Trained Eye


Cutting Floor Trusses


You recently did a home inspection for me on an older bungalow in East Kildonan and I have now taken possession of the house. It has funny floor structure with steel trusses under small wooden floor joists. I have heating ducts that hang down so low that I constantly bump my head on them. I would like to move the ducts up inside the steel trusses and wonder if I can cut the cross members in the trusses to do this? If I can’t cut them, can I run other ducts in between the webs?

Doug Perrier, Winnipeg


I remembered the floor structure in question almost immediately after receiving the query, due to its unusual nature. This type of construction is very uncommon in local homes, but is more seen regularly seen in commercial buildings. This type of design will be very strong and will allow for long spans without intermediate support. If I remember correctly, the trusses spanned the entire floor without a main beam or posts for additional support. This may be the reason for the unusual construction in the first place.

Steel floor trusses are engineered and manufactured to specific standards and should never be cut or altered. The cross braces or webs are integral to the strength and removing or cutting these without additional support can cause damage to this area. If alteration is absolutely necessary, I would recommend consulting a structural engineer to give advice and specific recommendations for additional support. This may be costly and not within reason for such a small alteration.

To answer the second part of the question, I can’t think of a good reason that new ducts cannot be run in between the webs of the trusses to save space. A few things should be taken into consideration before beginning the alteration.

The first thing to take into consideration is the size and design of the new ducting in relation to the original. The new ducting should be able to accommodate the same volume of air, or more, to allow for proper heating. Every new elbow or change in direction will considerably restrict the airflow. If the new ducting is a different shape or is corrugated, it may cause further restriction.

This is an ideal time for a good evaluation of the original heating system design and how well the heat is distributed. Many older forced-air heating systems were poorly designed, allowing for uneven heat distribution to different rooms in a house. Often dampers installed inside the ducting, to provide better control of airflow, are improperly adjusted or broken. I would recommend delaying any alterations until Mr. Perrier has lived in the house for a period of time to determine if the heating system is delivering the desired level of heat necessary.

Much research has gone in to HVAC (heating, ventilation, air conditioning) systems in the last 10-15 years and our knowledge is constantly increasing. Concerns with health as well as energy conservation have encouraged a more urgent desire to create better ventilation and heating systems. Newer homes are much tighter and allow for very little air infiltration, compared to older ones. This has caused a whole range of previously unheard of problems as the energy conservation technology grew faster than improvements in our HVAC systems. The homebuilding industry is beginning to close this gap as new systems come into use and are tested.

A good suggestion would be to consult a licensed HVAC contractor before beginning any alteration of the ducting in question. The insight and suggestions provided may be able to improve the performance and efficiency of the existing system.




P.O. Box 69021
#110-2025 Corydon Ave
Winnipeg, MB
R3P 2G9