosely filled with rock wool.

Upside-Down Ceiling V.5 Flitch Beam2.jpg

The inner leaf ceiling frame will need to be reinforced on either side of the main beam with two steel 152x89x16mm universal beams in order to support the inner ceiling in a single span. These will need to be pre-drilled in order to attach the connecting timbers.

Upside-Down Ceiling V.5 Flitch Beam.jpg

The inner leaf ceiling will be built using the same ‘inside – out” design as the walls. The ‘backbone’ will support timber framed modules which will be capped with one layer of 12.5mm cement board and one layer of 18mm OSB with green ‘glue’ in between.

Upside-Down Ceiling 1.jpg
Upside-Down Ceiling 2.jpg

The existing window will be replaced on the outer leaf with a fixed one, glazed with 16mm laminated glass and one on the inner leaf using 12mm glass, conforming as closely as possible to the design in chapter 5 of the second edition of Rod Gervais’s book. The existing door will be removed and two, triple sealed doors (similar to Rod’s “super door”) will be installed with closers.

Window&Doors.jpg

Which brings me to HVAC – but I think that needs a post all on it’s own.

© John Steel 2020

Mass Law, Resonance and coincidence

The Mass Law of sound insulation provides a guide by which the level of transmission loss (TL) of a structure (how much sound energy it inhibits) can be predicted. It tells us that, in a single leaf wall or partition, TL increases by 6dB with every doubling of mass at a given frequency (this is a theoretical figure – in the real world it’s nearer to 5dB) and with every doubling of sound wave frequency, TL also increases by 6dB. This illustrates two important facts that we need to bear in mind when designing a studio. Firstly, low frequencies are harder to control than higher frequencies and if you intend to control low end frequencies with a single leaf construction, it will need to be dense and very, very heavy. That also means it will be extremely expensive to build and, given that the ceiling needs to be the same density as the walls, incredibly difficult to construct.

Fortunately, there’s a way to dramatically improve TL without hugely increasing cost, time spent and effort. This requires building a ‘room within a room’ structure, sometimes referred to as a MAM (mass air mass) or MSM (mass spring mass) system. It’s important to know that in order for a MSM partition to work effectively, the two leaves must be completely separate from each other structurally i.e. they cannot connect at any point. If they do, this will form a ‘flanking path’ which will turn the structure (at least partially) into a resonator rather than an isolator. Look at the cross-sectional wall diagram below. This represents a top down view of wooden framed, stud walls of varying construction, clad with plasterboard and shows the rather contra-intuitive way in which MSM isolation works. The STC (Sound Transmission Class) measurement is used in the building industry to quantify isolation in houses and offices etc. It’s not very useful in music applications because it only considers a limited range of frequencies but is illustrative in this instance.

The addition of insulation in the example second from left (rock wool or fibre glass flock) in the cavity of a basic wall (far left) improves the sound isolation by providing ‘damping’ of the plasterboard cladding (i.e. it reduces vibration). Notice how doubling the mass of the wall in the same configuration (third from the left) produces only a modest improvement (STC 40) and removing one inner layer of plasterboard increases this to STC 50. Better still, removing both inner layers increases this to STC 57 (second from right) and doubling up the plasterboard on the outside faces of the wall (far right) gives a rating of STC 63. The materials used are identical to those in the STC 40 wall but by arranging them differently in the STC 63 wall, insulation is massively improved at exactly the same cost. The take away from all this is that if you want a reasonable level of sound insulation, build an air-tight, two leaf MSM system of appropriate mass with loose cavity insulation for the best, most cost effective result.

So far so good! But there other factors to consider (this is often the case in acoustics). Strictly speaking, mass law applies directly to non-rigid partitions or ‘limp mass’ (insert your own single entendre here). Building materials invariably have an inherent rigidity, so mass law is only really an indicator of possible T/L. The actual sound insulation of a given structure depends on the interaction of the mass involved, the stiffness of the materials and how effectively they are damped. As well as this, mass law is subject to resonance at lower frequencies (we already know that low frequencies are harder to contain) and losses at higher frequencies due to the coincidence effect or coincidence ‘dip’.

Every panel you build will have a natural range of of frequencies at which they will vibrate more easily than others. These are called resonant frequencies and consist of a fundamental frequency and multiples of this known as harmonics. The critical frequency of a wall is that at which the bending waves within the wall match the frequency of a sound wave impacting on it. When this coincidence happens it enables more efficient transfer of sound wave energy across the partition. The effect of resonance and coincidence dip can’t be eliminated but you can engineer your design to have the lowest possible fundamental frequency and highest possible critical frequencies. The algebraic formulae for calculating these effects on transmission loss are listed on the table below this paragraph if you want to do your own calculations. The link below that is for an online transmission loss calculator. It was built by ‘Gregwor’ who is a moderator of The John Sayer’s Recording Design Forum and I recommend that you subscribe to it if you have any interest in building your own studio. The moderators and senior members of the forum have a great deal of sage advice on this and other related topics and I’m very grateful to Greg for his permission to use the link.

TL = 14.5*LOG(M*0.205)+23dB
where M = surface density of leaf
F0 = C((m1+m2)/(m1*m2*d))^0.5
where C = constant for empty or insulated cavity
where m1 and m2 are mass of each leaf (kg/m^2)
where d = cavity depth in metres
F<F0 = 20LOG(f*(m1+m2))-47
F0<F<F1 = R1+R2+20LOG(f*d)-29
F>F1 = R1+R2+6
where f = frequency you want to check TL of
where m1 and m2 are mass of each leaf (kg/m^2)
where d = depth of cavity
where F1 = 55/depth of cavity
MASS LAW = 20LOG(f*(m1+m2))-47.2

© John Steel 2020

HVAC

(Heating, ventilation, air conditioning)

As I have already mentioned, this is the aspect of studio design that is most complex and time consuming (at least for me). I recently realised the way I have been thinking about it is mostly wrong!! More about that in a paragraph or two but this is a subject that gets complicated very quickly. In essence, what you are trying to do is ensure that:

  • Sufficient fresh air is brought in to your insulated space
  • Humidity is controlled in order to make heating & cooling efficient and
  • Exhaust air is vented properly
  • Transmission loss is maintained throughout the ventilation system

Broadly speaking, there are two types of HVAC systems you can choose from. The first type is a ducted, forced-air system that circulates via a heat exchanger and can include a dehumidifier unit. The total volume of air in the system is cycled 6-8 times an hour and is ‘topped up’ with 30% fresh air. The boiler will need to be housed in a separate room, the ducting will need to be large and the silencers even larger!

The second type is a ‘ductless’* mini – split system. This is where air circulation and heating / cooling are separate and only needs to move 30% volume of air compared to a forced air system (the fresh air you need to breath). This type of HVAC is less efficient and will require a drain pipe to be installed from the AC condenser unit through the inner & outer leaves to the outside, but has the significant advantage of being cheaper to install and requiring smaller silencers.

My studio is a single room, so adhering to the KISS maxim (“keep it simple stupid”), I have opted to build a ductless HVAC system. Please note that when I say “cheaper to install” it’s still actually fairly costly – it’s a relative term to describe money well spent.

The first step is to calculate the volume of your room or rooms. The quantity of supply air needed to ventilate the room adequately in cubic feet per minute (CFM) is: 30% of 6 x room volume (in cu feet) ÷ 60. (6 = the minimum number of air changes per hour).

In my instance this works out to be: 30% of 6 x 1600 ÷ 60 = 48 CFM (It took me 3 years to properly understand this and I’m eternally grateful to Gregwor from Alberta, Canada [designer of the MSM calculator and moderator of the John Sayers’ Recording Studio Design Forum] for making the penny drop with one of his many clear, concise posts on the subject).

Once you have this figure, you have to consider the velocity of the air entering the room. Why? – because if the air velocity is too great when it arrives at the supply register for the inner leaf, you will hear an audible ‘whoosh’ due to turbulence which, obviously defeats the whole project. Conventional wisdom has it that air velocity as it comes into the inner leaf must be no more than 300 cubic feet per minute, ideally it should be even less. The only way to control this is by adjusting the dimensions (specifically the cross sectional area) of the duct and silencer boxes you are going to build. *(Yes, ‘ductless’ mini-split systems do, confusingly, often have an amount of ducting in them).

You will be intuitively familiar with the general principles involved here. What helped me was to visualise using a garden hose. We all know that if you partially block the flow of water with your thumb, the speed of the jet (velocity) increases. When you un-block the hose (increase the cross sectional area or CSA) the speed of the water falls. This is the effect we’re looking to achieve in our HVAC silencers.

Making sure the dimensions of all the elements of your HVAC system are correct is critical! We already know our desired air volume (in my case 48 CFM) and we also know that we have to keep the velocity to under 300 feet per minute. The maths for calculating the minimum CSA of your supply is:

CSA = CFM ÷ 300 [48 ÷ 300 = 0.16 ft (1.92 inches)]

Remember, we really want the air flow to be slower than this, so let’s metaphorically take our thumb further away from the flow and increase this to 4 inches (100mm).

Great! I know the minimum CSA of my system and with that figure, now I can scale all the other parts of the path. Starting at the beginning, I’m going to need an air intake something like this – often referred to as a ‘louvre’ (nothing to do with the Parisian museum).

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Something to keep in mind is that louvres have a defined ‘open’ or ‘free’ area. This refers to the unobstructed region of the grille which is typically between 35% – 60% of the total area. I’m going to connect my louvre to a filter box and then a short length of round steel ducting – so the maths goes like this:

50mm x 50mm (duct radius) = 2500mm²

x 3.142 = 7855 mm²

multiply by 2 to give double the surface area (assuming 50% open area):

= 15710mm² (square root of this will give the louvre size)

= 125.339539mm

Filter Box without lid. undefined

Filter Box with latched lid.undefined

So let’s round that up to 150mm. This is connected to the filter box which in turn, connects via a circular flange to a short run of round steel duct where it feeds into the outer leaf silencer. In order to slow the airspeed, the CSA of the silencer must be twice the CSA of the supply duct. For round duct this area = 3.142 x r(adius)² (Pi x r squared).

My duct has a 5cm radius: 5 x 5 = 25cm² 25 x 3.142= 78.55cm² (CSA)

So doubling and rounding up gives us a silencer CSA of 160cm². Remember that this is the space for the air to flow through, not the over all dimensions of the silencer. The silencer has to be made of materials that match the surface density of the leaf that it’s attached to so they’ll need to be fairly massive and include three or four internal baffles for adequate insulation. At the point that the silencer goes through the outer leaf it should be via a sleeve of identical density. Inside the void, the connection to the inner leaf silencer can be made with conventional ducting.

The CSA of an inner leaf silencer should be greater again than the outer leaf silencer (thumb well and truly away from the end of our imaginary hose). So I intend to build mine with a cross section of 324 cm² (18cm x18cm). The attached sleeve goes through the inner leaf and into the room through a diffuser grille directly above the intake of the AC condenser unit. This will help to mix the fresh air with room air and maintain an even temperature.

Outer leaf silencer (open).

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The exhaust side of the ventilation system is almost identical – only in reverse. It should be located on the opposite side or end of the room to avoid the re-circulation of stale air. The one extra component needed is a fan to pull air through the path. The fan can be fitted on the supply side to ‘push’ instead. The main difference is that this will create positive air pressure in the room and locating the fan on the exhaust side will cause negative pressure. The advantage of having negative pressure in your room is that it acts on the doors (I’ll cover doors and windows in another post) and helps to increase insulation by pulling them into the seals.

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The fan has to be powerful enough to meet the airflow requirements of your room and also overcome the static pressure inherent in any HVAC system. Every component added to the path will add a small amount of resistance to airflow or static pressure. This can be cumulatively significant and is tricky to work out although there is a very helpful app available from ASRHAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers) which helps to detail static pressure for specific HVAC components. Static pressure is usually measured in inches of water column, which is often shown as an abbreviation such as “in. wc,” “in. wg” or “in. H2O”. Even experienced designers will concede there is at least a small element of educated guessing involved in calculating the static pressure of a sound insulated HVAC system. This is partly because of the requirement to maintain transmission loss. If you were simply installing ventilation and AC, you would keep the ducting as straight as possible to lower static pressure and turbulence, but we need to include baffled silencers which are made up of 90° & 180° turns. The method I used to estimate the static pressure of my silencer design is called “equivalent duct length”. This asserts that a 90° turn in ducting is equivalent to the duct diameter x 60. If you are using a combination of round and square ducts, you can use this online calculator to work out the equivalent diameters:

https://www.engineeringtoolbox.com/equivalent-diameter-d_205.html

For my design I calculated that the small silencers would have an equivalent round duct diameter of 14.2cm and the large ones 19.9cm.

Equation for converting a sharp 90 degree bend into an equivalent length of straight duct is:

Duct Diameter x 60

14.2 x 60 = or 852cm for each 90 degree turn x 8 for each silencer = 68.16 metres (223.622ft) of straight duct for each outer leaf silencer

and

20 x 60 = or 1200 cm (3.937008 ft) for each 90 degree turn x 11 for each inner silencer = 132 metres (433.0709 ft) of straight duct for each outer leaf silencer.

Using the engineeringtoolbox.com friction or head loss calculator for air ducting this results in:

Friction Loss (inH2O): 0.072
Friction Loss (inH2O/100 ft): 0.0302
Air velocity (ft/min): 282
Air velocity (ft/sec): 4.6899999999999995
(Outer leaf silencer)

and

Friction Loss (inH2O): 0.0216
Friction Loss (inH2O/100 ft): 0.005
Air velocity (ft/min): 138
Air velocity (ft/sec): 2.29

(Inner leaf silencer)

Giving a combined loss for all 4 silencers of 0.14882 (inH2O) or 37 pascals and apparently achieving an air velocity of 138 ft/min at the point it enters the inner leaf.
Using the ASHRAE app I added in the losses for 2 outer louvres with filters [2x 0.05 in.wg], 2 plenums[2 x 0.01in.wg] and 1 metre of additional 4” ducting[0.02 in.wg]
Total = 0.28882 in.wg or 71.9 pascals.

So, in short, I know that a fan that will move up to 300 CFM through 100m duct against a static pressure of 0.3 in.wg will be fine.

© John Steel 2020

Starting to build

By February 2020 I was finally ready to start building. There was a fair amount of tedious preparation work such as removing rawl plugs, screws, nails and other fixings from the brickwork / woodwork. I was careful to mark where I need to back fill holes with chalk circles as they’re not that easy to see and failing to block them with mortar will affect sound insulation. I moved on to stripping out the old wiring and neon lighting and a temporary power supply was rigged up by an electrician.

“Crime scene”

I commissioned a steel flitch plate to reinforce the existing beam and the work was carried out by a local fabricator (to an excellent standard I might add – it was engineered to millimetre accuracy). undefined

To build the flitch beam, I hired a local carpenter called Sean Butler (www.seanbutler.co.uk), who I already knew was great, very easy to work with and reliable. Because of the weight of the additional beam and steel plate, he brought his father Alan (also a very experienced carpenter and dry humourist – seeing the numerous chalk circles on the wall, he said he felt as though he was working at a “crime scene”) as well as an assistant. Before it could be fitted the supporting ‘pier’ needed attention because it wasn’t quite as tall as the one at the opposite end of the room. When the beam was originally installed it was leveled by packing the gap with blocks of wood(!!). undefinedSo props were placed under the beam and the packing removed, meanwhile engineering blocks were cut to size and cemented in place. undefinedundefinedundefinedundefined By the next day the mortar was firm enough to take the props away and the main structural reinforcement done! undefinedNext I set about sealing the outer leaf walls. To do this, I intended to fill the numerous drill holes in the brickwork and then apply two thick coats of masonry paint. Then, just as I was getting started, I encountered the first big setback. I had assumed that because the garage is built in to the earth at one side, this was a good thing. It has to be, doesn’t it? WRONG!!!!!! For the six months up to February, SE England had seen very high rainfall (for the previous 3 months it had been 155% of average and for February it was 250% of average). It seemed to rain non-stop, almost with no let up. Overnight on the 4th of March it was particularly bad, really knocking down to the point that the ground surrounding my house was completely saturated (there was standing water on the lawn). Inevitably, rain water found it’s way into the garage between the bottom of the wall facing the hill and the concrete floor. The chalk line in the photos indicates roughly where the ground level is on the outside of the wall.undefinedundefinedundefined

I arranged for a drainage contractor to visit with a view to installing a French drain (named after it’s inventor rather than the country) and was waiting for the quote when the next big setback – covid-19, arrived. The resulting shut-down meant that it was impossible to buy materials for the project, although given the significance of world events, this didn’t actually seem very important at the time.

Fast forward to May 2020 and I had used the enforced delay to refine my design (largely as a way of occupying myself) and also gathered some very useful information about combating damp. I was in touch with a studio builder in Bristol called Tom, who built his studio on heavy clay soil next to a river which has been known to overflow. He recommended tanking the garage, whatever else I decided to do. So, as building materials became available again, this was the first job I tackled. I applied a coat of Construction Chemicals tanking slurry (as far as I can tell it’s a mixture of cement and glass fibres) up to one metre above ground level (said to be the limit of capillary rise in brick walls). Then added a wedge of mortar (or ‘fillet’) around the perimeter of the room where the slab meets the wall. When the mortar had hardened sufficiently, I painted on a second coat and left it to cure (the manufacturer recommends a full week). Not my tidiest work, but at least it will be hidden! undefinedundefinedundefinedMeanwhile I raised the two joists nearest the gables to make room for the inner leaf silencers, moved another one other to create more space between the rafters and pinned the others with M12 bolts. The raised joists will be reinforced (or ‘collared’) before any increased load is placed on them.undefinedundefinedundefinedundefined Then I started to make a framework from softwood batten inside the gable timbers which is set in 30mm from the edge. I’m going to attach the Cement board and OSB3 cladding to this (after caulking it of course). I originally intended to pin the boards directly to the gable but doing it this way will increase the inter-leaf cavity by the thickness of the cladding.undefined undefinedundefined

A side-note that may be of interest to other project builders is that you can save a considerable amount of money on materials by bulk ordering. I found suppliers that beat trade prices by quite a large margin (£4 per sheet of OSB3, nearly £5 per sheet of MDF and over £6 per sheet of cement fibre board).undefined

© John Steel 2020