top of page
  • Writer's picturelynn tan

Renovation in a designated flood plain

Updated: Nov 7, 2022

This project locates in southland in Vancouver BC, a designated floodplain area, RS-6 Zoning. See Figure 01. Lot size is 66’ * 130’. As known, buildings on designated flood plains have to subject to Flood Plain Standards and Requirements. The most representative is Flood Construction Levels abbreviated as FCL, which means, the underside of a floor system (wood or steel) or the top of a concrete slab of any building used for habitation, business or storage of goods shall not be lower than 4.6m from GVRD datum2 in designated flood plains. But there is a few exemptions, for example, alteration of an existing building to increase the building area by less than 25 per cent of the total building area, if

i) the number of dwelling units is not increased,

ii) there is no further encroachment into setbacks required by this By-law, and

iii) there is no further reduction in the flood construction level.

After a series of consulting work and a long time of consideration the owners finally decided to conduct a renovation with a certain addition at this property.

The existing building is a two stories single house which had a 2,600 sqft floor area, with a 500sqft attached double parking garage. See Figure 02. There was no further information we could find but the built year was 1952, and ever had a renovation in 1960s. Because the family wanted to live in finally, they pursue a better life quality more than the economic benefits, so they do not much mind if this house has a maximum permitted floor area or not. Besides, the well maintained garden was one of the most important reasons why this family purchased this property, now a renovation would protect the existing condition at a maximum degree. The architect provided a proposed floor area of 3200sqft, with an addition of the upper master suite. We completed the designing work and submitted the Development Permit application in the mid-April in 2016, got the Development Permit on 28 August, 2016.

It was really a challenge and a valuable experience to us all. I was a newcomer, the architect, and even the structural engineer, they both had rarely met this kind of project before. Nobody knew what had happened on this site, only the eastside neighbor who had lived there for over 30 years told me that there was a flood in the basement at least 20 years ago. All these factors made the work extremely difficult.

Clueless, fearless, we started the demo on September 07. The expected total construction duration was 8~10 month, the estimating cost was $500,000. But actually it took us more than 18 months to complete the whole project, it cost double the money. During the construction process, almost every phase had its own unforeseen problems. Ignoring the subjective reasons, here I list them as 6 most impressed issues which highly blocked the process, and by adopting what methods we finally overcame these difficulties.

1, Insufficient preparation.

According to the approved plans, demolish should be limited to non-structural components, somewhere the framing losing it structural strength could be restored, and somewhere the structural components connected the new addition could be reconstructed. So, most of the wood framing and concrete assemblies were planned to be remained. There was a few unexpected problems happened as follows:

i) After a certain period when the framing had been proceeding to the mid-term, the owner found that the main entry canopy was too short and old compared to the main building framing, as well as the front brick steps, she required to remove both and reconstruct them lately.

------ this was not a big problem but a time and money waste, a mini excavator had to be rented again and the operator’s wage charged by hour. The operation plus the waiting time spent three days.

ii) Concrete block foundation wall exposed lately, before the demo started, we did not know the foundation wall all around the building had a 2’ height on the top was made of concrete blocks instead of concrete, not to mention reinforced, because there was a smooth cement finishing on the exterior surface. It was when the demo of the interior of exterior wall almost finished, blocks and mortar joints between the blocks exposed. At somewhere the weakest in strength of the concrete wall, those blocks could be actually shifted when they were pushed slightly.

------ Worrying about the high expenses of the professional shoring, we ever planned to remove the blocks and replace with reinforced concrete one short piece by one short piece. But nobody could guarantee there was no risk of collapse to be happening, after all, the framing assembly had been over sixty years old, finally we had to accept the support of the professional shoring.

iii) An extra concrete slab was found under the existing concrete slab. To add a new strip footing, we had to cut the existing concrete slab to locate the new footing and connect the existing slab to new footing meanwhile. After the concrete cutting finished, the mini excavating of the footing trench in the basement started and terminated immediately by an astonishing situation. As the first shoveling, a concrete hard flooring was exposed which had been hidden under a 6” sand layer for at least decades. Through a further investigation, this concrete flooring was a 6” slab, obviously constructed before the existing slab and had been abandoned by its deformation or some whatever reasons.

------ Ignoring the apparently high cost, the owner required to remove both layer of the incomplete slabs, to build a brand new concrete slab with the new strip footings. It would be simple for me to do my job accordingly, however, by the geo-tech report, the base soil was native brown/ grey clayey silt, and the water table was seen pretty high. It was easy to predict that, under this low bearing capacity, a heavy equipment would definitely trap in the basement without an effective method to be pulled out, unless the whole demo of concrete was totally completed by manpower. Furthermore, I was really afraid that there were some more complicated situations arouse underneath this old slab. So, we took a compromise to remove the upper slab by manpower keeping the lower incomplete slab.

2, Winter construction.

i) Winter usually comes in the end of October and continues to the end of March of next year in Vancouver, we surely had the psychological preparation when we started this project. The formworks and rebar had both been ready for the concrete pouring by December 07, only we had to wait to the temperature going up to 5 which is the minimum allowed temperature of the concrete pouring. The forecast showed the temperature on December 20 was 3 ~ 9 , every related parties had got ready, concrete was booked on site at 12pm. The concrete finishing guys, one framer and I got to the site before 8am that day, we had been so confident that we could finish the job smoothly until we found that in the footing trenches, under the melt 1 ½” water there was a 1” thick layer of ice! See figure 03. It is well known that the concrete pouring is not allowed to mixed with ice, we had no other option but rushing to clear all the ice immediately. Worrying the ice not completely cleared, we urgently rented a trigger torch kit to heat with flame everywhere to avoid any possible hidden trouble. It was fortunate that we only delayed one more hour than we expected to finish the whole concrete job.

------ Ice always floats above the water which is the common sense, who can imagine a ice layer is under the water? In a practical construction project, people sometimes cannot draw a conclusion by their knowledge. They must must conduct a complete investigation in order to get the real condition and find a correct solution.

ii) In cold winter, construction trades always take off a certain period because of the heavy rain or snow, which is easy to understand. But it was apparently that I had not had sufficient relevant experience, which was once the lumbers got wet by being exposed to the rain or snow, no framer liked to use them again. Framers always chose the dry and clear lumbers at first. So, after the framing had been finished, there were lots lumbers left and wasted that were wet or with nails on it.

------ At least this was my experience, winter not only prolonged the construction period, but also led to direct materials waste. Storage of materials in winter is a lesson for construction people.

3, Shoring.

As I mentioned in a previous paragraph, we had to shore the whole framing to rebuild the foundation wall. The scaffolding company sent their P.Eng to visit the site firstly on October 26, 2016, followed by two more reviewing in the design stage. It took three weeks to complete their designing, lately their three labors started to install the scaffolding on November 22 and finished in the next day. As we can see on the drawing, the framing of the east side main floor living room and the south part consisted of garage and upper kitchen were not shored. Because the living room framing was permitted to reconstruct with 2*6 together with the above new addition by the city inspector when he inspected the site someday, and the south part has short solid concrete footing which was not necessary to reconstruct the foundation. To the shoring, first of all, leaving a 3’ working space is one of the important factors for the engineer’s consideration. Besides, somewhere the shoring should go up to the roof edge to support two floors, because the existing framing lost its function at some specific places. See Figure 04, 05.

In this project, we had rented most of those shoring components for 40 days, and kept a few key components for extra one month for the security. The total cost of the shoring company was $15,390.00, including the designing fee $1,470.00, the rest included the rental fee and the labor of installation and taking off.

5, Waterproofing.

i) It is obviously that waterproofing performs an important role in a construction project in this area. Neither the property owner nor the general contractor can skip this step. We did spend a certain period of time to find a local professional waterproofing trade and a building envelope engineer to engage in this project. One week after his site visiting, the building envelope engineer provided his designing drawing, which was not a problem to be understood from a theoretical perspective. See figure 8. But, under the situation of a high water table of this area, the whole basement slab is immersed in the water for over 6 months every year, none of us could guarantee there would be no water issues happening in the future until we finally decided to install drain tile under the full basement slab which is normally called mechanic defense. We hired the draintile sub-trade to dig the trenches and installed draintiles all over each rooms running by gravity gathered to one pipe extending out of the wall and connected to the exterior draintile. Thus, there are four protection measures of the waterproofing applied in this basement slab, except the draintile, the other three are called passive waterproofing. The first line of defense was surely the draintile, the second was the fractions of original slab fixed with new poured concrete with Kryton waterstop grout at all cold joints. The third was a waterproofing membrane with a coating which is called ECOSHIELD-E & 60MIL (DRY) ECOLINE-S. The fourth was the basement floor slab.

Although the waterproofing trade we hired is a professional local company with over 30 yrs experience in metro Vancouver, they still had some problems at some specific construction details and materials. Such as at the joints of the penetrated pipes of the concrete slab, see figure 9, the waterstop had moved and was no longer around the pipes. The engineer recommended that we should remove the concrete around and the swelling waterstop to re-install a new one and to be filled with drypack within these areas.

6, The advantages & disadvantages of warmboard.

Generally speaking, warmboard is a new product which can substitute the concrete slab of the radiant heating assemblies. It is a 1 1/8” of 7-ply plywood stamped with a 0.025” thick aluminum radiant panel. It can be installed as a structural subfloor which interfaces with all types of finished floors. You can find there are over 10 advantages listed on their website, however, to my experience in this project, ignoring an amount of $25,500 material cost upon 2100sqft, its advantages can be listed as below:

i) self weight is light, 3.2psf, standard concrete slab 1 ½” self weight is about 18psf. This merit is very important to some projects which are sensitive to their structural load bearing;

ii) construction process is weather unlimited, site is clean and tidy;

iii) energy efficiency is predictable, because the heating tubes are directly attached to the above finished flooring, and the conductivity of aluminum is much high than the light concrete;

iv) more comfortable because of more even floor temperature created by the evenly flow heat and the module tubing circuits.

And, the disadvantages are listed below:

i) although its size is 4’ * 8’, same as the common plywood, but the weight of each piece is 102.4lb, quite more heavy than a most common used ½” thick plywood board which is 50lb, or a ¾”thick plywood board which is 75lb, definitely it is not possible for one worker to carry;

ii) as a subfloor, it cannot be as level as a poured concrete because of the underneath joists or beams are not assured perfectly level, it leaves behind a level problem to the finishing flooring;

iii) a correct porter cable router is needed for toilets locating and somewhere has to change the tubing circuits directions, but most of the professional framers do not have it. That is to say, it is not only an extra cost but also takes time for framers to get used to it;

iv) at any of the new routings, the small zigzag thorns of the 1060 aluminum sheet are always make people worry that the sharp edges will scratch the tubes;

v) the sound proofing value is comparatively lower than the concrete slab.

8, Insulation & building envelope.

To reach the current effective insulation RSI value requirements, architect proposed the exterior wall assemblies as Figure 13 shows, existing or new 2*4 framing was filled with R14 Batt, inside following by rigid insulation taped to the 2*4 wood studs. It was correct theoretically. But it was practically quite difficult for drywall workers to find the back framing to nail in, and it is apparently reducing the strength while the nailing penetrating through the 1 ½” rigid insulation board. With the approval of the city inspectors, we changed the insulation approaches. We installed R14 Batt in between the existing 2*4 framing, and installed 2 ½” ROXUL COMFORTBATT exterior insulation board attached to the exterior Tyvek. In the following, it was ½” thick, 2” wide strapping fastened through the 2 ½” insulation Batt to the wall sheathing, and then stucco finishing. The owners were happy to have this revised insulation method although it was quite expensive than the proposed method, but they know this will highly improve the insulation effect and will save money from energy consumption comparatively in the long term.

However, this 2 ½” insulation Batt brought us a trouble in the construction of the building envelope. The typical exterior wall finishing, such as wood siding, stucco, their thickness is about 1 ½” ~ 1 ¾”. And the typical window casing on the exterior wall is an exactly good stop of these kinds of finishing to form a smooth rain screen. But, this was a different situation with this additional 2 ½” thickness.

First of all, we had to add 2*4 treated lumbers around the window’s opening to fit this ROXUL COMFORTBATT, as well as all the openings of various purposes, such as duck vents. Second, for each of the electric power panels, low voltage panel which were mounted on the exterior wall, we had to add an extra fascia board to fit the thickness of this ROXUL COMFORTBATT as well. Third, the metal flashings over openings should be installed directly on the Tyvek under the ROXUL COMFORTBATT, see figures 18, 19. Because this insulation batt is specifically used as an exterior insulation material being allowed a certain moisture content, but a reduced efficiency will occur if this moisture content reaches a certain level. Furthermore, the risk of decay of the adjacent materials is predictable if there is no excess of the moisture.

We did the right thing but still ignored a detail that ROXUL COMFORTBATT exposed to the air within the small gap between the metal flashing and stucco stop, people could easily see it when they approached near a certain distance. We spent a lot time to find a proper way to make it nice looking, an experienced builder provided his suggestion that adding a black metal flashing into this narrow gap to cover this nude insulation batt which was proved effective for the looking, but the problem is the excess of the moisture is blocked. As an afterthought, I would insist my opinion that the correct resolution is that the black bug screen should be installed behind the insulation batt and wrap the bottom of the batt together with the strapping ends.

This project finally passed the final building inspection on May 22, 2018, and the family moved in within the next couple days. People can never foresee every single problem until it happens, but serious and adequate preparations always help to reduce the risks.

Special thanks to the consultants of this project and this article:

Ian Guan, Architect AIBC, Gradual Architecture Inc.

David Dai, P. Eng, RJBC Consulting & Project Management Ltd.

Wei Chen, P. Eng, Tri-Can Consulting Ltd.

July 23, 2018

---The end ---

Lynn Tan

Dual C Construction & Management Inc.

Tel: 778-870-7158

Fax: 604-438-4803

8 views0 comments


bottom of page