CivDesignr Journal

A collection of articles and ideas on construction and maintenance. By Alan Howard BSc. Civil (UWI - Trinidad), MEng. Civil (U of Toronto)
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Some beautiful Ceiling panels put up by an australian company

Asker Anonymous Asks:
How much concrete is needed for a floor slab that is 10'x 20'
civdesignr civdesignr Said:

To determine the amount of concrete required (or any of the materials that make up the concrete) we start with a calculation for volume.

A typical ground floor slab cast on solid ground (cast “at grade” in engineering terms) is between 4 and 5 inches thick or between 100 and 125 millimetres in metric.

Assuming a 4” thick floor first convert everything to feet and multiply length x width x thickness.

Volume of Concrete = 20’ x 10’ x 0.33’ = 66 cubic feet

You can also Try my Concrete Material Calculator on my calculations page here which can determine how much cement, sand and stone in cubic meters you would require.

Also bear in mind that a 94lb bag of cement holds 1 cubic foot (0.0281 cu. metres)

Here is a sample picture of my calculator below.

                     Click HERE to use calculator shown above

How a short term fix becomes a long term solution

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As I was driving home today I saw a house with a typical split system unit. The indoor portion (evaporator) of the unit had all the lines running through a hole in the window back to the outside portion (condenser).

Seeing that made me wonder how a situation so obviously “makeshift” could have become a permanent solution to a problem and I remembered all the conversations i’ve heard consultants and contractors have over the years about how soon they would “get back to that” and “this only temporary” only to find that years down the road the temporary solution was lost in the crush and no-one got back to it.

Never let a consultant/contractor on a site push temporary fixes as a way to “put to bed a thorny issue” or brush aside a concern. That little extra effort to make a permanent fix in the first instance will pay dividends later in your project when you realise you don’t have the time or energy to enact the permanent fix.

How should I manage the change process.

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I was reading a forum online about a home owner in Australia building their new home with a good process to manage changes in place.

However, things still went wrong. The client requested a number of changes, the contractor submitted updated drawings that were approved by the client BUT the drawings didn’t have all the requested changes.

Here is a copy of the post in question and full forum thread:

Thanks alot of the help guys. I have since released the progess payment to them in good faith before this was fully resolved, though now things have just become a bit more interesting. As since signing the final contract drawings, which had the eaves going all the way around the house. The builders then asked me to sign a new drawing designs plans for a variation to our driveway. However unbeknown to us and under no instruction, the builder also changed and took the eaves off the side section of the the house in the drawings. 

So would the builder be able to say that I’m liable to pay for alterations to put the eaves on and to make the house compliant with the estate covenants? even though in the variation letter/request which only details changes to the driveway and makes no mention whatsoever of the eaves. And just that by me signing a later design drawing plan, it’s my fault for not picking up on their mistake, or is it that because there’s no mention of the eaves in the detailed variation letter, that the eaves should have been built according to the final contract, and not the later signed display drawings (especially given that I was only looking for changes to the driveway in the new drawings as per the variation letter)?

Any help would be greatly appreciated as the builder is putting everything on the basis that they’re just going off the drawing plans that were signed off after the initial final contract binding ones, even though there was no variation letter/request for those changes.

Ultimately I agreed with the guys on the forum. Quickly correct your mistake and restate your position. You as client can’t catch all of your contractors mistakes and they really aren’t your responsibility.

Really, the lesson here is to take your time when reviewing changes and act quickly if you make an error.

One other factor to consider with changes is thinking them through before enacting them. Mull them over, discuss them with your contractor or anyone else involved in your construction decision-making process, make sure that the change is what you want. You need to do this each time because all changes have implications on time and cost and you need to be aware of what you are getting into beforehand.

Changes will always happen on every project; they are part of life. Having a good process to manage them is essential to a successful project.

How to Deal with Changes in Structure

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The “structure” in a building can be considered equivalent to the bones or skeleton of the building. The structural components are those parts of the structure designed to keep the building up.

Like the bones in your body from head to toe there needs to be firm connection from foundation to roof in the structure.

Aspects of renovation which require changes to structure therefore must be carefully considered, planned and executed. You should engage an engineer or a contractor with engineers on staff when dealing with major modifications to your structure.

During the planning phase the engineer will advise the contractor of how final structural modifications will look like and ALSO what the structural modifications will look like during the construction phase. It is vital that this second point isn’t overlooked because the contractor can’t just cut structure without making sure that structural loads are properly transferred during the construction phase. Failing to do so can lead to dangerous structural settlement or worse a collapse.

Avoid Site Supervision Pitfalls

How can you be sure that the workmen on your site are being productive and giving you value for money? Should that cabinet really take 2 days to install? With the high costs of labour nowadays every hour affects “the bottom line” and clients are justified in being concern about effciency.

The key to ahceiving efficiency is choosing a good on-site supervisor. Most workers will fall in line behind a good no nonsense site supervisor but how do you find one?

1. Past on time performance is a good indicator of how well a supervisor will perform on your job. Ask questions about past customers and whether projects ended on time.

2. If you are in North America check your local better business bureau ( or equivalent agency in your area for information on your builder and his past interactions with clients.

3. Beware if your contractor is a one man show. Progress should still occur if the “main supervisor” isn’t there or is sick. 

4. A good site supervisor plans. He should know what will happen if he is not there and he must have a second-in-command if he is not there.

Keep these key tips in mind for a project which progresses as it should.

Liability in Construction Contracts

Buck Stops Here sign


Who is responsible? where does the buck stop? where does it all end? These are all important questions to be answered before entering into any contract with a builder or consultant because having clear expectations at the start of any project is a sure way to avoid disappointment later.

Liability or Responsibility should be placed squarely in the lap of the person best able to deal with the associated risks. I have discussed this before in a previous post however the reality of liability extends beyond the terms of the contract. The safety of persons on the site is one such liability shared by both the home owner and the builder. For example, a home owner shouldn’t knowingly allow a builder to run an unsafe site and a builder shouldn’t know better than to run an unsafe site as well.

Since this liability exists and since the contractor is in charge of the work on site then damage caused by site accidents should be covered by the contractors insurance instead of being passed on to the home owner directly. As a client, the home owner should therefore make sure that all necessary third party insurances are in place to prevent this.

How To Build Your Home To Resist Extreme Wind Events


In my previous post I mentioned two major considerations in hurricane resistance. Today I will tackle the final piece of the puzzle.

3. Design

The shape and type of roof is a third factor which affects how resistant a home is to extreme wind events. A few design characteristics make a roof more susceptible.

First of these is the slope. A flatter sloped roof increases suction on the roof as air passes over it in the same way an aircraft wing would. This uplift is capable of ripping the sheets or removing the entire roof. Roof pitch should therefore ideally  be in the region of 15 to 20 degrees of slope.

Secondly, the amount of overhang at the edges (or eave) can also make a roof easier for the wind to lift. Overhangs should be limited to 12 to 18 inches to reduce the likelihood of this occurring.

While these tips should shed some light on what you ought to look for in a good design they are in no way exhaustive and should not prevent you from engaging an engineer to provide additional details to ensure that your structure is safe during hurricanes.

How to tell when something is wrong on your project


There are three quick “rules” i use to quickly assess whether there is a problem on your construction project.

Visit site regularly - There is no way around it you need to visit your site to monitor progress. Take pictures so you can record progress as well. This will be important to protect your investment should your contractor try to pull a fast one.

Beware of crowds - You should be wary of crowds on your site. Especially if crowds disperse on your arrival. This is a tell tale sign that something is up on your project. Ask questions and take a first hand look at the area.

Stalled progress in one area - Another bad sign, this usually means that the contractor is stuck for one reason or another. Ask a question here as well.

Just remember these quick and simple rules and you will be able to pretty quickly figure out whenever something is up.

How build your home to resist extreme wind events


We’ve all see the post hurricane photos like the one above shot in Miami, Florida or Galveston, Texas or any of the islands of the Caribbean. Homes flattened by hurricane force winds. Sadly, many of the structures, especially those in lesser developed countries, really weren’t constructed to resist the winds that they should be able to. What makes it even more sad is that these buildings can be constructed to withstand most winds. 

One place where buildings fail in high winds is the roof and this failure can occur in many ways.

1. Connection Failure

A connection failure is a failure of the fasteners which hold your roof together. Roofs of buildings in hurricane prone areas should be held together with hurricane straps at the appropriate areas, the roof eave and peak/ridge. You should insist that your builder install these at the appropriate areas.

The builder must also install fasteners to connect the roofing material to the structure of the roof. These fasteners should be structural screws or any fastener designed against pull-out.

2. Material failure

The materials used in a roof must be chosen to be compatible and strong enough to resist hurricane winds. In the Caribbean islands we are fortunate enough to have access to south american hardwood like purple/green-heart which is much stronger than southern pine. If Galvanised sheets are used these must be of the correct thickness to prevent the roof fasteners from just pulling right out.

3. Design… (to be cont’d)