Outward opening windows have been recorded as falling from buildings, creating near misses, causing injury and even death of pedestrians below.
There are no rules specifically forbidding the use of outward opening windows, but their risks above that of alternative opening styles (i.e. sliders, open in etc.) require careful consideration in a risk assessment to ensure that no greater danger is posed by an outward opening window than that of the equally available alternatives, as by their very nature, outward openers always project an element beyond the line of the façade or building face in their open condition.
One reason outward openers may be identified as being advantageous over that of the alternative styles is the ventilation area offered, as for similarly sized windows, outward openings may provide more free open area for ventilation than their inward opening counterparts.
Following the introduction of Approved Document O (ADO), new overheating requirements are likely to result in the amount of open area required in most residential buildings increasing dramatically. This could favour outward opening windows.
Increased use of these types of windows to prevent overheating could lead to situations where many more outward opening windows are left open accidentally, including during extreme weather conditions. If several windows are now available per room, for example, if all are open, will an occupant or user remember to always close all of them? To meet ventilation requirements, windows may have to be fitted at lower levels (with appropriate guarding). Such windows could be left open on a regular basis as background ventilation, especially if, by virtue of having guarding in place, they offer little security risk while leaving the building unoccupied for periods of time and are potentially more awkward to open and close than more traditionally located windows.
If outward opening windows are to be used, consideration of accidental conditions in which windows are left open during storms or high wind conditions need to be addressed in line with the usage of the building. It would appear there are two general approaches, design of restrictor/stay, hardware and/or other components to maintain an accidental or extreme loading condition in the case of being left open. Or design the window, frame, and hardware for the condition where the restrictor/stay or similar becomes released in an accidental situation, that prevents the falling of the casement or any other components that may cause injury.
BS EN 14351-1 gives some consideration to testing a restrictor/stay, hardware and other components in place through the requirement of the system to hold a load of 350 N over a period of 60 seconds. It is not clear how the magnitude of this load was chosen, but it does lie between the point load values for residential barriers (i.e. 250 N to 500 N for domestic categories) from BS 6180.
It does not, however, appear to approach the load one might expect on a window during high winds or storm conditions. Even the most common small factor opening windows used in the UK when applied with the CWCT minimum wind loading (800 Pa) would still result in a load on the casement that is greater than 350 N. Therefore, consideration must be given to ensuring that the window is closed during extreme weather events if following BS EN 14351-1. If this cannot be controlled adequately, consideration could be given to providing restrictors/stays or other components that can withstand the expected wind loading in the accidental conditions as described previously.
Following the approach from BS EN 14351-1 “…threshold strength shall be demonstrated by means of tests carried out as described in EN 14609 or EN 948 (reference methods), or by calculation.”, it would seem reasonable that this approach could be considered for an accidental wind load condition as well. Bearing in mind that the test is not only checking the restrictor, but also the hinges, frame and associated other hardware, in which case a static calculation assessment should follow suit.
As it is an ‘accidental’ condition, one could reasonably expect to adopt this as an Ultimate Limit State condition, so deformation of the components, but ability to continue carrying the load may not result in a ‘failure’.
Revision 1, 25 May 2023
Duncan Grover - October 25, 2024
Schüco solutions deal specifically with this topic, including detailed planning guidelines when designing outward opening windows safely.
Whilst failure is not limited to dynamic movements and fatigue on the fittings caused by positive and negative pressures, it is considered a primary cause often associated with the building being unoccupied. A range of standard fittings including an engagement catch, concealed into standard locking components, will remove to risk of dynamic movement of the vent. In the event of total fittings failure, and the vent becoming detached, a safety catch which will stop the vent from falling. All part of the standard fitting range backed up by internal testing due to no relevant test standards existing.
So whilst there are challenges with outward opening solutions on high rise buildings, the design and planning guidelines, along with standard fittings offered by Schüco, start to address the risks directly, dealing with the concerns raised above in (3) making these opening types relevant without the expensive control measures.
Dan Gleeson - October 25, 2024
Schueco has several safety features for top hung windows that specifically address this issue.
Should a top hung window be subjected to positive and negative wind forces that cause the window to oscillate, an auto engaging restrictor can be specified to catch the window as it is forced into the closed position preventing it from being re-opened by negative pressures. Additionally should the window become detached in abnormal storm conditions, a mechanical tether can also be specified as a further enhancement so as to restrict its movement and prevent it from falling.
Graham Hurrell - October 25, 2024
Good content on a tricky topic for which there is currently no magic answer or test. Should there be one? Probably also worth mentioning that outward opening windows can also lead to compromised internal safe cleaning strategies due to reach issues unless their size and position is very carefully controlled, which is not my experience to date
Robert Simcock - October 25, 2024
We have historically had a great deal of difficulty with this topic – As the risk this poses simply has not been considered (or not considered as the risk is passed onto the Main Contractor/Facade Contractor to solve) – As a Main Contractor we normally inherit the design for stage 5 delivery and so have to try to influence the design at stages 3 or 4 (if we are appointed that early) to design these out where we can – however it is not always possible by the time we are engaged into contract (or it could be a CM or TRAD contract so are in a position to influence only). Our position going forward however is relatively straight forward; 1. Design out if possible 2. If they cannot be removed, look at alternatives – Such as making inward opening or louvered/ventilated 3. If not able to be designed out; a. Ensure there is a robust method in place to close them when they reach their design wind limit (ideally electrical chain drive linked to BMS/wind device) – not being reliant upon the rooms “user” to close (as they dont) b. Add a tested window tether with a re-enforced window frame c. review the glass choice and its method of attachment to ensure safe breakage and retention in case of failure d. Ensure the wind limit for the windows design is absolutely clear during the design phase and in the O&M Manual and coordinated with M&E team for BMS activation e. Ensure the wind limit the windows are design to is tested, especially to dynamic wind affect – as this can place load horizontally which can buckle stays – most likely a bespoke test. Luckily, in more recent days, we have managed to design out this concern or look at alternatives – the cost of the items listed in (3) make the control measures expensive – who pays for this?